Wilted plants of Valencia market-type peanut (Arachis hypogaea L.) were found in two fields in August 2006 and three fields in September 2007 in Curry County, New Mexico. Plants had extensive, light brown discoloration and interstices of greenish tissue on blighted stems and branches across plant canopy levels. Disease incidence was less than 1% with infected plants in groups of two to five within each field. Five 4-cm stem segments were taken from each of five diseased plants in each field, submerged for 5 min in 0.5% NaOCl, rinsed in sterile distilled water, cut into 0.5-cm pieces, and plated on acidified potato dextrose agar (APDA). Mycelial colonies, recovered from plant tissues and incubated on APDA at 25°C under a 12-h photoperiod, were white and floccose with light green-yellow areas becoming visible within 7 to 10 days of incubation. Black stromata formed, spreading in a concentric pattern or scattered as large masses on APDA. Ostiolate and rostrate pycnidia with long beaks more than 500 μm were observed. Alpha conidia exuded from pycnidia in creamy-to-yellowish drops and were ellipsoid and biguttulate with an average length of 6.6 μm and width of 2.10 μm. Colonies were identified as Phomopsis longicolla Hobbs (1). PCR amplification of the internal transcribed spacer (ITS) region of rDNA of three isolates using primer pair ITS4/ITS5 (3) was followed by sequencing and BLAST analysis and showed a 95% homology with the sequence of the ITS region of rDNA of P. longicolla (1). Digestion of PCR-amplified DNA with AluI yielded two restriction fragments of sizes consistent with those reported for P. longicolla (2). Koch's postulates were established with three isolates tested for pathogenicity on Valencia peanut cv. Val-C at the four- to six-leaf stage using stem and root inoculations. Stems were injected with conidial suspension (106 conidia/ml) with a hypodermic needle or stabbed at the cotyledon axils with a sterile toothpick dabbed into an exuded conidial drop. Control plants were stem injected with distilled water or stabbed with a sterile toothpick. For root inoculation, plants were uprooted, washed free from soil, and inserted up to the crown into a 50-ml plastic test tube containing 40 ml of conidial suspension (25,000 conidia/ml) or sterile distilled water. For each method, eight plants were inoculated with each isolate, and four plants served as control. All inoculation methods were performed on the same day and repeated three times. Inoculated plants were covered with a clear plastic bag that was removed after 4 days. Plants were placed at 30°C under a 14-h photoperiod for 2 weeks. On stem-inoculated plants, light-to-dark brown discoloration formed at the sites of inoculation and expanded up and down the stems, which became brown, resulting in plant death within 10 to 14 days. On root-inoculated plants, browning of crown areas progressed up the stems, followed by plant death. P. longicolla was recovered from all inoculated plants. To our knowledge, this is the first report of P. longicolla on peanut in New Mexico and the United States. This report demonstrates the association of P. longicolla with peanut and its ability to cause stem blight. The occurrence and extent of this disease may be of a concern, because on other crops, Phomopsis diseases can cause significant reduction in seed germination, plant vigor, and yield.
References: (1) T. W. Hobbs et al. Mycologia 77:535, 1985. (2) A. W. Zhang et al. Plant Dis. 81:1143, 1997. (3) A. W. Zhang et al. Phytopathology 88:1306, 1998.
