Genetic Diversity Among Ophiosphaerella agrostis Strains Causing Dead Spot in Creeping Bentgrass

Dead spot is a new disease of young creeping bentgrass (Agrostis stolonifera L.) putting greens in the United States. The disease is incited by Ophiosphaerella agrostis Dernoeden, M.P.S. Câmara, N.R. O’Neill, van Berkum, et M.E. Palm, a previously undescribed pathogen that first was isolated in 1998 from a golf course in Maryland (1,3). In the same year, the disease was confirmed on creeping bentgrass putting greens in five states. Since 1998, O. agrostis has been found in at least 13 states and has been isolated from several additional turfgrass species, including hybrid bermudagrass (Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy) and colonial (A. capillaris L.) and velvet (A. canina L.) bentgrasses (1,11,14). Dead spot generally appears within 1 to 2 years following establishment of creeping bentgrass putting greens. The disease also may occur on renovated putting surfaces following fumigation with methyl bromide. In the mid-Atlantic region of the United States, disease symptoms may appear as early as May, and disease activity often peaks between July and August (10). Initially, the disease appears as small, copper or reddish-brown spots approximately 1 to 2 cm in diameter that may increase to approximately 6 to 8 cm throughout the summer months. Once infection occurs, turfgrass in the center of dead spots dies and pits or depressions often form which adversely affect the putting surface. Regrowth of bentgrass in infected spots is slow, and dead spots often remain present throughout the winter until bentgrass growth resumes in the spring. Other species within the genus Ophiosphaerella (O. herpotricha J. Walker, O. korrae Walker & Smith (synonym Leptosphaeria korrae), and O. narmari Walker & Smith (synonym L. narmari)) cause various turfgrass diseases (2,4,5,15,24,26, 31,32). The aforementioned Ophiosphaerella spp., however, have not been reported to infect creeping bentgrass. Unlike other Ophiosphaerella spp. found in association with turfgrasses, the production of pseudothecia by O. agrostis is common. Field observations revealed that O. agrostis is capable of producing prodigious numbers of pseudothecia in necrotic leaf, sheath, and stolon tissues (3,11). Growth chamber studies confirmed that pseudothecia development can occur within 4 days and that mature ascospores may be produced in as little as 1 week when subjected to a range of temperatures (13 to 28°C) and constant light (12). In a Maryland field study, pseudothecia developed in the greatest numbers during the summer months (June to August), but occasionally developed as late as September (10). Additionally, immature ascospores were found in small numbers within pseudothecia throughout the winter (10). The pathogen’s ability to proliferate sexually during the summer months increases the chance for genetic recombination. The capability of O. agrostis to undergo numerous meiotic cycles in a single season ultimately may lead to increased pathogen fitness due to a more diverse population. Genetic analyses, based on amplified fragment length polymorphism (AFLP) fingerprinting of O. agrostis isolates collected in 1998, revealed an 87% or greater similarity among isolates from five different states (1). Previous studies revealed that a single isolate (OpOH-1) exhibited differences in colony color, sequence within the internal transcribed spacer (ITS) 2 region of ribosomal DNA, and fungicide sensitivity when compared with the type species (1,9). Unlike the rose-quartz or pink colony color of most isolates collected in 1998, O. agrostis isolate OpOH-1 exhibited white to olive-gray mycelia when grown on potato dextrose agar (PDA) at 25°C (1,12). Although within the range (75 to 150 μm) reported by Câmara et al. (1) in the original description of O. agrostis, ascospores of isolate OpOH-1 produced on PDA were shorter (68 to 98 μm) and had fewer septa (6 to 8) than most other isolates (9). Additionally, rDNA sequencing of three O. agrostis isolates revealed a single base pair (bp) insertion of thymine nested between a cytosine and a five-thymine repeat in two different locations of the ITS2 region in OpOH-1 (1). Finally, in vitro experiments indicated reduced sensitivity of the isolate OpOH-1 to the fungicide iprodione (3-(3,5dichlorophenyl)-N-(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboxamide) (9). Additional AFLP fingerprinting from a larger population is needed to elucidate genetic diversity within O. agrostis. Since the initial description of the pathogen, additional isolates exhibiting varying colony characteristics have been collected ABSTRACT Kaminski, J. E., Dernoeden, P. H., Mischke, S., and O’Neill, N. R. 2006. Genetic diversity among Ophiosphaerella agrostis strains causing dead spot in creeping bentgrass. Plant Dis. 90:146-154.