First Report of Leaf Spot of Kentucky Bluegrass (Poa pratensis) Caused by Nigrospora oryzae in Ontario
Authors: Zheng L. , Shi F. , Kelly D. , Hsiang T. 
Published: March 2012

Kentucky bluegrass (Poa pratensis L.) is an important cool-season perennial grass in Ontario. It is native to Europe and can form an attractive and durable turf. In late September 2011, distinct leaf spots were observed on a Kentucky bluegrass lawn in Guelph, ON. Symptoms ranged from small lesions that were chocolate brown and oval or circular up to withered leaves. On potato dextrose agar (PDA) amended with streptomycin and tetracycline, a fungus was consistently recovered from symptomatic leaf samples after surface sterilization for 1 min in 1% sodium hypochlorite. On PDA, cultures were gray with an irregularly distributed, wool-like, fastgrowing aerial mycelium, showing a dark back side as the colony changed to darker brown after 7 days at 25°C. On diseased leaves, conidia were observed after moist incubation, borne on a hyaline vesicle at the tip of each conidiophore. Conidia were single celled, black, smooth, spherical, and 11.2 to 15.5 μm (average 13.8 μm) in diameter. The pathogen was identified as Nigrospora oryzae based on previous descriptions (1,2). Genomic DNA was extracted from a representative isolate, 11201, and the internal transcribed spacer (ITS) region of the ribosomal DNA was amplified by the primers ITS1 and ITS4 (4). The ITS sequence showed 99.8% similarity in the overlapping 508-bp portion with N. oryzae (GenBank No. GQ328855). Pathogenicity tests were performed in the laboratory with the isolate on 3-week-old, sand-based, Magenta box-grown plants of three cool-season turfgrass species, P. pratensis, Agrostis stolonifera, and Lolium perenne, by inoculating with fungal plugs. A 5-mm-diameter plug from 5-day-old PDA cultures was directly placed onto leaves in each of four replicate boxes per species, and then removed after 48 h of incubation. Plants treated with sterile agar plugs served as controls. Magenta boxes containing treated turf were covered loosely with their plastic lids and incubated at 23°C. Three days after inoculation and 1 day after inoculum removal, typical chocolate brown spots were observed on inoculated leaves from all three turfgrass species, but no symptoms were seen on agar plug-treated control plants. Koch's postulates were fulfilled by reisolation of N. oryzae from diseased leaves. The pathogenicity tests were carried out twice with the same results. This is an indication that N. oryzae causing leaf spot of Kentucky bluegrass in Ontario was not hostspecific, and could potentially affect other cool-season turfgrass species. Review of the literature revealed that N. oryzae is known as a pathogen on maize, rice, sorghum, cotton, weeds, and several other hosts, but has not been reported on any species of turfgrass (3). To our knowledge, this is the first report of N. oryzae infecting Kentucky bluegrass in Ontario or worldwide.

References: (1) M. B. Ellis. Dematiaceous Hyphomycetes, CAB, Kew, Surrey, England, 1971. (2) H. J. Hudson. Trans. Brit. Mycol. Soc. 46:355, 1963. (3) R. W. Smiley et al. Compendium of Turfgrass Diseases. 3rd ed. APS Press, St Paul, MN, 2005. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.