Since 2002, symptoms of an unknown disease have been observed in ‘El Toro’ zoysiagrass (Zoysia japonica Steud.) in several locations across North Carolina. Symptoms become evident in the spring as the zoysiagrass comes out of winter dormancy. Circular or irregularly shaped patches, 10 to 30 cm in diameter, remain dormant as the surrounding turf resumes growth. These patches eventually collapse and die, leaving sunken depressions in the turf stand. After the initial appearance of symptoms, the zoysiagrass slowly recolonizes the patches by spreading inward from the perimeter. Microscopic observation revealed necrotic stolon and root tissue that was colonized by ectotrophic fungal hyphae, whereas leaf and sheath tissue was colonized by species of Curvularia, Colletotrichum, Alternaria, Ascochyta, Drechslera, or Fusarium. Sections of necrotic root and stolon tissue were washed under flowing tap water for 10 min, submersed in 0.6% NaOCl for 5 min, rinsed with sterile dH2O, blotted dry, and placed on ¼ strength potato dextrose agar amended with 100 μg/ml each of streptomycin sulfate and chloramphenicol. A total of 50 isolates were obtained from four locations during 2002 and 2003. A fungus resembling Ophiosphaerella spp. was consistently isolated and was confirmed to be Ophiosphaerella korrae by species-specific PCR assays (3) and rDNA internal transcribed spacer (ITS) sequencing. Pathogenicity tests were conducted in the field on ‘El Toro’ zoysiagrass at the Lake Wheeler Turfgrass Field Laboratory in Raleigh, NC. Autoclaved rye grain (Secale cereale L.; 200 g of grain, 5.75 g of CaCO3, and 220 ml of H2O) was infested with one of eight O. korrae isolates. Plots (1 × 1 m) were inoculated on 13 October 2004 by removing an 11-cm-diameter core from the center of each plot to a 5-cm depth, placing 10 cm3 of infested rye grain in the bottom of the hole, and replacing the core. Noninoculated and uninfested rye grain treatments served as controls, and each treatment was replicated eight times in a randomized complete block. No symptoms were observed in the experimental area during 2005. In April 2006, five isolates (Zrr20, Zrr36, Zrr57, Zrr58, and Zrr59) incited spring dead spot symptoms in at least four of eight inoculated plots. The average diameter of patches induced by these isolates ranged from 7.9 to 11.4 cm. In April 2007, three isolates (Zrr20, Zrr36, and Zrr57) incited symptoms in at least four plots, with average patch diameters ranging from 14.5 to 16.0 cm. These inoculation success rates and patch diameters were similar to those resulting from O. korrae inoculation of bermudagrass conducted on the same date (L. P. Tredway, unpublished data). No symptoms were observed in noninoculated plots or those amended with uninfested rye grain. O. korrae was consistently reisolated from symptomatic stolons and roots in May 2007 to complete Koch's postulates. To the best of our knowledge, this is the first report of spring dead spot of zoysiagrass caused by O. korrae in the United States. Previously, O. herpotricha was shown to induce spring dead spot symptoms on zoysiagrass in Kansas (1), and O. korrae was reported as a zoysiagrass pathogen in Japan (2). To date, we have only observed spring dead spot on the Zoysia japonica ‘El Toro’.
References: (1) D. E. Green et al. Plant Dis. 77:1040, 1993. (2) T. Tani. Color Atlas of Turfgrass Diseases. Ann Arbor Press, Chelsea, MI, 1997. (3) N. A. Tisserat et al. Phytopathology 84:478, 1994.
