SUPPRESSION OF DOLLAR SPOT BY HYPOVIRULENT ISOLATES OF SCLEROTINIA-HOMOEOCARPA

Selected hypovirulent isolates of Sclerotinia homoeocarpa were evaluat ed for efficacy in suppressing dollar spot of turfgrass under growth r oom and field conditions. Under growth room conditions, hypovirulent i solates Sh12B, Sh09B, or Sh08D of S. homoeocarpa caused 3.4 to 30.4% d iseased turf in comparison to virulent isolates Sh48B and Sh14D, which caused 80.2 to 90.2% disease. In treatments that received both virule nt and hypovirulent isolates, only hypovirulent isolate Sh12B signific antly reduced disease as compared with the central with virulent isola tes alone. In a field experiment in 1993 on swards of creeping bentgra ss artificially inoculated with a virulent isolate of the pathogen, al l treatments containing hypovirulent isolate Shl2B applied as a myceli al suspension, granular mix, or alginate pellets developed significant ly less disease (6.3 to 20.8% diseased turf) compared with their respe ctive formulation controls (23.8 to 31.2%). Suppression of dollar spot by treatment with mycelial suspensions of isolate Shl2B was evident u p to 45 days postinoculation, and disease suppression was still signif icant 1 year after application when compared with the water control. A pplications of hypovirulent isolate Sh09B did not reduce dollar spot i n any treatments. Significant suppression of dollar spot by isolate Sh 12B was also observed in the experiment conducted in 1994. In addition , suppression of dollar spot by hypovirulent isolate Sh12B was evaluat ed on swards with naturally occurring inoculum during 1994. Treatments with a mycelial suspension and alginate pellets of hypovirulent isola te Shl2B significantly reduced dollar spot compared with their respect ive formulation controls. With few exceptions, there was no statistica l difference between treatments with hypovirulent isolate Shl2B and th e fungicide chlorothalonil (Daconil 2787). Multiple applications of th e hypovirulent isolate did not result in greater suppression of dollar spot as compared with a single application. The results indicate that hypovirulence has potential as an effective strategy for the manageme nt of dollar spot.