First Report of Pyricularia grisea Causing Gray Leaf Spot on Kikuyugrass (Pennisetum clandestinum) in the United States.

Abstract

Kikuyugrass (Pennisetum clandestinum) is a warm-season turfgrass that has been adopted for use in fairways and roughs in a number of subtropical areas including southern California, Mexico, Australia, and South Africa. During August 2003, a foliar disease of Kikuyugrass was reported from a number of golf courses in southern California. Examination of diseased plants showed the presence of dark, olive green-to-brown lesions on the foliage. Incubation of these plants in a moist chamber for 12 h led to the production of numerous pyriform conidia from these lesions that were characteristic of Pyricularia grisea. Single-spore isolates of the fungus were obtained from infected kikuyugrass samples by transferring conidia to acidified 1.5% water agar and then transferring single, germinated conidia to one-quarter-strength potato dextrose agar. Colony morphology and conidia production were consistent with that described for P. grisea (1). Koch's postulates were performed separately for two single-spore isolates (OSGC-1 and CCCC-1) obtained from infected kikuyugrass. For each isolate, 2-week-old, glasshouse-grown seedlings of kikuyugrass (cv. 'AZ-1') and perennial ryegrass (Lolium perenne) grown in 75-mm pots in soilless media were inoculated with conidia from either OSGC-1 or CCCC-1. For each test, six pots of both kikuyugrass and ryegrass were inoculated, and the tests were conducted three times for each isolate. Conidia were obtained from isolates grown on clarified V8 agar in 100-mm petri plates for 14 days at 25°C. Suspensions were made by adding 10 ml of sterile distilled H2O (sdH2O) to the plates, scraping the surface of the media to dislodge the conidia, filtering the suspension through cheesecloth, and then adjusting the final concentration to 1 × 106 conidia/ml with sdH2O. Seedlings were inoculated with the conidial suspensions with an aerosol applicator, placed in plastic boxes lined with wet paper towels, and sealed to provide adequate moisture for infection. Boxes were incubated at 28°C for 48 h after which time the covers were removed and the plants maintained in ambient glasshouse conditions at approximately 28°C. In all three replicated experiments, kikuyugrass seedlings inoculated with OSGC-1 or CCCC-1 developed symptoms of disease approximately 5 days after inoculation, while inoculated perennial ryegrass did not, even 14 days after inoculation. Symptomatic kikuyugrass leaves were taken randomly from plants from each of the three replicated tests, surface disinfested in 0.3% sodium hypochlorite for 30 s, rinsed with sdH2O, blotted dry, and placed onto acidified water agar in petri plates. Twenty-four hours later, abundant sporulation was observed from symptomatic tissue with conidiophores bearing conidia typical of P. grisea. To our knowledge, this is the first report of gray leaf spot being caused by P. grisea on Pennisetum clandestinum in North America. Reference: (1) M. B. Ellis. Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, Surrey, UK, 1971.