In August of 2005, a golf course in Las Vegas, NV reported turf loss from an unknown disease on perennial ryegrass fairways. Samples from this course were examined, and diseased plants were found covered with lesions and sporulation typical of gray leaf spot as caused by Pyricularia grisea (Cooke) Sacc. With petroleum jelly, sporulating leaves were attached to the inside top surface of 100-mm petri dishes filled with 15 ml of 1.5% water agar. Conidia were allowed to drop onto the agar surface and 24 h later, individual germinating pyriform conidia were transferred to petri dishes containing one-quarter-strength potato dextrose agar (¼-PDA) with the aid of a fine needle and stereomicroscope. Isolates of the fungus were maintained at 28°C with constant fluorescent light. Isolates were examined 7 to 10 days later, and morphology and conidia production were consistent with that described previously for P. grisea (1). Koch's postulates were performed using a single isolate (SSGC-1.1) grown for 14 days on ¼-PDA. The petri dish was flooded with 15 ml of sterile distilled water plus 0.05% Tween 20 and conidia dislodged into the solution with a rubber policeman to obtain a solution of approximately 5 × 103 conidia per ml. Using a modified thin-layer chromatography plate sprayer, the solution was misted onto six pots of 6-week-old perennial ryegrass (a mixture of approximately 33% each of varieties ‘Kokomo’, ‘Cabo’ and ‘Secretaria’), seeded at a density of 2 kg per 93 m2 grown in 4- × 4-cm plastic pots filled with University of California soil mix. As a control treatment, six pots of perennial ryegrass (grown as previously described) were treated with water plus 0.05% Tween 20 only. Pots of plants were placed into closed, translucent, plastic containers lined with wet paper towels to provide a moist environment and held at 30°C for 48 h. Pots of plants were transferred to an incubator set at 30°C and 80% relative humidity with 12 h of alternating light and dark cycles. Four days after inoculation, plants misted with conidia developed symptoms typical of gray leaf spot. Plants were again placed into closed plastic containers lined with wet paper towels for 24 h, at which time, lesions on symptomatic plants developed abundant conidia characteristic of P. grisea. Water-only treated plants did not show any symptoms or signs of disease. P. grisea was reisolated from sporulating leaves as described above. The disease has been spreading in the midwestern and northeastern United States since first reported in 1991 on perennial ryegrass in Pennsylvania. It has only recently been found on turfgrass in California (2), and to our knowledge, this is the first report of this pathogen on perennial ryegrass in Nevada.
References: (1) M. B. Ellis. Dematiaceous Hyphomycetes. CMI, Kew, Surrey, UK, 1971. (2) W. Uddin et al. Plant Dis. 86:75, 2002.
