5.5.2
RELATIVE VIRULENCE OF ISOLATES OF SCLEROTINIA HOMOEOCARPA

TOM HSIANG, LIN YANG and WAYNE BARTON

Department of Environmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

Background and objectives
Dollar spot disease caused by Sclerotinia homoeocarpa is the most common disease of high maintenance turf in the Great Lakes region. Recently there have been reports of resistance to demethylation-inhibiting (DMI) fungicides for this pathogen. The purpose of this study was to sample naturally occurring isolates of S. homoeocarpa varying in sensitivity to propiconazole (a DMI fungicide), and to test whether this variation was correlated with fitness as represented by growth rate in culture and field virulence on creeping bentgrass in the absence of the fungicide.

Materials and methods
In 1994, isolates of S. homoeocarpa were collected from diseased turfgrass in eight locations across southern Ontario, Canada, where DMI fungicides reportedly had never been used [1]. Isolates were tested for sensitivity to propiconazole using an amended agar assay. Forty isolates representing a range of propiconazole sensitivities were chosen for field testing. Field test plots were laid out in a 0.5 m x 0.5 m grid pattern on a native sand putting green. There were three replicate plots per isolate in a fully randomized design. Plots were inoculated and then rated weekly for 6 weeks by counting the number of dollar spot patches per plot. The field experiment was first conducted in summer 1995 and then repeated again in summer 1996. AUDPC (Area Under the Disease Progress Curve) was calculated to represent disease progression.

Results and conclusions
Higher disease severity was seen in our test plots than in uninoculated controls and in areas just outside our plots, and this indicated that the disease in our plots resulted at least partly from our inoculations. Disease severity, as represented by AUDPC, was highly correlated for the different isolates between the two years of the field test. Disease severity in 1995 was not signficantly correlated with in vitro growth rate, but it was in 1996. There was no statistically significant relationship between fungicide sensitivity (log-EC50 value) and isolate growth rate. Some previous studies on fungicide resistance have found that DMI-resistant isolates are slower growing in culture than their sensitive relatives. Growth rates in culture have been used as predictors of fitness in other studies, but for S. homoeocarpa, this assessment must be approached with caution.
There were significant negative relationships between AUDPC and log-EC50 value in both years. This implied that isolates of S. homoeocarpa with reduced sensitivity to propiconazole showed a lower virulence. The practical implication of this result is that in the absence of propiconazole use, isolates with less sensitivity to propiconazole could be out-competed by isolates with more sensitivity to propiconazole but greater virulence. However, further study is required to determine the time frame for this to occur, and whether DMI-resistance prevention strategies can feasibly be based on the existence of resistance-related fitness costs.

References
1. Hsiang T, Barton W and Yang L, 1997. Eur. J. Plant Pathol. 103:409-416.