Colonization of Nonwounded and Wounded Creeping Bentgrass by Virulent and Hypovirulent Isolates of Sclerotinia Homoeocarpa.

Sclerotinia homoeocarpa causes dollar spot disease of turfgrass. Hypovirulent isolates of S. homoeocarpa contain the mitochondrial virus Ophiostoma mitovirus 3a (OMV3a). In this study, colonization and lesion production by S. homoeocarpa on nonwounded and wounded leaves of creeping bentgrass were characterized using light microscopy. On nonwounded leaves, hyphae of virulent and hypovirulent isolates infected the leaf by forming appressoria along cell walls and over stomata by 48 hours post-inoculation (hpi). Inter- and intracellular hyphae were not common until the nonwounded leaves were heavily colonized at 96 hpi.  The colonization front grew ahead of lesion development for 50 hpi on both nonwounded and wounded leaves. On wounded leaves, mycelia grew directly into and infected wound sites as soon as 8 hpi, and inter- and intracellular hyphae rapidly developed into a colonization front.  In contrast to the virulent isolate, the hypovirulent isolate seldom colonized nonwounded or wounded leaves beyond 25 mm from the inoculation point after 240 or 168 hpi, respectively.  Fluorescent microscopy demonstrated noticeable callose accumulation around infection hyphae and along epidermal cell walls under appressoria, indicating a plant defense response. The above results indicate that S. homoeocarpa is a hemibiotrophic plant pathogen that colonizes and causes disease more rapidly on wounded than nonwounded grass, an important consideration for turfgrass management. The etiology of virulent and hypovirulent isolates was similar, except for slower progression of hypovirulent colonization and disease. The combination of slower and less vigorous growth by hypovirulent isolates combined with the formation of defense-related structures are likely reasons for reduced symptom development by hypovirulent isolates.