Plant wounding and Ophiostoma mitovirus 3a (OMV3a) influence infection of creeping bentgrass by Sclerotinia homoeocarpa

Colonization and lesion development by virulent, asymptomatic and hypovirulent isolates of Sclerotinia homoeocarpa on nonwounded (NW) and wounded (W) leaves of creeping bentgrass were characterized. Hypovirulent and asymptomatic isolates contain the fungal virus, Ophiostoma mitovirus 3a, and virulent isolates are virus-free. On NW leaves, all isolates infected leaves with appressoria along cell walls and through stomata by 48 hours post-inoculation (hpi). Inter- and intracellular hyphae formed on heavily colonized, NW leaves at 96 hpi. Wound-inoculated grass had a colonization front characterized by inter- and intracellular hyphal colonization within nonsymptomatic tissues at 8 hpi by direct infection of the wound site. The colonization front grew ahead of symptom development for 50 hpi on both NW and W leaves. In contrast to virulent and asymptomatic isolates, the hypovirulent isolate seldom colonized more than 30% of leaf tissue of the NW or W leaves. Callose accumulation under fungal appressoria, oxalate oxidase activity, and hydrogen peroxide accumulation at lesion borders were also demonstrated. The results indicate that S. homoeocarpa is capable of colonizing creeping bentgrass without causing visible symptoms. In addition, the fungus causes disease more rapidly on W than NW grass, and the etiology of virulent, asymptomatic and hypovirulent isolates was similar. The results of this study contribute to our understanding of the basic biology of S. homoeocarpa and may influence integrated management practices.