February 7-8, 2010 - Orlando, Florida
Implications of fungicide application timings and irrigation on disease control and peanut yield
J. AUGUSTO (1), T. Brenneman (1)
(1) Dept. Plant Pathology, UGA, Tifton, GA, USA
Phytopathology 100:S199
Night application and/or fungicide redistribution with irrigation may improve control of stem rot (Sclerotium rolfsii) and increase peanut (Arachis hypogaea L.) yield by enhancing fungicide penetration to the lower canopy. Four applications of chlorothalonil (1.26 kg a.i./ha), prothioconazole plus tebuconazole (0.23 kg a.i./ha), tebuconazole (0.21 kg a.i./ha), flutolanil plus propiconazole (0.45 kg a.i./ha) or pyraclostrobin (0.21 kg a.i./ha), and two applications of fluoxastrobin (0.17 kg a.i./ha) or azoxystrobin (0.31 kg a.i./ha) were applied either early morning (AM = 3 - 5 a.m., with folded leaves) or during daylight (PM = 10 a.m. - 12 p.m., with unfolded leaves) in irrigated and nonirrigated plots to evaluate disease control and peanut yield in 2008 (dry year) and 2009 (wet year). In 2008 leaf spot control was similar regardless of spray timings, fungicides, or irrigation. The AM application of all systemic fungicides except fluoxastrobin decreased stem rot in nonirrigated plots, but only azoxystrobin and prothioconazole plus tebuconazole decreased stem rot more in AM than in PM sprays in irrigated plots. Yields were higher with AM sprays of tebuconazole and prothioconazole plus tebuconazole in nonirrigated plots, and with flutolanil plus propiconazole, pyraclostrobin, tebuconazole and prothioconazole plus tebuconazole in irrigated plots than with PM sprays. In 2009, leaf spot was severe and spray timings with systemic fungicides gave similar control regardless of irrigation; pyraclostrobin had the lowest ratings. The AM sprays of pyraclostrobin, flutolanil plus propiconazole and prothioconazole plus tebuconazole had lower stem rot and higher yields than PM sprays, irrespective of irrigation. The effects of spray timings and irrigation on fungicide efficacy are not the same for all products.
Bacterial leaf scorch of blueberries: A new threat to the southeastern industry
P. M. BRANNEN (2), L. Nissen (2), T. Denny (2), C. Chang (3), M. Tertuliano (1)
(1) Entomology Dept., Univ. of Georgia, Tifton, GA; (2) Plant Pathology Dept., Univ. of Georgia, Athens, GA; (3) Plant Pathology Dept., Univ. of Georgia, Griffin, GA
Phytopathology 100:S199
The Xylella fastidiosa bacterium is the causal agent of bacterial leaf scorch (BLS) of blueberry, predominantly a problem on southern highbush cultivars (Vaccinium corymbosum interspecific hybrids), but also more recently confirmed to be present in rabbiteye (Vaccinium virgatum) cultivars. Symptoms include marginal leaf scorch, leaf drop, yellowing of stems, and eventual plant mortality. Initial typing of the blueberry strain places it in an A-type category, not closely related to Pierce’s disease (G-type) strains. The glassy-winged sharpshooter, Homalodisca vitripennis, a known insect vector of other X. fastidiosa diseases, has been established as a potential vector of the bacterium in Georgia, since it is the most prevalent sharpshooter found in commercial blueberry plantings. A 2008 survey determined the prevalence of BLS in Georgia, and 71.1% of farms were positive for BLS in at least one field. Field resistance or tolerance was observed among some cultivars. However, highly susceptible cultivars are predicted to incur complete loss within 10 years of planting.
Efficacy of fungicides applied in furrow for peanut disease control
T. B. BRENNEMAN (1), J. Augusto (1)
(1) Department of Plant Pathology, University of Georgia, Tifton, GA
Phytopathology 100:S199
Prothioconazole (0.20 kg/ha), azoxystrobin (0.11 kg/ha), or penthiopyrad (0.35 kg/ha) were applied to Tifguard peanut (Arachis hypogaea) in furrow (spray volume 35 L/ha) at planting in two replicated trials in 2009. All plots received only chlorothalonil during the season for foliar diseases. Data for the trials could be combined, and none of the treatments affected plant stands. Expanding leaves were bioassayed with Sclerotium rolfsii after emergence but no residues were detectable. However, prothioconazole reduced leaf spot at harvest whereas other treatments did not. Prothioconazole and penthiopyrad each reduced stem rot at both midseason and harvest, but only penthiopyrad significantly increased yield (512 kg/ha greater than the nontreated control). In furrow fungicides are known to reduce diseases of seedlings and roots, but can also reduce foliar diseases of peanut.
Evaluation of Pasteuria usgae as a biological control of sting nematode (Belonalaimus longicaudatus)
J. H. CAMPBELL (1), J. L. Starr (1), K. L. Ong (1)
(1) Dept. of Plant Pathology & Microbiology, Texas A&M University, College Station, TX, USA
Phytopathology 100:S199
The plant-parasitic sting nematode (Belonalaimus longicaudatus) is one of the most devastating nematode pests on turfgrass. In recent years, the turfgrass industry has seen a number of chemical control measures taken off the market leaving no effective alternative. The effectiveness of a commercial formulation of Econem (Pasteuria usgae, an obligate bacterial parasite specific to sting nematodes) was tested on a bermudagrass putting green in Texas. A complete-block design with five replicates of each treatment was used. Granular applications of 100 thousand (30 g product formulation) or 200 thousand spores (60 g product formulation) per 16 square foot plot were applied monthly from April through July 2009. Effects of treatments on nematode population densities, root health and length, turf color and turf density were evaluated over time. There was no effect of treatments on sting nematode populations, root health or turf density. Turf color was significantly greater at both the 100k and 200k levels from the untreated controls at the 0.05 level. Average root length was statically greater at the 200k level than the other treatments. Less than two percent of nematodes in bacterial treated plots were encumbered by P. usgae endospores at the termination of the experiment.
Components of resistance to Cercospora arachidicola in medium maturity peanut varieties with moderate early leaf spot resistance in the field
E. G. CANTONWINE (2), A. K. Culbreath (1), D. Baskin (2), D. Murphy (2)
(1) University of Georgia, Tifton, GA, USA; (2) Valdosta State University, Valdosta, GA, USA
Phytopathology 100:S200
Until recently, resistance to early leaf spot, caused by Cercospora arachidicola, has been linked to late-maturity in peanut (Arachis hypogaea). An experiment was conducted to evaluate the components of resistance to C. arachidicola in two medium maturity peanut cultivars with moderate field resistance, Georgia-03L and Tifguard. Their responses to inoculation were compared to those of Georgia Green, a susceptible medium maturity cultivar, and two resistant late-maturing cultivars, Georganic and York. Leaves taken from the second position of flowering plants were detached, placed in beakers of saturated sand, and inoculated with spores of C. arachidicola. Leaves were maintained in a dew chamber at 24°C, 100% relative humidity, and 12-hr photoperiod for 32 days. Infection frequency, lesion diameter, incubation period, latent period, and the number of spores per lesion area were compared for the genotypes. The only resistance component observed for Georgia-03L was a reduced infection frequency, 0.35 lesions/cm compared to 0.53 lesions/cm for Georgia Green. Tifguard had a lower infection frequency (0.29 lesions/cm), smaller lesion diameter, longer latent period, and fewer spores per lesion area than Georgia Green. Infection frequency was also lower for York (0.23 lesions/cm) and Georganic (0.40 lesions/cm) than Georgia Green, and York and Georganic had smaller lesions than Georgia Green. York had fewer spores per lesion area than all cultivars tested.
Effect of cotton cultivar selection on soil populations of Fusarium oxysporum f. sp. vasinfectum
S. CHAWLA (1), J. E. Woodward (3), T. Wheeler (2)
(1) Department of Plant and Soil Science, Texas Tech University, Lubbock, TX, USA; (2) Texas AgriLife Research, Texas A&M System, Lubbock, TX, USA; (3) Texas Tech University, Lubbock, TX, USA & Texas AgriLife Extension Service, Texas A&M System, Lubbock, TX, USA
Phytopathology 100:S200
Fusarium wilt, caused by the soilborne fungus Fusarium oxysporum f. sp. vasinfectum (Fov), is an important disease of cotton (Gossypium hirsutum L.) in portions of West Texas. A microplot study was conducted over the 2008 and 2009 growing seasons to investigate the influence of planting susceptible and/or resistant cotton cultivars, FiberMax (FM) 9058F and Stoneville (ST) 4554B2F, respectively on soil population of Fov. Fibermax cultivars, when planted 2 consecutive years resulted in large increase of Fusarium wilt. The hypothesis was that cultivars can affect population density of Fov in the soil. Microplots (75 cm diameter × 45 cm deep) were augmented with field soil naturally infested with Fov and Meloidogyne incognita. Treatments consisting of rotation schemes containing ST 4554B2F and FM 9058F were arranged in a randomized complete block with nine replications. Baseline soil populations (46.2 cfu/g soil) were enumerated for each microplot via soil dilution plating on a semi-selective medium. It was observed that FM 9058F planted in sequential seasons increased Fov populations (79.4 cfu/g soil); however, populations in microplots planted to ST 4554B2F over two seasons remained constant (45.8 cfu/g soil). Soil populations in microplots initially planted to FM 9058F followed by ST 4554B2F remained unchanged (44.2 cfu/g soil); whereas, Fov populations in microplots initially planted with ST 4554B2F followed by FM 9058F were not different from those planted to FM 9058F for two seasons. Results from this study may be useful in developing long term management strategies that can be implemented into integrated programs for sustaining the production of cotton in fields infested with Fov.
Effects of chlorothalonil and dodine applied alone and in combination with systemic fungicides on late leaf spot of peanut
A. K. CULBREATH (1), T. B. Brenneman (1), R. C. Kemerait (1)
(1) University of Georgia, Tifton, GA
Phytopathology 100:S200
In the southeastern U.S., control of late leaf spot, caused by Cercosporidium personatum, of peanut (Arachis hypogaea) requires multiple applications of fungicides. Recent renewed interest in the fungicide dodine for leaf spot control prompted comparison of this fungicide to chlorothalonil alone and in combination with four systemic fungicides in a randomized complete block field experiment at Tifton, GA in 2009. All fungicides were applied seven times at ca.14 day intervals starting at 37 days after planting. Leaf spot epidemics were severe, with final leaf spot severity ratings (Florida 1-10 scale) of 9.3 (> 95% defoliation) for the nontreated control. The dodine (0.45 kg a.i./ha) treatment had final leaf spot severity ratings of 8.1 compared to 5.1 for 1.26 kg a.i./ha of chlorothalonil (LSD = 0.7). Final leaf spot ratings for mixtures of dodine (0.3 kg a.i./ha) with propiconazole (0.063 kg a.i./ha), thiophanate methyl (0.2 kg a.i./ha), tetraconazole (0.063 kg a.i./ha) and cyproconazole (0.03 kg a.i./ha) were 7.2, 7.5, 6.1 and 6.6, respectively, compared to 4.8, 5.0, 5.1, and 5.2 (LSD = 0.7) for chlorothalonil (0.84 kg a.i./ha) for those respective treatments.
Comparative evaluation of the survivability of Acidovorax avenae subsp. citrulli in stored seeds
B. DUTTA (1), R. Walcott (1)
(1) University of Georgia, Athens, GA, USA
Phytopathology 100:S200
Bacterial fruit blotch (BFB), one of the most economically important bacterial diseases of cucurbits worldwide, is caused by Acidovorax avenae subsp. citrulli (Aac). Infested seeds are the primary source of inoculum and under favorable environmental conditions, up to 100% yield loss can occur. Recent reports indicating that Aac can be transmitted to seedlings from infested seeds stored for more than 38 years, suggest that the bacterium can withstand desiccation during storage. However, no detailed studies have been conducted to dissect the mechanisms of long term bacterial survival in seeds. Hence, the objective of this work was to compare the ability of Aac to survive on host and non-host seeds with Xanthomonas campestris pv. campestris (Xcc), Pantoea stewartii subsp. stewartii (Pnss) and Ralstonia solanacearum (Rs). Watermelon, tomato, cabbage, and corn seeds (n = 100 g) were artificially inoculated (separately) with suspensions containing 108 CFU/ml of each of the four bacteria. Inoculated seeds were air-dried overnight and stored at 4°C and 50% R.H. The bacterial populations on five replicated samples (n = 1 g of seed) from each treatment were estimated weekly for 3 months on semi-selective media. In two independent trials, the Pnss was undetectable on all seed types at the end of 3 months. In contrast, the populations of Aac and Xcc declined to 102 to 103 CFU/g of seeds whereas Rs populations declined to 104 to 105 CFU/g of seeds, irrespective of seed type. Seed type was not a significant factor in bacterial survival. These data suggest that Aac, Xcc, and Rs are more tolerant to desiccation than Pnss. The data also suggest that it is likely that the ability of Aac to survive for 38 yrs in stored seed is due to the location of the bacterium in the seed rather than some unique characteristic of the bacterium.
Current status and future of HLB
T. GOTTWALD (1), M. Irey (2), A. Bergamin-Filho (3), R. Bassanezi (4)
(1) USDA-ARS, Fort Pierce, FL; (2) Southern Gardens Citrus, US Sugar Corp., Clewiston, FL; (3) ESALQ, Universidade de São Paulo, Brazil; (4) Fundecitrus, Araraquara, Brazil
Phytopathology 100:S200
Results from studies on the increase in HLB incidence and spread in China and Reunion Island indicate a rate of disease increase leading to a multi-year epidemic requiring 7 to 10 years for infection to approach an asymptote of 100%. In contrast, more recent studies in Brazil, Vietnam, and Florida suggest a much more rapid rate of disease increase and spread. An HLB epidemic was examined in a plantation of over 4,800 ha in South Florida where no new citrus had been introduced for 10 y and thus spread was entirely dependent on psyllid transmission. The level of psyllid infestation was unprecedented compared to previously recorded psyllid infestations. The psyllid vector was relatively newly introduced to Florida and thus lacks the biological and environmental constraints found in its native range. Consequently the HLB epidemic in Florida is undoubtedly one of the worst on record. Stochastic Markov-Chain Monte Carlo models indicated a prevalence of secondary spread with occasional primary spread from outside the plots. Interpretations of the stochastic models combined with survival analyses show spread over multiple scales from local to regional are occurring simultaneously and continually in Florida. Edge effects analyses indicate a prevalence of infections that accumulate at the transition of plantings and areas devoid of citrus such as the plantation perimeter, internal roads, canals, ponds, etc. This edge effect diminishes rapidly toward the interior of the planting and is generally well described by an inverse power function.
MeloCon WG® and SoilGard 12G® used in a program as a methyl bromide alternative to control nematodes and soil borne diseases in fruiting vegetables
H. B. HIGHLAND (1)
(1) Certis USA, Nokomis, FL
Phytopathology 100:S200
With the advent of the Montreal Accord of 2007 on restricting ozone depleting gases, and as a result of further state led restrictions, the use of methyl bromide and other fumigants in agriculture has been on a steady decline. As such effective and safe alternative treatments are being investigated, labeled and used in commercial production. The loss of fumigants is especially deleterious to the production of fruiting vegetables, primarily tomatoes and peppers, in the southeastern US, where soil borne diseases and nematodes can be of particular concern. A program of MeloCon® WG and SoilGard® 12 G, marketed by Certis USA, have been shown to be very effective when used alone or in combination to control nematodes and soil pathogens in field trials in the US. MeloCon® WG is a naturally occurring and beneficial soil fungus (Paecilomyces lilacinus strain 251) that controls a wide range of plant parasitic nematodes. MeloCon® WG has been shown in replicated field trials to control both southern root knot nematodes (Meloidogyne incognita) and stubby root nematodes (Trichodorus spp. and Paratrichodorus spp.), as well as many others. SoilGard® 12G is also a naturally occurring and beneficial soil fungus (Gliocladium (Trichoderma) virens strain GL-21) that controls a wide range of soil borne pathogens, including southern blight (Sclerotium rolfsii), Fusarium crown rot, and pepper blight (Phytophthora capsici). Replicated field trials using tomatoes with these products in conjunction with soil applied herbicides resulted in improved plant growth, increased survival, and increased yields, similar to methyl bromide and other chemical standards.
Yield loss associated with sheath blight disease of rice
C. A. HOLLIER (1), D. E. Groth (2)
(1) Department of Plant Pathology and Crop Physiology, LSU Agricultural Center, Baton Rouge, LA, USA; (2) Rice Research Station, LSU Agricultural Center, Crowley, LA, USA
Phytopathology 100:S201
Sheath blight is one of the most important rice diseases in the southern USA rice-producing area. Yield loss estimates are made annually but accurate measurements need to be taken. Fungicides were used a tool to influence sheath blight development in small plots. Applications were made with the aid of CO2– pressurized sprayers delivering 93L/ha of solution at various times to halt or delay disease development to determine the affect of disease development on yield at different growth stages. Early disease development on enclosed canopy rice reduced yield greater than epidemics that were halted until late stages of crop development. Yield losses ranged from 7.83% for heading stage development (late developing disease), 16.65% for boot stage development (intermediate developing disease) to 28.63% for green ring stage development (early developing disease).
Management strategies for Pierce’s disease: An increasing threat to grape production in the southern US
D. HOPKINS (1)
(1) University of Florida, Mid-Florida REC, Apopka, FL
Phytopathology 100:S201
Pierce’s disease (PD) of grapevine, caused by Xylella fastidiosa, affects grape production across the southern U.S. and is especially damaging in the Southeast, where it is the primary factor limiting the development of a grape industry based on the high-quality Vitis vinifera grape. PD is increasing in severity in the southeastern U.S. as a result of warmer winter temperatures, increasing the risk of PD in the Piedmont region. Currently, the only feasible control for PD in most of the southeastern U.S. is genetic plant resistance. Management strategies currently being used or tested include vector control, removal of reservoir hosts, various transgenes in grape cultivars or rootstocks, and biological control with benign strains of X. fastidiosa. In Temecula CA, an area wide leafhopper vector management program has been credited with saving the grape industry from a 100% loss to PD. Several transgenic grape lines are ready for field trials to evaluate resistance to PD, including lines containing transgenes for anti-microbial proteins, for programmed cell death, and for diffusible signal factor. In Florida, injection of a benign strain (EB92-1) of X. fastidiosa into transplants has controlled PD in a Vitis vinifera cv. Cabernet Sauvignon planting for 13 years. This control could be available for commercial use in 2–3 years.
Occurrence of boscalid-insensitive isolates of Didymella bryoniae in commercial watermelon fields in South Carolina
A. P. KEINATH (1), E. Fillippeli (1)
(1) Clemson Coastal REC, Charleston, SC, USA
Phytopathology 100:S201
Insensitivity to boscalid in Didymella bryoniae, causal agent of gummy stem blight on cucurbits, was found in Georgia in 2007. In 2009, isolates were collected in South Carolina from watermelon leaves with symptoms of gummy stem blight in four commercial fields in three counties and one research plot. All five sites had been sprayed with a boscalid-pyraclostrobin premixture (Pristine) in 2009 and in prior years. Sensitivity of these isolates to boscalid was compared to sensitivities of isolates that had never been exposed to boscalid collected from watermelon in 1998 or from muskmelon in 2002 and isolates previously exposed to Pristine collected from watermelon in 2005. When possible, the 2009 isolates were collected from sites sampled in 2005 and 1998. Suspensions of conidia and ascospores of 57 isolates were placed on water agar amended with 0, 0.01, 0.10, 1.0, or 10.0 mg/l technical grade boscalid. On each plate, spore germination was counted after 24 h. Relative percentage germination (germination on amended media/germination on nonamended medium) was regressed against the logarithm of fungicide concentration to calculate EC50 values. Insensitive isolates were found at all five sites sampled in 2009. Of 30 isolates collected in 2009, 13 had EC50 values >10 mg/l, 11 had EC50 values >1 mg/l, and 6 had EC50 values <1 mg/l. EC50 values for all 27 isolates collected in 1998 to 2005 were <1 mg/l. Spores of 27 of the 30 isolates collected in 2009 germinated on agar amended with 10.0 mg boscalid per liter compared to only 1 of the 27 isolates collected in 1998 to 2005. Ten isolates collected in 2009 were insensitive to 10 mg/l, based on germination of >50% of conidia and ascospores on amended medium. Isolates of D. bryoniae from South Carolina are now insensitive to both pyraclostrobin and boscalid.
Baseline sensitivity to fluopicolide in Phytophthora capsici isolates from the eastern United States
A. P. KEINATH (1), E. Fillippeli (1), M. K. Hausbeck (2), C. Kousik (3)
(1) Clemson Coastal REC, Charleston, SC, USA; (2) Michigan State University, East Lansing, MI, USA; (3) USDA, ARS, Charleston, SC, USA
Phytopathology 100:S201
Fluopicolide was registered in 2007 to control diseases caused by Oomycete pathogens such as Phytophthora capsici on cucurbits and peppers. In this study, 69 isolates of P. capsici from Michigan (24 isolates), South Carolina (17), Georgia (14), Florida (11), and North Carolina (3) recovered from watermelon (22), pepper (11), bean (10), squash (9), cucumber (6), or unknown hosts (11) were tested to determine their sensitivities to fluopicolide. In three assays, isolates were grown on V8 agar amended with technical grade fluopicolide dissolved in DMSO. For the mycelial growth assay, concentrations were 0, 0.03, 0.10, 0.30 and 1.0 mg/l. For the sporangia production assay, concentrations were 0, 0.03, 0.10, and 0.30 mg/l with a few isolates also tested at 0.01 mg/l. For the zoospore germination assay, isolates were initially tested at 0.10, 1.0, and 10.0 mg/l; some isolates then were tested at 0.03 or 31.6 mg/l. Percentage colony diameter, zoospore germination, and sporangia production relative to the nonamended control was regressed against the logarithm of fungicide concentration to calculate EC50 values. All isolates of P. capsici tested were sensitive to fluopicolide in all three assays. EC50 values for each assay were non-normally distributed. The median concentration was 0.28 (range 0.11 to 1.56), 0.04 (<0.01 to 0.14), and 2.08 (0.14 to 13.74) mg/l in the mycelial growth, sporangia production, and zoospore germination assays, respectively. The ratio between the least and most sensitive isolates was 14 for mycelial growth and sporangia production. For zoospore germination, the ratio was 98 across all isolates but ranged from 3 to 44 for isolates within states. For mycelial growth and zoospore germination, isolates from Michigan had a higher mean EC50 value than isolates from other states (P < 0.05). Zoospore germination was much less sensitive and sporangia production was much more sensitive to fluopicolide than mycelial growth was.
Redbud yellow ringspot disease: Thirty years of research
A. G. LANEY (1), R. C. Gergerich (1), I. E. Tzanetakis (1)
(1) University of Arkansas
Phytopathology 100:S201
In the 1970s a disease was found infecting eastern redbud, Cercis canadensis. Symptoms include chlorotic ringspots, oak-leaf, and vein chlorosis in mature leaves and are usually expressed early in the season. Previous work revealed the presence of virus-like double membrane-bound bodies in diseased plants. Similar bodies have been found associated with several diseases including rose rosette, high plains disease, fig mosaic, European mountain ash ringspot, and thistle mosaic. Recently, the genomes of the viruses associated with Fig mosaic (FMV) and European mountain ash ringspot (EMARaV) were sequenced, and found to be negative sense ssRNA viruses related to tospoviruses. We have obtained sequence information of a virus found in yellow ringspot diseased plants, provisionally named Redbud yellow ringspot-associated virus (RYRaV). Detection protocols have been developed and used in a survey of symptomatic redbud trees. RYRaV was found closely associated with diseased trees as more than 90% of tested material was infected with the virus. Potential field alternative hosts were surveyed and a several herbaceous hosts were inoculated mechanically and by grafting. Transmission studies using eriophyid mites are under way.
Genetic diversity of the Sclerotinia homoeocarpa population in Florida
D. LIBERTI (1), G. T. Cooper (1), J. A. Rollins (1), P. F. Harmon (1)
(1) University of Florida, Plant Pathology Department, Gainesville, FL
Phytopathology 100:S201
Dollar spot disease of turfgrass, caused by the fungus Sclerotinia homoeocarpa, is the most important turfgrass disease occurring world-wide on all cool and warm season turfgrass species. The ribosomal DNA (rDNA) sequences were obtained from twenty-six isolates collected from Floridian warm season turfgrass species including bermudagrass (Cynodon dactylon (L.) Pers.), seashore paspalum (Paspalum vaginatum Sw.), St. Augustinegrass (Stenotaphrum secundatum (Walter) Kuntze) and zoysiagrass (Zoysia japonica Steud.) and from the Floridian cool season turfgrasses, rough bluegrass (Poa trivialis L.) and creeping bentgrass (Agrostis palustris Huds). Isolates were collected from 26 distinct golf courses and other turfgrass swards in 12 Florida counties between 2004 and 2009. These data plus 14 other S. homoeocarpa rDNA sequences from GenBank were subjected to phylogenetic analysis using the neighborg-joining method and choosing Sclerotinia sclerotiorum (Lib.) De Bary and Poculum henningsianum (Plottn.) T. Schumach. as outgroups. Phylogenetic reconstructions based on sequences of internal transcribed spacer 1 (ITS1) and internal transcribed spacer 2 (ITS2) indicated that twenty out of twenty-six Floridian isolates clustered in a group that represents a newly identified biotype of S. homoeocarpa. Further characterization of this Floridian biotype is in progress.
Effect of southern root-knot nematode (Meloidogyne incognita) on cotton growth, yield and fiber quality
P. LU (2), R. C. Kemerait (2), C. D. Perry (2), R. F. Davis (1)
(1) USDA-ARS, Tifton, GA, USA; (2) University of Georgia, Tifton, GA, USA
Phytopathology 100:S202
Southern root-knot nematode (SRKN) (Meloidogyne incognita) is a major pest of cotton worldwide. Much research has been devoted to impact of SRKN on yield; less information is available regarding impact of SRKN on fiber quality. Objectives were to assess impact of SRKN on growth, yield, and fiber quality at five sites planted to cotton in Georgia in 2008 and 2009. Three sites were planted to DPL 555B/RR and two were planted to Fiber Max 9063B2F and Stoneville 4554B2RF. A randomized complete block design with 4-6 replications was used at each site. Risk management zones for SRKN were established at three locations based upon characteristics to include elevation, slope, soil electroconductivty and NDVI from bare soil reflectance. Nematicides (aldicarb, 3-6 lb/A), 1,3-dichloropropene (3-6 gal/A) and two seed treatment nematicides were used to create differential populations of SRN. Growth of the crop, soil populations of root-knot nematodes, and damage to the plants were assessed. Cotton growth and yields were often significantly and negatively correlated to root gall ratings, populations of nematode juveniles and the number of SRKN eggs extracted from root samples. However, most fiber quality parameters were not correlated to soil nematode populations or damage to the cotton plants. However, in fields with higher populations of SRKN, fiber quality properties tended to be more strongly correlated to nematode populations and subsequent root damage than in fields with lower populations. Often, plant growth, yield and fiber quality were significantly different between the high and low risk management zones; however the impact of SRKN populations was not always clear.
Quantitative modeling of the effects of temperature and wetness duration on germination and infection of cantaloupe by Pseudoperonospora cubensis
K. N. NEUFELD (1), P. S. Ojiambo (1)
(1) Department of Plant Pathology, North Carolina State University, Raleigh, NC
Phytopathology 100:S202
Cucurbit downy mildew caused by Pseudoperonospora cubensis is considered the most damaging disease of cucurbitaceous crops worldwide. Three response surface models were developed based on independent experiments in which cantaloupe plants were inoculated with P. cubensis and exposed to a range of leaf wetness durations (2–24 h) and fixed temperatures (5–30°C) in growth chambers. Germination was assessed at the end of each wetness period and infection was recorded 5 days after inoculation as percent leaf area with chlorotic and necrotic symptoms. Models were evaluated for their ability to predict germination and infection of P. cubensis. Optimum germination and infection was observed at 16.5 and 20.6°C, respectively, while little germination or infection occurred at 5 or 30°C. Optimum infection for wetness periods 4–8 h was observed at t = 20°C, but wetness periods > 8 h had broader optimum curves. Model 1 of the form f(w,t) = f(t)•(1-exp{-[Bw]D}) resulted in smaller asymptotic standard errors and yielded higher correlations between observed and predicted germination and infection data than either model 2 of the form: f(w,t) = A{1-exp[-f(t)•(w-C]D} or model 3: f(w,t) = [1-exp(-Bw)2] / cosh[(t-F)G/2]. Models 1 and 2 had non-significant lack-of-fit statistics while a lack-of-fit test was significant for model 3 for both germination and infection data. These models accounted for up to 98% of the total variation in the data. Risk threshold charts were developed to estimate the potential risk of cucurbit downy mildew epidemics in the field based on temperature and duration of leaf wetness.
Efficacy of strobilurin fungicides and host resistance for control of gray leaf spot of corn
M. A. NEWMAN (1)
(1) University of Tennessee, Jackson, TN, USA
Phytopathology 100:S202
Gray leaf spot (GLS) of corn, caused by Cercospora zeae-maydis is a common foliar disease that reduces corn yields in Tennessee and many other states. Two strobilurin fungicides (azoxystrobin and pyraclostrobin) have shown a high degree of control of GLS in tests conducted over the last three years (2006–2008) at the Research and Education Center at Milan, TN. Each fungicide was sprayed at (01.1 kg/ha a.i.) with Penetrator Plus @ 0.125% v/v as an adjuvant. Four-row plots 30’ long were randomized and replicated four times. Rows were on 30” centers and planted no-till in a field infested with GLS. The following three Pioneer corn hybrids with different levels of resistance to GLS were used: susceptible P 32T22, moderately susceptible P 33R76 and tolerant P 33V14. Each fungicide was sprayed once over the top at the VT growth stage (tassel) in 20 gallons of water per acre. Yield increases over the untreated control for the three-year period were significantly greatest for the susceptible hybrid for both fungicides. For the susceptible hybrid, the average three-year yield increase over the untreated was 1613 kg/ha with azoxystrobin and 1545 kg/ha with pyraclostrobin. The average three-year yield increase using the moderately susceptible hybrid was 1210 kg/ha with azoxystrobin and 470 kg/ha with pyraclostrobin. For the tolerant hybrid, the average three-year increase in yield was 538 kg/ha with azoxystrobin and 403 kg/ha with pyraclostrobin respectively. These results indicate that spraying strobilurin fungicides can increase corn yields, especially on the more GLS susceptible hybrids.
Screening Gulf Coast forest species for susceptibility to Phytophthora ramorum
J. A. PREUETT (2), D. J. Collins (2), T. L. Widmer (1), D. G. Luster (1)
(1) USDA/ARS Foreign Disease-Weed Science Research Unit, Fort Detrick, MD, USA; (2) Urban Forestry Program, Southern University and A&M College, Baton Rouge, LA, USA
Phytopathology 100:S202
Phytophthora ramorum, the causal agent of sudden oak death, is an emerging pathogen of California oak woodlands. This pathogen poses a threat to woody plants in many areas of North America, due to the broad host range of the pathogen and the wide distribution of hosts. The US Gulf Coast area is considered a high risk due to the suitable climate, but the question remains whether Gulf Coast woody understory species represent possible hosts for the pathogen. The following woody plant species, native to the Gulf Coast forest: yaupon (Ilex vomitoria), spice bush (Lindera benzoin), southern magnolia (Magnolia grandiflora), and eastern baccharis (Baccharis halmifolia) were tested for their reaction to P. ramorum. This study was conducted at the USDA/ARS plant pathogen containment greenhouse facility at Ft. Detrick, MD. Foliage of four test plants was inoculated with 50,000 zoospores per ml until the foliage was completely wet. The test was repeated three times for each plant species. Inoculated plants were placed in a dew chamber at 20°C for 4 days. After this incubation period, the leaves were detached, scanned on a flatbed scanner, and the leaf lesion areas were assessed for disease using ASSESS 2.0 software. Yaupon and southern magnolia appeared to be susceptible to P. ramorum. The average percentage of lesion leaf area was 4.9, 0.2, 28.1 and 32.1% for inoculated spice bush, eastern baccharis, yaupon and southern magnolia plants, respectively. This is compared to 1.2, 0.4, 0.1 and 0.6%, respectively, for the non-inoculated controls. We plan to continue this research to analyze additional Gulf Coast forest plant species for reaction to P. ramorum.
Characterization of rice blast resistance gene Pi-z(t) in rice germplasm using DNA markers and pathogenicity assays
M. ROY-CHOWDHURY (1), Y. Jia (2), A. Jackson (2), M. Jia (2), R. Fjellstrom (2), R. Cartwright (1)
(1) University of Arkansas, Cell and Molecular Biology Program, Fayetteville, AR, USA; (2) USDA-ARS, Dale Bumpers National Rice Research Center, Stuttgart, AR, USA
Phytopathology 100:S202
The Pi-z(t) gene in rice confers resistance to a wide range of races of the rice blast fungus, Magnaporthe oryzae. The objective of the present study was to identify Pi-z(t) in 131 worldwide rice germplasm using DNA markers and pathogenicity assays. Four simple sequence repeat (SSR) markers (RM527, AP4791, AP5659-1, AP5659-5) closely linked to Pi-z(t) were first used to predict the existence of Pi-z(t) in rice germplasm and results were verified using pathogenicity assays with an avirulent IE1k / two virulent races, IB33 and IB49. A total of 98 germplasm containing one to four SSR alleles for the Pi-z(t) gene was found to be resistant to IE1k and susceptible to IB33 and IB49, suggesting these germplasm contain different Pi-z(t) haplotypes. Eighteen germplasm containing one to four SSR alleles were found to be resistant to all three races, suggesting the presence of other R gene(s) in addition to Pi-z(t). Five germplasm containing three to four SSR alleles were found to be susceptible to all races, indicating the absence of the R gene(s) or presence of non functional components of Pi-z(t) in these germplasm. Six germplasm containing one to four SSR alleles, with one having novel alleles, were found to be resistant to IB49 and IE1k but susceptible to IB33, suggesting that other R gene in these germplasm confer resistance to IB49. Three germplasm containing two to three SSR alleles were found to be resistant to IB33 and IE1k and susceptible to IB49, suggesting the presence of additional R gene(s) to IB33 in these germplasm. Finally, one germplasm with novel SSR alleles was found to be resistant to all races, suggesting the presence of Pi-z(t) independent R gene(s) in this germplasm. These characterized germplasm should be useful for genetic studies and marker assisted breeding for improving blast resistance worldwide.
Zebra chip of potato: Current status and future outlook
C. M. RUSH (1), D. C. Henne (1), F. Workneh (1) and N. Gudmestad (2)
(1) Texas AgriLife Research, Amarillo, TX; (2) North Dakota State University, Fargo, ND
Phytopathology 100:S203
In Texas, potatoes are grown in the Rio Grande Valley, the Winter Garden area near San Antonio, and the Panhandle. In 2000, potatoes from the lower Rio Grande Valley displayed brown necrotic flecks and streaking of the medullary rays. In fry tests, chips from these tubers exhibited dark brown blotches and stripes, which were initially referred to as Texas Defect, but later renamed Zebra Chip (ZC). Zebra Chip affects all market classes of potatoes by reducing yield and quality, and now has been identified in California, Colorado, Kansas, Nebraska, New Mexico and Wyoming. In areas where ZC has become established, it is the most economically important impediment to profitable potato production. Recently, a phloem-restricted proteobacterium, Candidatus Liberibacter, has been associated with ZC. Two newly named species, Ca. Liberibacter psyllaurous and Ca. Liberibacter solanacearum, have been reported as etiological agents of the disease. Both are transmitted by the potato psyllid Bactericera cockerelli, but sequence analysis suggests that the two are likely the same species. However, slight differences between the two suggest the possibility of strains. No genetic resistance to ZC has been identified, insecticides for psyllid control are often ineffective, and factors which impact disease epidemiology are largely unknown. In response to this national threat to the potato industry, a multidisciplinary, multistate team of researchers and extension specialists initiated a program with the goal of reducing losses from ZC to economically sustainable levels by development of a comprehensive, environmentally responsible disease management program. To accomplish this, an advisory board of farmers and representatives from ag-industry, together with the participating scientists, identified seven priority focus areas (Disease Etiology and Vector/Pathogen Diversity, Epidemiology, Pest Management, Breeding, Economics, Risk Assessment and Technology Transfer), each with a number of specific objectives, which together constitute an integrated systems approach to resolving the ZC problem.
Etiology of zoysiagrass diseases in northwest Arkansas
T. N. SPURLOCK (1), E. A. Milus (1)
(1) University of Arkansas, Fayetteville, AR, USA
Phytopathology 100:S203
The increased use of zoysiagrass in Northwest Arkansas has raised awareness of its susceptibility to a range of pathogens. The most destructive and widespread disease is large patch caused by Rhizoctonia solani AG 2-2 (LP). The disease occurs on warm season turfgrasses in the transition zone and is very destructive to zoysiagrass in Northwest Arkansas. Symptoms include irregular patches of dying turf up to several meters in diameter with distinct lesions on the leaf sheaths and stems. Management of large patch with fungicides is expensive and control is variable, possibly indicating that other microorganisms are associated with the disease. The objectives of this study were to determine if other microorganisms isolated from large patch areas contribute to disease severity. Fungi and oomycetes were isolated from leaf sheaths, stems, and rhizomes of zoysiagrass with large patch symptoms from fairways of three golf courses in Northwest Arkansas. Isolates were grouped by morphological characteristics and frequency of isolation recorded. R. solani AG 2-2 (LP) was the most frequently isolated fungus from all sampling locations. Isolates representative of other morphological groups were tested for pathogenicity on Zoysia japonica cv. Meyer and Zoysia matrella cv. Cavalier. Individually, R. solani and Gaeumannomyces graminis var. graminis increased the proportion of shoots with lesions (DS) and decreased biomass (B) over the non-inoculated control in both cultivars. When zoysiagrass was inoculated with another isolate in addition to R. solani, a Fusarium isolate and a sterile white basidiomycete caused significantly less disease (DS and B) in Cavalier than R. solani alone. However, the addition of a Pythium isolate with R. solani decreased B over R. solani alone in both cultivars and DS in Meyer. Disease severity and growth for the combination of R. solani and G. graminis var. graminis was not significantly different from these pathogens inoculated singularly.
Comparison of seed treatments for control of soybean seedling diseases in field soil at three temperatures
K. E. URREA (1), J. C. Rupe (1), C. S. Rothrock (1)
(1) University of Arkansas
Phytopathology 100:S203
Seedling diseases frequently reduce seed germination, seedling emergence, stand, vigor, and yield, and sometimes require replanting. Since seedling disease severity depends on the pathogens present in the soil and the environmental conditions, evaluation of seed treatments and cultivars in the field can be very erratic. The objectives of this research were to evaluate selective and broad spectrum fungicide seed treatments and cultivars in field soil under controlled environmental conditions. Three cultivars were treated with six fungicide treatments or not treated and planted in soil collected from two fields in April, May and June in 2008 and 2009. Tests were conducted in growth chambers at 21°C (April soil), 25°C (May soil) or 28°C (June soil) and soils were watered when matric potentials reached –30 J/kg. After two weeks, the tests were rated for stand, root rot and plant growth and isolations were made from the roots. In 2008, seed treatments resulted in greater stands than the control at all three temperatures only for the cultivar Archer. In 2009, seed treatments resulted in a significant increase in stands for low and high quality seed of the cultivar Hutcheson, but not for HBK4924. Allegiance® was the most effective fungicide tested across all the temperatures in increasing plant stands. For all temperatures and soils, Pythium sylvaticum followed by Fusarium oxysporum were the most frequently isolated pathogens from roots. These results indicate that by controlling environmental conditions seed treatment fungicides and the importance of different seedling pathogens can be efficiently evaluated.
Cellular mechanisms that indicate needle health of seedlings of loblolly pines
C. WALKINSHAW (1)
(1) USDA Forest Service, Pineville, LA
Phytopathology 100:S203
Health of needles on seedlings is readily apparent in pines. Symptoms of disease are easy to recognize by observing the whole leaf or thin sections of needle tissue. In this study histology techniques were used on both healthy and diseased needles that were fixed, sectioned and examined for tannin and other variables, including necrosis of resin ducts. The largest variation in number of starch grains and cells with excess tannin occurred in the resin ducts. These were lined with epithelial cells and had an outer layer of parenchyma cells that were often torn during normal growth. Sporadic healing occurred in areas near phenol cells. Energy for repair of these cells appeared to originate from the collenchyma. Bands of phloem within the needle traces had high starch content. Phenol oxidase, acid phosphatase and peroxidase enzymes as measured by histochemical techniques, combined to hydrolyze cell contents. These observations describe the biology of a number of cellular changes that are associated with susceptibility of loblolly pine needles to decline in the greenhouse and field.
Development of a screening protocol for assessing baseline sensitivity to fungicides for Phakopsora pachyrhizi, the soybean rust pathogen
N. A. Ward (1), R. W. SCHNEIDER (1), C. G. Giles (1), C. L. Robertson (1)
(1) Department of Plant Pathology & Crop Physiology, Louisiana State University Agricultural Center, Baton Rouge, LA
Phytopathology 100:S203
Soybean rust, caused by Phakopsora pachyrhizi, was first discovered in the continental United States in November 2004. Since then it has been detected throughout the U.S. and Ontario, Canada. The disease has been particularly severe in the Southeast where producers are forced to apply fungicides. These continual applications have the potential to select for fungicide resistant strains of the rust pathogen, and these strains could easily overwinter along the Gulf Coast on kudzu and other alternative hosts. The purpose of this study was to develop a sensitive and repeatable assay for establishing baseline sensitivity concentrations for two classes of fungicide chemistry, namely triazoles and strobilurins. The following fungicides were included in this study: tetraconazole (Domark®), flutriafol (Topguard®), azoxystrobin (Quadris®), and pyraclostrobin (Headline®). Freshly produced urediniospores were collected by brushing and discarding existing spores from infected leaves with an artist’s paintbrush. These leaves were then incubated in a moist chamber (25°C) for 2 days after which hyaline urediniospores were produced in abundance on the lower leaf surface. These leaves were placed (lower surface down) on a grid suspended over a plastic petri dish, and the upper leaf surface was gently tapped. This dislodged the urediniospores into the dish, and the spores were then dabbed with the brush to break apart clumps. Water agar plates amended with various concentrations of the fungicides were inoculated by touching spores in the spore collection plates with the tip of the brush and then touching the surface of the amended agar plates with the brush. Spore germination was assessed after incubation for 4 hours in the dark at 25°C.
Pod yield of peanut breeding lines from fields infested with Sclerotinia minor or Verticillium dahliae
J. E. WOODWARD (1), M. R. Baring (3), C. E. Simpson (4), T. A. Baughman (2)
(1) Texas AgriLife Extension Service, Lubbock, TX, USA; (2) Texas AgriLife Extension Service, Vernon, TX, USA; (3) Texas AgriLife Research, College Station, TX, USA; (4) Texas AgriLife Research, Stephenville, TX, USA
Phytopathology 100:S204
Diseases such as Sclerotinia blight (Sclerotinia minor Jagger) and Verticillium wilt (Verticillium dahliae Kleb.) can drastically reduce peanut (Arachis hypogaea L.) yields in Texas. Field trials were conducted to evaluate the performance of advanced peanut breeding lines in a field naturally infested with S. minor. Pod yields were increased by 2457, 1391, 1226 and 981 kg ha–1 for breeding lines TX-3, TX-2, TX-1 and TX-4, respectively, when compared to the commercial standard ‘Flavor Runner 458’. Yield for these breeding lines were equivalent to or greater than that of the partially resistant cultivar ‘Tamrun OL07’. Separate trials were conducted on the Southern High Plains to evaluate the performance of breeding lines TX-3, TX-4, TX-5 and TX-6 in fields infested with V. dahliae. Pod yields were greatest for breeding line TX-3 and ‘Flavor Runner 458’, 5038 and 4960 kg ha–1, respectively, whereas yield was lowest for breeding line TX-5 (3966 kg ha–1). Pod yields for the cultivars ‘McCloud’, TX-6, ‘Tamrun OL02’ and ‘Tamrun OL07’ where intermediate ranging from 4476 to 4254 kg ha–1. Results from these studies indicate that there are varying levels of resistance to S. minor and V. dahliae among the breeding lines evaluated. Breeding line TX-3 may be suitable for fields co-infested with both pathogens.
Characterization of interacting genes with the rice blast fungus avirulence gene AVR-Pita
J. Xing (1), S. Lee (2), Y. JIA (2), L. Yuan (1)
(1) Central South University, Changsha, P.R. China; (2) Dale Bumpers National Rice Research Center, Stuttgart, AR, USA
Phytopathology 100:S204
The AVR-Pita gene in Magnaporthe oryzae determines the efficacy of the Pi-ta blast resistance gene. AVR-Pita encodes a predicted metalloprotease with 223 amino acids. AVR-Pita176 with deletion of 57 amino acids at the amino terminus was previously shown to be involved in Pi-ta mediated blast resistance as a putative effector protein. In order to study the role of AVR-Pita in fungal pathogenicity and blast resistance, AVR-Pita176 was used as bait to identify interacting genes from a yeast two-hybrid library constructed using mRNAs isolated from a U.S. tropical japonica cultivar Katy leaves at different time points after inoculation with M. oryzae. Identified AVR-Pita interacting proteins will be verified using in Vitro binding techniques. In addition, three predicted proteins, AVR-Pita223, AVR-Pita176, and AVR-Pita166 will be used to examine interaction specificity in the yeast two-hybrid assays. The roles of AVR-Pita interacting proteins in fungal pathogenicity and blast resistance will be investigated and progress will be presented.
Histopathology of ‘rapid blight’, a disease caused Labyrinthula terrestris on cool-season turfgrasses
K. YADAGIRI (1), J. L. Kerrigan (1)
(1) Entomology, Soils, and Plant Sciences, Clemson University, Clemson, SC, USA
Phytopathology 100:S204
Rapid blight is a disease on cool-season turfgrasses, caused by a microorganism known as Labyrinthula terrestris. Symptoms of rapid blight include water-soaked lesions and browning or bronzing of foliage that lead to yellowing and death of the infected turf. So far, eleven states in the US on both coasts have reported rapid blight, in addition to other countries including the United Kingdom, Spain and Argentina. Saline irrigation water and soil are favorable for disease causation. Rapid blight is more severe on salt-sensitive varieties of turfgrasses that are mostly cool-season turfs, such as rough bluegrass (Poa trivialis), perennial ryegrass (Lolium perenne), annual bluegrass (Poa annua) and colonial bentgrass (Agrostis tenuis). Labyrinthula terrestris is an unusual pathogen on turf; it belongs to a group of organisms commonly referred to as marine net-slime molds, which have been primarily known to cause diseases on sea grasses. Details of the host-pathogen interactions of Labyrinthula terrestris on turfgrasses have not been investigated. We are, therefore, documenting the infection processes and life cycle using light and electron microscopy to better understand the pathogenicity of this organism and, ultimately, apply these finding to provide better means of controlling rapid blight disease.
Epidemiology of soybean rust in soybean sentinel plots in Florida
H. M. YOUNG (2), J. J. Marois (2), D. L. Wright (2), D. F. Narváez (1), G. K. O’Brien (2)
(1) Monsanto Co., St. Louis, MI, USA; (2) University of Florida, NFREC, Quincy, FL, USA
Phytopathology 100:S204
Since its discovery in 2004 in the Southeastern United States, soybean rust (SBR) severity has been variable from year to year. It is important to understand the epidemiology of the pathogen in Florida as it may serve as an inoculum source for other areas of the country. This study examined the incidence and severity of SBR in relation to prevailing weather data, growth stage, and maturity group (MGIII, MGV, MGVII) in soybean plots (15 m square) across north Florida that were part of the national sentinel plot network from 2005 through 2008. On average, plots became infected 30 days earlier in 2008 than 2005. Precipitation was the principle factor affecting disease progress, where disease increased rapidly after rain events and was suppressed during dry periods. In 2008, there was a significant increase in disease incidence and severity as reflected in the area under the disease progress curve. This was associated with the occurrence of Tropical Storm Fay, which deposited up to 290 mm of water in the plot locations. Results from this study may lead to a better understanding of the impact of weather on the epidemiology of this pathosystem.
Effect of temperature on latent period of Stagonospora nodorum blotch on winter wheat under field conditions
A. D. ZEARFOSS (1), C. Cowger (2), P. S. Ojiambo (1)
(1) Department of Plant Pathology, North Carolina State University, Raleigh, NC, USA; (2) USDA-ARS, North Carolina State University, Raleigh, NC, USA
Phytopathology 100:S204
Stagonospora nodorum is the causal agent of Stagonospora nodorum blotch (SNB) and yield losses from severe disease epidemics can be as high as 50%. To establish a model for SNB development based on the effects of temperature on pathogen latent period and life cycle relative to the host, batches of two winter wheat cultivars (AGS 2000 and USG 3209) were inoculated with pycnidiospores of S. nodorum at weekly intervals over a 1 year period. After an incubation period of 72 h, plants were exposed to field conditions where prevailing temperatures ranged from 5°C to 28°C with a mean batch temperature of 9°C to 24°C. Latent period until the first visible symptoms ranged from 8 to 34 days. The relationship between development of lesions with pycnidia and accumulated thermal time will be described using a shifted cumulative gamma distribution model using estimated base temperatures. These results will provide valuable data that link crop growth models with the progress of SNB and facilitate the establishment of disease development models for use in timing fungicide applications.
