June 13-16, 2006 - Boise, Idaho
Posted online October 2, 2006
Vegetative compatibility in Aspergillus fumigatus and Neosartorya fischeri. E. BECKER (1), W. C. Nierman (2), and P. J. Cotty (1). (1) USDA-ARS, Dept. Plant Sciences, University of Arizona, Tucson AZ, 85721; (2) The Institute for Genomic Research, Rockville, MD 20850.
Aspergillus fumigatus is a common soil-borne fungus that can cause invasive aspergillosis in immune-compromised patients. Nitrate non-utilizing mutants for eleven isolates of A. fumigatus and five isolates of Neosartorya fischeri, a closely related species used for comparison, were selected on chlorate medium and subjected to vegetative compatibility analyses in a manner similar to that used previously for A. flavus. Of the 11 A. fumigatus isolates studied, 7 belonged to unique vegetative compatibility groups (VCGs) and 4 belonged to a single VCG. The sequenced isolates Af293 and CEA10 belonged to unique VCGs. All N. fischeri isolates, including the sequenced NRRL181, belonged to unique VCGs. Vegetative compatibility analyses with nit mutants may facilitate studies on the epidemiology and population biology of A. fumigatus.
The National Plant Diagnostic Network - Role in agricultural biosecurity. R. M. BOSTOCK, C. S. Thomas, R. W. Hoenisch, and A. Coggeshall. Dept. Plant Pathology, University of California, Davis, CA 95616.
The National Plant Diagnostic Network (NPDN) was created to enhance the capabilities of existing diagnostic laboratories in the nation to detect and report introduced pathogens, pests and weeds of high consequence to plant agriculture and natural ecosystems. The network coordinates diagnostic and scientific expertise at land grant universities, state departments of agriculture, agencies within the USDA (CSREES and APHIS), and other organizations involved in agricultural production and security. The program, established in 2002, is funded and administered through the USDA. Responsibilities of the NPDN include the compilation and establishment of diagnostic protocols for priority agents, the development and administration of a web-based diagnostic and reporting system for the nation, the provision of up-to-date information on priority plant pests, the development of analytical tools to exploit these data, and the training of first detectors. The network is organized into 5 regions. The mission and design of the NPDN and progress towards meeting network objectives will be presented and discussed.
Emergence of bacterial blight of crucifers in California and the U.S. C. T. BULL (1), and S. T. Koike (2). (1) USDA/ARS, Salinas, CA; (2) Univ. Calif. Coop. Extension, Salinas, CA.
Since its initial appearance in 1995 on broccoli raab (Brassica rapa subsp. rapa) in the Salinas Valley of California, Pseudomonas syringae pv. alisalensis has been shown to cause bacterial blights on a variety of crucifers in California and other states. In addition to published reports of outbreaks in commercial field plantings of broccoli raab, broccoli (Brassica oleracea subsp. botrytis) and arugula (Eruca sativa) it has recently caused disease on conventionally and organically produced nursery transplants and other crops in the field. The 2005 outbreak of bacterial blight on crucifer transplants may have been due to the development of resistance to copper used for disease control. All strains of P. syringae pv. alisalensis isolated from diseased transplants from this outbreak were resistant to at least 100 µg/ml copper. It is likely that bacterial blight of crucifers is more widespread than currently reported and may occasionally be misidentified as P. syringae pv. maculicola though these organisms can readily be differentiated.
Strategies for controlling increasingly important stripe rusts of wheat and barley. X. M. CHEN. The USDA-ARS Wheat Genetics Unit and Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430.
Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici, has historically been a destructive disease in the western United States, but has become increasingly important in the Great Plains and southeastern states since 2000. The disease has caused yield losses of more than 231 million bushels in the last six years plus multimillion dollars spent on fungicide application in the U.S. Stripe rust of barley, caused by P. striiformis f. sp. hordei, has become established and caused significant yield losses in the western U.S. since 1991 when the pathogen was first reported in southern Texas. The diseases have been monitored through trap plots and field surveys by collaborators throughout the nation. Virulence changes of the pathogens have been determined by testing stripe rust samples on the sets of wheat and barley differential genotypes. Wheat and barley germplasms have been evaluated in fields under natural infections and in the greenhouse with selected races of the pathogens to provide effective sources of resistance to breeding programs. High-temperature, adult-plant (HTAP) resistance has proven to be durable and effective in control of stripe rusts of wheat and barley in most epidemic regions. Combining genes for HTAP and effective all-stage resistance should be the best approach to achieve sustainable control of stripe rust. Molecular markers for resistance genes have been identified and used to improve the efficiency of developing resistant cultivars. Timely use of effective fungicides is still needed to reduce yield losses when susceptible cultivars are grown and when previously resistant cultivars become susceptible.
Development of a real-time, quantitative PCR for detection of potato purple top phytoplasma in plants and beet leafhoppers. J. M. CROSSLIN (1), G. J. Vandemark (1), and J. E. Munyaneza (2). (1) USDA-ARS, Prosser, WA 99350; (2) USDA-ARS, Wapato, WA 98951.
A quantitative, real-time “TaqMan” polymerase chain reaction assay (qPCR) was developed which was capable of detecting and quantifying a group 16SrVI phytoplasma in infected tomatoes, potatoes, and beet leafhoppers (Circulifer tenellus Baker). Primers and probe were designed from the 16S rRNA gene of the Columbia Basin potato purple top phytoplasma. The detection limit in phytoplasma-infected tomato DNA was approximately 50 pg. The detection limit of the assay with a plasmid clone was approximately 500 ag. Phytoplasma concentration varied considerably among purple top-symptomatic potatoes and individual infected leafhopper vectors. The assay also detected aster yellows (group 16SrI) and pigeon pea witches’-broom (group 16SrIX) phytoplasmas. The qPCR was at least as sensitive as the commonly used and more labor-intensive nested PCR for detection of the pathogen. To our knowledge this is the first report of qPCR of phytoplasma in potatoes and beet leafhoppers, and only the second report of qPCR of a group 16SrVI phytoplasma.
Susceptibility of sweet corn varieties grown in the Columbia Basin to High Plains virus. N. L. David (1), G. H. Clough (1), P. B. Hamm (1), S. L. GIECK (1), and M. A. Trent (2). (1) HAREC, Oregon State University, Hermiston, OR 97838; (2)Grant/Adams Area Extension, Washington State University, Ephrata, WA 98823.
Yield losses and reduction in quality of sweet corn due to infection with High Plains virus (HPV) continue to be problematic. Studies were conducted at two locations near Othello, Washington and one in Hermiston, Oregon during 2005 to determine the susceptibility of sweet corn varieties commonly grown for processing in the Columbia Basin to HPV. Four replications of ten varieties were tested in a randomized complete block design. Percentage of disease incidence was visually evaluated throughout the growing season and HPV infection was confirmed using ELISA. Overall, Sheba and Shaker exhibited high levels of infection, averaging 24.6% and 18.5% respectively. Chase and Marvel displayed intermediate infection levels, 11.6% and 6.5% respectively, while the other six varieties showed little infection (less than 1%). Unfortunately, those varieties showing apparent resistance to HPV often were not resistant to common corn smut (Ustilago maydis), another important disease of the region.
Occurrence of plant-pathogenic nematodes in green pea fields in the Columbia Basin of Washington and Oregon. N. L. DAVID (1), L. D. Porter (2), R. E. Ingham (1), K. E. Merrifield (1), and P. B. Hamm (1). (1) Dept. Botany and Plant Pathology, Oregon State University, Corvallis, OR 97330; (2) USDA ARS, Prosser, WA 99350.
Irrigated green pea fields in the southern Columbia Basin of Oregon and Washington often exhibit areas of stunted plants occurring in circular patches. During 2005, circular patches were sampled from thirty-six pea fields. Pea root and soil samples were taken within each patch and from healthy-looking areas adjacent to the patches and assayed for plant-pathogenic nematodes. Root-lesion, (Pratylenchus neglectus and P. penetrans), root-knot (Meloidogyne sp.), stubby-root (Paratrichodorus sp.), and stunt (Tylenchorynchus sp.) nematodes were recovered from 8, 5, 4, and 5 sites in Oregon and 10, 9, 3, and 11 sites in Washington, respectively. Additionally, 1 and 4 sites from Washington contained pin (Paratylenchus sp.) and ring (Mesocriconema sp.) nematodes, respectively. When present in a field, plant-pathogenic nematode densities were comparable in healthy and stunted areas, suggesting nematodes were not likely responsible for the observed damage within these fields.
Evaluation of fungicide seed treatments for control of Phoma lingam in brassica vegetables. M. L. DERIE and L. J. du Toit. Washington State University - NWREC, Mount Vernon, WA 98273.
Black leg of brassicas, caused by Phoma lingam, is a seedborne disease of economic significance. With the recent withdrawal of the fungicide benomyl in the US, alternative vegetable seed treatments are needed for management of this disease. Eleven treatments were evaluated on a cauliflower seed lot that had 6% P. lingam, and eight treatments were evaluated on a cabbage seed lot with 28% P. lingam. Assays for seed germination, seed health, and transmission of the pathogen were completed for the treated seed. None of the treatments reduced final seed germination significantly, except fludioxonil in one cauliflower trial. All treatments reduced seed transmission of P. lingam significantly, except azoxystrobin, a low rate of thiabendazole, and thiophanate-methyl. Boscalid (with or without pyraclostrobin), and a high rate of thiabendazole consistently provided the most significant control of P. lingam, and appear to be effective alternative seed treatments to benomyl.
Survey of pathogenic fungi in commercial seed garlic. F. M. DUGAN and S. L. Lupien. USDA-ARS Western Regional Plant Introduction Station, Washington State University, Pullman, WA 99164-6402.
Commercial seed garlic (Allium sativum) was purchased from sources growing the product in mainland China or the states of California, Idaho, North Dakota, New York, Oregon, or Washington. Twenty-five bulbs of each of these seven lots were individually wrapped in aluminum foil and incubated at 28°C until appearance of symptoms. Excised tissues and/or fungi sporulating on symptomatic tissues were transferred to half strength V8 agar amended with antibiotics. Potentially pathogenic fungi were further transferred to appropriate diagnostic media and were inoculated to garlic cloves together with appropriate controls for confirmation of pathogenicity. All lots contained pathogenic fungi. Frequently represented were Penicillium hirsutum and Embellisia allii, but Fusarium proliferatum and F. oxysporum f. sp. cepae were also prevalent. Aspergillus ochraceus, reported as a pathogen in India, was recovered from lots originating in Oregon or Washington, but isolates were not consistently aggressive in pathogenicity tests. Botrytis sp. and numerous saprophytic fungi were also recovered.
Identification of the plant-parasitic nematodes Pratylenchus neglectus and P. thornei by PCR. S. A. EASLEY, C. J. W. Watson, R. W. Smiley, and J. G. Sheedy. Oregon State University, Pendleton, OR 97801.
The root-lesion nematodes Pratylenchus neglectus and P. thornei are widely distributed across the Pacific Northwest. High populations (>2000/kg soil) of these plant-parasitic species have been associated with significantly reduced wheat yields, particularly in dryland annual cropping systems. Genetic resistance and tolerance reactions of wheat cultivars differ for each Pratylenchus species, and it is difficult to differentiate these species using morphological characters. Previously published species-specific primers were used to analyze DNA extracted from laboratory-maintained pure cultures and field soil samples. The two forward species-specific primers evaluated, PNEG and PTHO, were from the internal variable portion of the D3 expansion region of the 26S rDNA. The reverse primer D3B was used for both procedures. Each primer produced a unique amplicon from its respective target, enabling the PCR to successfully differentiate P. neglectus and P. thornei in pure cultures and in mixed extracts. Identification of these species in field-based soil samples has not yet been definitive.
Corn stunt. C. A. FRATE (1), C. G. Summers (2), and D. Opgenorth (3). (1) University of California Cooperative Extension, 4137 S. Laspina St, Tulare, CA 93274; (2) Department of Entomology, UC Davis 95616; (3) California Department of Food and Agriculture, Sacramento, CA 95832.
Corn stunt, caused by Spiroplasma kunkelii and vectored by the corn leafhopper Dalbulus maidis, has been observed in California’s southern San Joaquin Valley since the 1960’s, with periodic outbreaks lasting 1–2 years. In 1996 the disease was detected using ELISA but, unlike previous outbreaks, it is now found on a yearly basis and both the pathogen and vector appear to be increasing in incidence and distribution. Corn stunt reduces silage quality and yields and may make corn unacceptable for silage. Studies using PCR confirmed that infected leafhopper adults survive southern San Joaquin Valley winters on volunteer and unharvested corn, alfalfa, winter forage, and in riparian areas. Increased corn and winter forage acreage and an extended growing season are factors thought to contribute to vector survival and the continued yearly existence of corn stunt. Foliar and seed treatment insecticides were evaluated for control. The most significant management tools, however, are to plant early, harvest all corn by November 1, and eliminate volunteer corn plants.
Emergence of Iris yellow spot virus in the western U.S. D. H. GENT (1), L. J. du Toit (2), S. K. Mohan (3), H. R. Pappu (4), and H. F. Schwartz (5). (1) USDA-ARS, Corvallis, OR 97331; (2) Dept. Plant Pathology, Washington State University, Mount Vernon, WA 98273; (3) Dept. Plant Sci., University of Idaho, Parma Res. Ext. Center, Parma, ID 83660; (4) Dept. Plant Pathology, Washington State University, Pullman, WA 99164; (5) Dept. Bioagricultural Sci. and Pest Management, Colorado State University, Fort Collins, CO 80523.
Iris yellow spot virus (Family Bunyaviridae, Genus Tospovirus) is an emerging pathogen of onion and other Allium spp. worldwide. The virus is vectored exclusively by onion thrips (Thrips tabaci), and can be devastating to onion bulb and seed crops. The disease was first observed in the Treasure Valley region of Idaho and Oregon in 1989, but since 2001 has been confirmed in most areas of onion production in the western U.S. and on numerous other hosts. Knowledge of disease and vector epidemiology is limited, but recent studies have provided insights into pathogen phylogeny, host range, overwintering, and integrated management approaches for IYSV and its thrips vector. A summary of current knowledge will be presented in relation to disease management considerations.
High Plains virus: An emerging disease of sweet corn in the Columbia Basin of Oregon and Washington. S. L. GIECK, P. B. Hamm, G. H. Clough, and N. L. David. Oregon State University, PO Box 105, Hermiston, OR 97838.
High Plains virus (HPV), vectored by the wheat curl mite Aceria tosichella Keifer, was first described in North America in the High Plains region in 1993, but was not found in the Pacific Northwest until 1998 when infection of sweet corn was reported in Washington. However, significant damage was not observed until 2003. HPV is now found throughout the region. Symptoms of HPV in sweet corn include stunting, streaking, mosaic, chlorosis and sometimes death. Ears of plants infected with HPV are often stunted and distorted, and they may not fill or fill abnormally and mature early, leading to yield and quality reductions. Oregon and Washington account for more than 35% of the sweet corn grown for processing in the United States, with approximately 100,000 acres planted in the Columbia Basin. In 2004, at least 1860 acres were significantly damaged due to HPV infection, causing a reported 45–50% reduction in yield. Additional damage was found in 2005. There is no labeled control for the vector, and control measures have been limited to cultural practices, particularly disruption of the vector life cycle and planting of resistant varieties.
Pitch canker in California: Is the invasion over? T. R. GORDON. Dept. of Plant Pathology, University of California, Davis, CA 95616.
Pitch canker, caused by Gibberella circinata, was discovered in California in 1986. Over the next several years, the disease became more widespread and it is now found in at least 18 coastal counties. Monterey pine (Pinus radiata) continues to be the most severely affected host species, with damage occurring in both native and planted stands. However, where pitch canker is well established, disease severity appears to have stabilized. This may be attributed to a loss of highly susceptible individuals and recovery of many trees that were once severely diseased. Recovery results from containment of existing infections, which are often obscured by vigorous growth of healthy branches subtending those that were symptomatic, and a lack of new infections. The absence of new infections can be explained by systemic resistance induced by prior exposure to the pathogen. This hypothetical explanation is supported by field observations and by results of experiments conducted under both field and controlled conditions. It remains to be seen how long trees showing recovery will remain resistant to pitch canker and it has yet to be determined if the observed resistance is specific to this disease.
Impact and biology of Phytophthora ramorum. N. J. GRUNWALD. Horticultural Crops Research Laboratory, USDA ARS, Corvallis, OR.
Phytophthora ramorum, causal agent of sudden oak death (SOD) on oaks and Ramorum blight on ornamentals such as rhododendron, is an emerging pathogen with significant impact on both natural oak forest ecosystems and the nursery industry. The pathogen was simultaneously discovered in Germany and California in the mid 1990s. Oak mortality in CA and OR forests, where SOD is either established or being eradicated, is severe. Nursery crops can also be severely affected, although levels of resistance to P. ramorum on ornamentals such as Rhododendron and Viburnum vary considerably. Nurseries are also impacted due to restrictions on interstate movements of SOD hosts. The P. ramorum population in the US is clonal and consists of three major lineages. Both the European and North American populations are reproductively isolated and have gone through a genetic bottleneck. There currently is no evidence for sexual recombination in the field.
Togninia: Newly discovered pathogen causes centuries-old disease. W. D. GUBLER, S. N. Rooney-Latham, and A. Eskalen. Department of Plant Pathology, University of California, Davis, CA 95616.
The grapevine disease esca was first described by the Greeks and Romans and was referred to in the Bible. For centuries the disease occurred in vineyards while etiology was unknown. The disease was described in California over 100 years ago. Early research attributed the disease to two basidiomycetes but symptoms could not be reproduced using the suspect pathogens. Recently, a group of fungi belonging to the genus Phaeoacremonium (teleomorph: Togninia) were shown to reside in grapevines as endophytes. Pathogenicity studies showed these fungi to be capable of causing both vascular and foliar symptoms of esca. Fruit symptoms also now have been reproduced with multiple species of Togninia. While spore trapping of the pathogens showed Togninia spp. spores were being released during periods of rainfall in the fall and winter, fruiting bodies had not been observed. Subsequently, perithecia of Togninia minima and 3 other species of Togninia were detected in pockets of decayed vascular tissue on grapevine. These same species were subsequently found on native trees in California; in rotted vascular tissue indicating that these fungi are endophytes in more than one host. For thousands of years these pathogens have gone undetected in grapevines while causing severe disease in Mediterranean climates. The disease cycle is now known and control measures are being investigated.
Resistance to Wheat streak mosaic virus in perennial wheat. L. A. HARRISON and T. D. Murray. Dept. Plant Pathology, Washington State University, Pullman, WA 99164.
Anticipated benefits of perennial wheat include reduced planting costs, improved wildlife habitat, reduced soil erosion and increased water retention. Wheat streak mosaic virus (WSMV) has potential to reduce vigor and regrowth of perennial wheat. The objective of this work was to identify perennial wheat lines with effective resistance to WSMV. Thirty-three lines derived from Thinopyrum sp. were evaluated in a growth chamber for resistance to WSMV. Lines 03JP031 and 03JP039 had reduced virus replication, as indicated by lower ELISA values, and less severe symptoms, compared to a susceptible control. In field plots, 03JP039 had no symptoms and WSMV was detected in only 7% of plants, compared with two susceptible annual wheat controls, in which 46 to 91% of plants were symptomatic and positive for WSMV by ELISA. In the second year, 59% of 03JP039 plants developed symptoms and virus was detectable when temperatures rose above 32°C, suggesting temperature-sensitive resistance, compared with 100% symptomatic plants in five other perennial lines. These results show that resistance to WSMV is present in perennial wheat and should be useful to plant breeders.
Strategies to ameliorate glyphosate immobilization of Mn and its impact on disease. D. M. HUBER. Botany & Plant Pathology, Purdue University, West Lafayette, IN 47907.
The widely used herbicide glyphosate [N-(phosphonomethyl)glycine] in root exudates changes the rhizosphere biology to reduce Mn availability for crop uptake. Mn deficiency compromises plant resistance mechanisms mediated through the shikimate pathway, and take-all, head scab, Corynespora root rot, citrus variegated chlorosis, and other diseases increase after glyphosate application. Research to minimize the soil and plant immobilization of Mn by glyphosate has evaluated Mn source, time of application, and glyphosate formulation on Mn utilization by glyphosate-resistant corn and soybean. The K salt of glyphosate was less reactive than the isopropylamine formulation, and inorganic salts were more reactive than chelated or organic micronutrient sources. Normal translocation and utilization was observed only when Mn was applied 8 or more days after glyphosate. Glyphosate reduced Mn-reducing organisms in soybean rhizospheres, increased Mn-oxidizers, and predisposed soybeans to Corynespora root rot. Lower rates and fewer applications of glyphosate should be considered to minimize impacts on the soil environment and predisposition to disease.
Influences of application timing on sporulation of the biocontrol product Aspergillus flavus AF36 in cotton fields of Arizona. R. JAIME-GARCIA and P. J. Cotty. USDA-ARS, Dept. Plant Sciences, University of Arizona, Tucson, AZ 85721.
Aflatoxins are toxic, carcinogenic metabolites produced by certain fungi in the genus Aspergillus. Atoxigenic A. flavus strains are used as biocontrol to reduce aflatoxin contamination. To be effective, atoxigenic strains must reproduce during crop development when environmental conditions are conducive to aflatoxin contamination. However, optimal timing of atoxigenic strain application has not been determined. Commercial AF36 biocontrol product was applied in five cotton fields in each of two locations in 2004 and three locations in 2005. Applications were made biweekly following emergence. Incidence of product sporulation and amount of spores produced were determined weekly for four weeks after each application. Results indicate significant differences among application dates. Applications before June had little sporulation. Optimal sporulation within one week occurred for applications during July and August following canopy closing. Prior to application, an environment favorable for fungal growth is required for optimal dispersal from the biocontrol formulations.
Wilt/decline of Acacia koa caused by Fusarium oxysporum in Hawaii. R. L. JAMES (1), N. S. Dudley (2), and R. Sniezko (3). (1) USDA Forest Service, Forest Health Protection, Coeur d’Alene, ID 83815; (2) Hawaii Agriculture Research Center, Aiea, HI 96701; (3) USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, OR 97424.
An important wilt/decline disease of native Acacia koa was first described in Hawaii in 1985; the disease currently causes varying impacts on the four major Hawaiian Islands: Oahu, Kauai, Maui, and Hawaii. Disease etiology was investigated using Koch’s Postulates; Fusarium oxysporum was repeatedly shown to cause disease symptoms. Other Fusarium spp., particularly F. solani, are also commonly isolated from diseased trees. Large survival differences among koa families in young field trials on infested sites indicate that genetic variation in disease resistance likely exists. Current efforts are underway to determine biological and non-biological factors associated with disease severity, determine disease intensity and impact throughout Hawaii, assess genetic variation and develop molecular markers for detecting and monitoring pathogens associated with the disease, and develop/implement screening protocols to rapidly evaluate a large number of koa families for genetic resistance to this disease.
Secondary spread of Verticillium wilt in perennial mint fields: Evidence from spatial analysis. D. A. JOHNSON (1), H. Zhang (2), and J. R. Alldredge (2). (1) Dept. of Plant Pathology, Washington State University; (2) Dept. of Statistics, Washington State University, Pullman, WA 99164.
Spatial patterns of mint plants with symptoms of Verticillium wilt caused by Verticillium dahliae were characterized in ten commercial mint fields in Washington using several spatial analysis methods. The disease was assessed in 0.76 × 0.76 m quadrats in randomly selected sections varying in size from 5–76 m wide by 57–396 m long. The variance-to-mean ratio identified aggregation of diseased stems within quadrats. There was generally more clustering within than across rows according to both doublets and runs analyses. Total number of wilt foci ranged from 5 to 170 per field, and mean size of foci ranged from 1 to 2.7 quadrats. In one field observed repeatedly, total foci increased from 24 to 104, and the mean size of foci increased from 1.0 to 1.3 quadrats in the same section of the field from one year to the next. Size of foci increased to 2.7 quadrats in a third year of sampling the same field. Mean focus size was larger within than across crop rows in 10 of 13 field sampling occasions. The proximity index indicated highly compacted disease foci. The statistical methods employed were useful in describing, quantifying and visualizing spatial patterns of infected mint. Verticillium wilt spread during the life of the perennial mint crop. Inoculum for much of the secondary increase likely did not directly originate from microsclerotia present in soil before the crop was planted or from infected rhizomes that were originally planted.
Understanding impacts of blackberry rust in the Pacific Northwest. K. B. JOHNSON (1) and W. F. Mahaffee (2). (1) Dept. of Botany & Plant Pathology, Oregon State University; (2) USDA-ARS Horticultural Crops Research Laboratory, Corvallis, OR 97331.
The blackberry rust pathogen, Phragmidium violaceum, first observed in spring 2005 on weedy Himalayan blackberry (Rubus armeniacus) plants along Oregon’s southern coast, is now present in western Oregon, western Washington and northern California, including Oregon’s Willamette Valley where commercial blackberry production is concentrated. In addition to infecting weedy blackberries, all fields of the commercial cultivar ‘Thornless Evergreen’ (Rubus laciniatus) have become infested with the pathogen, with several fields being impacted severely. Research has been initiated to address weather and climate influences on the disease cycle, disease management in susceptible cultivars, susceptibility of blackberry germplasm, and impact of the rust on blackberry weeds.
Leaf spot of Cercospora beticola: A new disease of safflower. R. T. LARTEY (1), TC. Caesar-TonThat (1), S. Ghoshroy (2), A. W. Lenssen (1), and R. G. Evans (1). (1) USDA-ARS, NPARL, 1500 N Central Ave. Sidney, MT 59270; (2) New Mexico State University, Dept. of Biology, Las Cruces, NM 88003.
Cercospora Leaf Spot of safflower by Cercospora carthami has only been reported in the old world. Unusual spot symptoms on safflower cv. Centennial, which is rotated with sugar beet at Sidney, MT in the Northern Plains, prompted our studies of safflower as a possible host of C. beticola, a pathogen of sugar beet. Using isolates C1, C2, Sid1 and Sid2 of C. beticola, we provided evidence that it causes leaf spot disease in safflower. Safflower plants produced spot symptoms after cross infection of sugar beet with the four isolates. The presence of C. beticola in spot lesions was confirmed by PCR. In addition, sequences of PCR amplicons confirmed the presence of C. beticola in safflower and sugar beet lesions when compared with C. beticola sequence from GenBank. Antibody from C2 also bound to samples from the lesion tissues. Finally, infection progression of isolates C2 and Sid1 in safflower with scanning electron microscope showed entry of the pathogen through stomatal openings. Lesions showed substantial hyphal mass and protruding hyphae from stomatal apertures in lesion areas, indicating presence of internalized hyphae after establishment of infection.
Molecular mapping of genes conferring non-race specific and race-specific resistances to stripe rust in spring wheat cultivar Alpowa. F. LIN (1) and X. M. Chen (1,2). (1) Dept. of Plant Pathol., Washington State Univ.; (2) USDA-ARS, Pullman, WA 99164-6430.
The spring wheat cultivar ‘Alpowa’ has both race-specific resistance and non-race specific high-temperature adult-plant (HTAP) resistance to stripe rust caused by Puccinia striiformis f. sp. tritici (PST). To identify and map genes for the resistances, Alpowa was crossed with ‘Avocet Susceptible’ (AVS). In the tests of seedlings under controlled greenhouse conditions with 15 races of the pathogen, AVS was susceptible to all races and Alpowa was resistant to only PST-1 and PST-21. One partially dominant gene was identified from tests of the parents, F(1), F(2), and F(3) progeny of the cross inoculated with PST-1. This gene was mapped on chromosome 1BS. To map the quantitative trait loci (QTL) for the HTAP resistance, parents and 136 F(3) lines were tested in three locations in eastern and western Washington under natural infection of PST races virulent on seedlings of Alpowa and all-stages of AVS. QTL mapping identified a QTL that explained 54.8% of the total variation based on the relative area under disease progress curve data and 48.9% of the total infection type variation. The QTL was mapped on 7BL. The chromosome locations and different race reactions indicate that the two Alpowa genes have not been previously described.
Current status and management of Fusarium wilt of lettuce in Arizona. M. E. MATHERON and M. Porchas. Yuma Agricultural Center, University of Arizona, Yuma, AZ 85364.
Fusarium wilt of lettuce, first recognized in Japan in 1955, has since been observed in the United States (California and Arizona), Iran, Italy and Taiwan. In Arizona, the causal agent, Fusarium oxysporum f. sp. lactucae, has been recovered from lettuce plants in at least 30 different fields through 2005. Evaluation of different types of lettuce revealed that in general, crisphead lettuce cultivars were most susceptible whereas romaine cultivars demonstrated the highest level of tolerance to Fusarium wilt. Disease severity also was related to planting time; highest in lettuce planted in early September, moderate in mid-October, and least severe in early December plantings. The severity of Fusarium wilt was correlated to soil temperature at seeding and early plant growth. The efficacy of preplant solarization of lettuce beds as a potential management tool for Fusarium wilt was evaluated in the summer of 2004 and 2005. After lettuce followed by wheat was grown, the field was flood irrigated, planting beds were prepared and beds to be solarized were covered with clear plastic. The incidence of Fusarium wilt when planted with the susceptible lettuce cultivar >Lighthouse= was reduced 42% after a 40-day solarization treatment in 2004 and 81% following 28- and 56-day solarization periods in 2005, compared to nonsolarized beds.
Improved formulations of Aspergillus flavus AF36 for management of aflatoxin contamination. A. K. McCLOSKEY, R. Jaime-Garcia, and P. J. Cotty. USDA-ARS, Dept. Plant Sciences, University of Arizona, Tucson, 85721.
Aflatoxin contamination is a serious problem throughout warm production areas worldwide for several important crops including corn, peanuts, pistachios, and cottonseed. Quantities of aflatoxins in crops can be decreased by a biocontrol that utilizes atoxigenic strains of A. flavus to competitively exclude aflatoxin producers. The most extensively used atoxigenic strain formulation is steam sterilized wheat colonized by the desired strain. This formulation provides nutrients to support growth and sporulation of the biocontrol agent after application. In order to stimulate earlier and higher sporulation of the product, colonized wheat seed was coated with several nutrient sources. In laboratory tests sporulation was evaluated after 1, 2 and 7 days. Certain additives, including chili powder and ammonium sulfate, significantly increased sporulation of AF36 on the colonized seed after just 2 days. Chili may also reduce product removal by mammals. The results indicate that modifications to the current formulation could improve efficacy of AF36 biocontrol products.
Consistent formation of sporulation deficient mutants during commercial scale production of Aspergillus. M. McDONALD and P. J. Cotty. USDA-ARS, Dept. Plant Sciences, University of Arizona, Tucson, AZ 85721.
Since 1999, over 1 million pounds of steam sterilized wheat colonized by atoxigenic A. flavus has been produced in Arizona for use in the biocontrol of aflatoxins. During manufacture, fluffy variants with reduced sporulation arise. Fluffy mutants overgrew the sporulating wildtype. Similar mutants developed in A. parasiticus, A. nomius and both the L and S strains of A. flavus. The L-strain mutated more rapidly than the S-strain. A. parasiticus isolates varied widely in mutation rate, while A. nomius had the lowest mutation rate. The time to mutant development varied among the isolates and ranged from 11–30 days in A. parasiticus, 9–14 days in A. flavus L-strain, and 8–30 days in A. flavus S-strain. Mutant phenotypes remained stable through multiple single spore transfers and through selection of nitrate non-utilizing mutants. Toxin production was greatly reduced in mutants of aflatoxin producing isolates compared to wildtypes. Incidence of sporulation deficient mutants within atoxigenic strain formulations may be reduced by using spores from young cultures and by excluding spores from cultures where mutants are visible.
Susceptibility of kernels of walnut cultivars to Aspergillus flavus and A. parasiticus. T. J. MICHAILIDES and M. A. Doster. University of California, Davis, Kearney Agricultural Center, Parlier, CA 93648.
We wanted to determine whether walnut kernels of cv. Tulare possessed resistance to infection and aflatoxin contamination by Aspergillus flavus and A. parasiticus. Research by others using triturated kernel tissues in agar media suggested that walnuts of cv. Tulare (which are high in tannins that after breaking down release gallic acid) inhibited the production of aflatoxin by A. flavus, relative to other widely grown walnut cultivars in California. However, studies were not extended to intact walnuts. Kernels of cvs. Tulare and Chico were inoculated with each A. flavus or A. parasiticus and incubated at 30°C for 4 weeks. Analyses indicated that Tulare and Chico kernels were contaminated with 580 and 446 ng/g (A. flavus) and 1,027 and 1,071 ng/g (A. parasiticus) of aflatoxins respectively. Also inshell nuts of Tulare and Chandler were inoculated, incubated, and analyzed as above in 2 years. Both Tulare and Chandler walnuts were contaminated with comparable levels of aflatoxins suggesting that gallic acid present in kernels of intact walnuts of Tulare does not confer significant resistance to aflatoxin contamination.
Emerging fungal diseases in fruit and nut crops in California. T. J. MICHAILIDES, D. P. Morgan, D. Felts, and H. Reyes. Department of Plant Pathology, University of California, Kearney Agricultural Center, Parlier, CA 93648.
Following a severe epidemic in 1998 of panicle and shoot blight of pistachio, caused by a Fusicoccum sp., increased incidences of diseases with the same cause were also found in walnut, almond, pecan, and a number of ornamentals. The disease in almonds was initially reported causing band canker on the trunks, but recently the same pathogen was found causing fruit blight and cankers in the canopy. In walnuts, what was described as branch wilt (caused by Hendersonula toruloidea (syn. Nattrassia mangiferae)) most likely is also caused by Botryosphaeria dothidea. Cedar trees planted along streets or on private properties very frequently show shoot blights caused by a Fusicoccum sp. similar to that causing panicle and shoot blight of pistachio. In grapes, a canker disease found several years ago continues causing problems, and is caused by Aspergillus sect. Nigri spp., especially A. niger. The fungus attacks vigorously growing grape cultivars and creates large canker areas in the vine trunks, killing either individual cordons or entire vines. In figs, a severe limb dieback was found to be caused by N. mangiferae.
Characterization of pathogenic Rhizoctonia solani and R. oryzae of the Pacific Northwest using real-time PCR. P. A. OKUBARA and T. C. Paulitz. USDA ARS, Root Disease and Biological Control Research Unit, Pullman, Washington, USA.
Rhizoctonia solani and R. oryzae are the principal causal agents of Rhizoctonia root rot, damping-off and bare patch in dryland cereal production systems of the Pacific Northwest. We developed SYBR Green I-based real-time quantitative PCR (Q-PCR) assays that specifically amplified the internal transcribed spacers (ITS1 and ITS2) and 5.8S ribosomal DNA of R. solani AG-2-1, AG-8 and AG-10, R. oryzae grps I, II/III and III, and a binucleate Ceratobasidium spp. closely related to AG-I. In silico duplex stability analysis indicated that our Q-PCR primers would recognize isolates from Australia, Japan and other countries. Quantification limits ranged from 10 to 30 fg (approximately 15 to 20 genome equivalents) for mycelial DNA from cultured fungi, 1 to 5 pg of pathogen DNA in soil extracts, and 0.2 to 10 pg in root extracts. In a survey of Rhizoctonia isolates collected from sites that sustained root rot and bare patch, R. solani AG-2-1, binucleate AG-I-like and R. oryzae group II/III were most prevalent. Progress in sampling soil for R. solani and R. oryzae in conventional, minimum and no-till systems will be presented.
Rapid blight disease of cool season turfgrasses. M. W. OLSEN. Dept. of Plant Sciences, The Univ. of Arizona, Tucson, AZ 85721.
Rapid blight is a disease of cool season turf grasses that has been problematic on golf courses over the past ten years. It has been reported in eleven states and in the United Kingdom to date. In most cases disease has occurred in turfgrass irrigated with high salinity irrigation water. It affects turf varieties used for overseeding Bermuda including rough bluegrass (Poa trivialis), perennial rye (Lolium perenne), annual rye (Poa annua) and colonial bent (Agrostis tenuis). In 2003 the pathogen was shown to be a new Labyrinthula species, an organism in a group referred to as the marine slime molds, and was named Labyrinthula terrestris. Early laboratory trials showed that as salinity of irrigation water increased, severity of disease increased; recent studies have shown that disease development is dependent on salinity stress but not by stress induced by a non-ionic agent, polyethylene glycol. Like its marine relatives, L. terrestris grows best with NaCl amended media, but unlike them, it does not grow at the salinity of seawater. Field assays show that Bermudagrasses are hosts for L. terrestris, but appear to be unaffected. This relationship explains the recurrence of disease from one season to another.
Mandipropamid: A new fungicide for control of late blight and downy mildews. L. PAYAN, T. Harp, G. Cloud, B. Minton, A. Cochran, and P. Kuhn. Syngenta Crop Protection, 410 Swing Road, Greensboro, NC 27409.
Mandipropamid is a new fungicide from Syngenta Crop Protection for control of late blight on tomato and potato, and downy mildews on cucurbits, leafy vegetables, brassica vegetables, bulb vegetables, hops, and tobacco. Mandipropamid is a representative of a novel class of chemistry, the mandelamides, and provides outstanding control of foliar diseases caused by oomycetes. The solo product is formulated as a 2.08 SC, and has a use rate of 5.5–8.0 oz per acre, with an application interval of 7–14 days. Several combination products are also under development. Mandipropamid belongs to Resistance Group 40, the carboxylic acid amides (CAA). Registration is anticipated late in 2007, with launch the following year.
Hop powdery mildew in the Pacific Northwest. A. PEETZ (1), W. F. Mahaffee (2), W. W. Turechek (3), C. S. Thomas (4), M. Nelson (5), C. M. Ocamb (1), D. Gent (2), and G. G. Grove (5). (1) Dept. Botany & Plant Path., Oregon State Univ., Corvallis 97331; (2) USDA-ARS, Corvallis, OR, 97330; (3) USDA-ARS, Beltsville, MD 20705; (4) Dept. Plant Path., Univ. California, Davis 95616; (5) Dept. Plant Path., Washington State Univ., Prosser, 93350.
Hop powdery mildew (Podosphaera macularis syn. Sphaerotheca macularis and S. humuli) was first found in Pacific Northwest fields near Toppenish, WA in 1997, and was reported in all hop growing regions of Idaho, Oregon, and Washington by 1999. Nearly 960 hectares of a highly susceptible cultivar were destroyed in 1997. Due to extensive cone damage and yield loss in susceptible cultivars, growers responded in 1998 with intensive fungicide programs (13 to 17 applications per season). Through intensive field and laboratory research efforts, an infection risk forecasting index and an integrated management system was developed that is currently used on approximately 75% of U.S. hop acreage. From 2002 to 2004, growers using the index to assist in timing fungicide applications reported making 2.5 fewer applications and had 55% less incidence of cone infection than growers who did not use the model. This management approach has helped reduce control costs from $980 to $230–$490 per hectare, depending on variety and grower risk aversion, and has helped to maintain the economic viability of the U.S. hop industry.
Identification of novel sources of white mold resistance in pea. L. D. PORTER, G. Hoheisel, and G. Coffman. USDA-ARS, Prosser, WA 99350.
White mold, caused by Sclerotinia sclerotiorum, is a common foliar pathogen of many legumes including peas. White mold often causes serious disease in irrigated and dryland peas in the Pacific Northwest and is considered a serious potential threat to the expanding pea production in the Midwest of the United States. Due to poor economic returns to pea growers, expensive foliar fungicides used to manage white mold are cost prohibiting. There are currently no known sources of white mold resistance in peas. Therefore, the identification of novel sources of resistance to white mold from pea accessions taken from the Pisum core collection located at the USDA-ARS, Regional Plant Introduction Station (RPIS), Pullman, WA were screened for resistance to white mold. Resistance was based on lesion expansion, when plants were inoculated and maintained at 100% RH at 18 to 21ºC, for three days. Of 487 pea accessions assessed, 1, 4, 39, 276, 143, and 24 accessions had lesion expansions of, 0 to 0.5, 0.6 to 1.0, 1.1 to 2.0, 2.1 to 5, 5.1 to 8, and 8 to 11 cm, respectively. Novel sources of resistance to white mold were identified.
Emergence of a resistance-breaking strain of Beet necrotic yellow vein virus. C. M. RUSH and R. Acosta-Leal. Texas Agric. Exp. Station, Amarillo, TX 79012.
Beet necrotic yellow vein virus (BNYVV), vectored by Polymyxa betae, causes rhizomania, the most economically important viral disease of sugar beet worldwide. Rhizomania was reported in the USA from California in 1984, and soon was present in every major sugar beet production region in the country. Fortunately, genetic resistance conferred by a single dominant gene, Rz1, was soon identified. Currently, resistance to rhizomania in all commercially available cultivars grown in the USA is based on Rz1. In 2002, sugar beets with symptoms of rhizomania appeared in the Imperial Valley of California in fields planted to rhizomania resistant cultivars. Subsequent tests verified that a new resistance-breaking strain of BNYVV had emerged. Isolates of resistance-breaking and wild type BNYVV were purified from infected root tissue and selected regions from RNA 2 and RNA3 were cloned and sequenced. All resistance-breaking isolates possessed a unique amino acid motif, V(67)L(68)E(135), in the P25 ORF of RNA 3 that differentiated these isolates from wild type BNYVV. This motif has not been identified from resistance-breaking isolates recovered from other production regions in the USA.
Depth distribution of Pratylenchus neglectus and P. thornei populations in dryland wheat fields. J. G. Sheedy, R. W. SMILEY, and S. A. Easley. Oregon State University, Pendleton, OR 97801.
A survey of root-lesion nematodes (Pratylenchus neglectus and P. thornei) in Oregon and Washington dryland field crops was based on soil samples collected to 10-cm depth. The accuracy of the shallow sampling procedure was evaluated by collecting replicated soil samples to 120-cm depth at three Oregon and three Washington locations during 2005. Both species of Pratylenchus were present at the maximum depth sampled at most locations. Populations exceeding the estimated economic threshold level (2,000 Pratylenchus/kg of soil) were recorded as deep as 90 cm. Populations ranging from 5,000 to10,000 Pratylenchus/kg occurred in no-till annual spring wheat and spring mustard fields. Populations exceeding 3,900/kg occurred in cultivated winter wheat-summer fallow rotations. Peak populations commonly occurred in the 0–30 and 30–60 cm depth intervals in annual spring wheat and in winter wheat-summer fallow rotation, respectively. Multiple soil samples should be taken to at least 30 to 45 cm depth to accurately assess Pratylenchus populations in fields planted to dryland crops in the Pacific Northwest.
North Central Idaho on-farm fungicide evaluation for control Ascochyta blight in chickpeas 2005. L. J. SMITH. University of Idaho Extension, Nez Perce County, Lewiston, ID 83501.
Five fungicide treatments for control of Ascochyta blight of chickpea were compared in a randomized complete block design in an on-farm test including: Quadris Opti with crop oil, Quadris with crop oil, Quadris without crop oil, Headline with crop oil, Bravo WS with crop oil, and a non-treated check. Headline with crop oil provided the best blight control and yielded significantly better at the 5 percent significance level than the non-treated check. Seed quality per treatment was graded by commercial enterprise and values per acre assigned per treatment. All treatments provided more seed yield value per acre than the non-treated check.
Bacteria and yeast associated with sugar beet root rot at harvest in the Intermountain West. C. A. STRAUSBAUGH and A. M. Gillen. USDA-ARS, Kimberly, ID 83341.
Sugar beet roots with bacterial-like rot are occurring at harvest time in fields throughout southern Idaho and eastern Oregon. To establish the organisms associated with this rot, recently harvested sugar beet roots were collected from piling grounds in 2004 and 2005. Tissue from the transition zone between healthy and rotted areas was sampled on 287 roots and streaked on KMB and YDC. Cell and colony characteristics, carbon source utilization tests, and sequencing (16S rRNA for bacteria and 28S rRNA for yeast) were used to identify 396 isolates. Averaged over the two years, Leuconostoc mesenteroides, acetic acid bacteria, and enteric bacteria were isolated 32, 29, and 17 percent of the time, respectively. Gluconobacter asaii represented 92% of the isolates from the acetic acid group. Enterobacter spp. represented 82% of isolates from the enteric group. Pichia fermentans and P. membranifaciens were the most frequently isolated yeast species and represented 11% of all isolates. The importance of these organisms is yet to be determined, but they likely decrease percent recoverable sugar in freshly harvested and stored beets, and slow factory processing.
Erwinia amylovora does not persist on calyxes or in skin punctures of healthy winter pear fruit when inoculated at a biologically relevant dose. T. N. Temple (1), V. O. Stockwell (1), P. L. Pusey (2), and K. B. JOHNSON (1). (1) Dept. of Botany & Plant Pathology, Oregon State University, Corvallis, OR; (2) USDA-ARS Tree Fruit Research Laboratory, Wenatchee, WA.
Phytosanitary concerns over fire blight prohibit export of U.S.-grown pears to some countries. As part of a multi-year effort to characterize the risk of movement of E. amylovora with mature fruit, we co-treated pear flowers with strain Ea153N mixed with1 µm fluorescent microspheres. The pathogen was detected on flowers and calyx regions of healthy, immature fruit up to late June, but not at harvest in mid-August. Subsequently, Ea153N was not detected on calyx tissue of these fruit during 7-weeks of cold storage, even with enrichment assays, but microspheres were seen on calyxes at all sampling times, indicating that the tissue was inoculated with the pathogen. In other postharvest experiments, Ea153N did not survive in small skin punctures on fruit inoculated with up to 1000 cells per wound and held at 2°C for 7 weeks. E. amylovora is a poor epiphyte on fruit. Mature symptomless fruits subjected to commercial handling and a cold period are unlikely to harbor this pathogen.
New and emerging viral diseases in grapevines grown in California. J. K. UYEMOTO and A. Rowhani. USDA-ARS and Dept. Plant Pathology, UC, Davis, CA 95616.
Due to a new strain of grape phylloxera, Daktulosphaira vitifoliae, the hybrid rootstock AXR#1, of choice for more than 30 years in California’s wine vineyards, began to fail; circa 1985. The industry then turned to other hybrid rootstocks resistant to the new phylloxera strain. However, some hybrid rootstocks (of varied Vitis speciation) expressed incompatibility symptoms and plant death when grafted with certain scion sources. We examined the use of hybrid rootstocks as potential indicators of new grape viruses and the differential host responses on these rootstocks detected several lethal graft-transmissible agents (LGTAs). One LGTA, identified as a variant of Grapevine leafroll associated virus 2 (GLRaV-2), was first discovered in Redglobe table grape and designated GLRaV-2RG. This variant does not cause leafroll in Cabernet franc or Cabernet Sauvignon, standard indicators, but is lethal on 3309C, 1616C, 1103P, 5BB, and 5C, and latent on another 13 rootstocks. The type strain of GLRaV-2 was not lethal on the same rootstocks. Based on responses in several trials, five additional sources with different LGTAs were found. The characterization of these unknowns is underway.
Identification of genes in Puccinia striiformis and use of gene-specific PCR primers to differentiate rust species, formae speciales, and isolates. M. N. WANG (1), P. Ling (2), and X. M. Chen (1,2). (1) Dept. of Plant Pathol., Washington State University; (2) USDA-ARS, Pullman, WA 99164-6430.
Stripe rust, caused by Puccinia striiformis (P. s.), is an important disease on wheat, barley, and grasses. To study functional genomics, we have constructed a full-length cDNA library from urediniospores of P. s. f. sp. tritici. Of 196 sequenced cDNA clones, 37.8% had significant homology to the sequences of genes with characterized functions, 16.8% had significant homology to hypothetical proteins, 19.4% had some homology with genes, and 26.0% did not have significant homology with any sequences in other fungi. A total of 51 different protein products were identified and they are involved in growth, defense, and virulence/infection. In order to develop specific primers to differentiate rust species, formae speciales, and isolates, 28 gene-specific primer pairs were designed based on sequences of some of the identified genes. Using these primers, we have identified polymorphic markers to distinguish P. s. from P. graminis, P. triticina, and P. hordei, and markers to separate P. s. f. sp. tritici from P. s. f. sp. hordei, P. s. f. sp. poae, and P. s. f. sp. dactylidis. We also identified primers to detect polymorphisms among isolates of P. s. f. sp. tritici.
Newly recognized postharvest fruit rot diseases of apple and pear in the U.S. Pacific Northwest. C. L. XIAO. Washington State University, TFREC, Wenatchee, WA.
Phacidiopycnis rot caused by Potebniamyces pyri (anamorph Phacidiopycnis piri), Sphaeropsis rot caused by Sphaeropsis pyriputrescens, and a fruit rot caused by Phacidiopycnis washingtonensis are three newly recognized postharvest diseases. Phacidiopycnis rot is common on d’Anjou pears, accounting for 20–30% of the decay in Washington State, but it is much less common on apple. Sphaeropsis rot is prevalent on apple and is responsible for 20% of the decay on Red Delicious apples. A fruit rot caused by P. washingtonensis has been seen on apples but not pears. P. washingtonensis occurs sporadically but has the potential to cause significant losses of fruit during storage. Instances of losses up to 24% of apple fruit during storage caused by either S. pyriputrescens or P. washingtonensis have been observed. One common characteristic of these diseases is their association with cankers and twig dieback of trees in the orchard and the ability to cause latent infection of fruit in the orchard leading to fruit rots during storage. The biology and epidemiology of these diseases and their implication in disease control will be discussed.
Genetic characterization and QTL mapping of high-temperature, adult-plant resistance in ‘Bancroft’ barley against Puccinia striiformis f. sp. hordei. G. P. YAN (1) and X. M. Chen (1,2). (1) Dept. of Plant Pathol., Washington State Univ.; (2) USDA-ARS, Pullman, WA 99164-6430.
Bancroft barley has been identified to have durable high-temperature adult-plant resistance (HTAP) to stripe rust (Puccinia striiformis f. sp. hordei). To map quantitative trait loci (QTL) for the HTAP resistance, Bancroft was crossed with susceptible Harrington barley. The parents and F(4) and F(5) progeny of the crosses were evaluated in three locations in 2004 and 2005, respectively. Infection type (IT) and disease severity (DS) were recorded three times during the growing season. Area under disease progress curve (AUDPC) was calculated for each line based on the DS data. Analysis of variance with the IT data showed no significant differences among the locations, indicating that the HTAP resistance is non-race specific. A linkage map was constructed with eight resistance gene analog polymorphism (RGAP) markers and mapped on the long arm of barley chromosome 3H with three microsatellite markers identified using 119 F(6) lines. QTL mapping identified a single gene that explained 95%, 76%, and 71% of the total variations using the IT, DS, and AUDPC data, respectively. The two flanking RGAP markers detected polymorphism in 23 of 26 barley cultivars, indicating that the markers are useful to incorporate the HTAP resistance into most of the cultivars.
Identification of infection loci of Spiroplasma citri in the San Joaquin Valley in California. R. K. YOKOMI (1), A. Mello (2), J. Fletcher (2), and J. C. Chen (1). (1) USDA, ARS, 9611 S. Riverbend Ave., Parlier, CA 93648; (2) Dept. of Entomology & Plant Pathology, Oklahoma State University, Stillwater, OK 74078.
Spiroplasma citri, a phloem-limited, Gram positive prokaryote lacking a true cell wall, is the causal agent of citrus stubborn disease and is vectored by the leafhoppers Circulifer tenellus and Scaphytopius spp. in California. We initiated a survey of citrus and other hosts of S. citri in 2005 to determine pathogen distribution and stubborn incidence in California. Detection was based on symptomatology, isolation and culturing of plant tissue. Lyophilized or silica gel-desiccated sample tissue was also tested by PCR using spiralin gene primers. PCR detection was reliable in warm weather when S. citri titer was high but resulted in frequent false negatives in late fall and winter presumably due to low pathogen titer or erratic distribution. DNA extraction and purification improved PCR detection but are impractical for large-scale sampling. Immunocapture PCR was developed using S. citri polyclonal antiserum and increased sensitivity 10(^4)- to 10(^5)-fold and will be used in future surveys. Field samples were collected from July to December 2005. S. citri was readily detected in symptomatic citrus trees in Tulare and Kern Counties and was also found in daikon (Fresno Co.) and in carrots (Fresno and Kern Counties) showing symptoms of purple top. Surveys for S. citri will continue in citrus and other hosts in California through 2007.
Diversity within communities of L and S strains of Aspergillus flavus in cotton fields in Texas and Arizona. J. X. ZHANG and P. J. Cotty. USDA-ARS, Dept. Plant Sciences, University of Arizona, Tucson, AZ 85721.
Aspergillus flavus frequently contaminates cottonseed with aflatoxins in both south Texas and Arizona. A. flavus varies in aflatoxin production, vegetative compatibility and morphology. The two main A. flavus morphotypes are the S strain with average sclerotial diameter < 400 µm and the L strain that forms either larger sclerotia or no sclerotia. The S strain produces much more aflatoxin on average than the L strain. In the current study, A. flavus S and L strains resident in Texas and Arizona cotton fields were compared utilizing sequences of fragments of the ver-1, ordA, and niaD genes. A total of 48 S and 53 L strain isolates were compared. Isolates from Arizona displayed less genetic diversity than those from Texas and were clearly resolved into two monophyletic groups containing either only S or only L strain isolates. In contrast, in Texas the L strain appeared to be polyphyletic with eight isolates forming a separate clade within the clade containing all the Texas S strain isolates.
