Sclerotinia sclerotiorum population variability directly affects Sclerotinia stem rot (SSR) resistance breeding programs. In the north-central United States, however, soybean germplasm selection has often involved only a single isolate. Forty-four S. sclerotiorum isolates from Illinois, Michigan, Minnesota, Nebraska, Wisconsin, Poland, and across 11 different host species were evaluated for variation in isolate in vitro growth, in vitro oxalate production, and in planta aggressiveness on the susceptible soybean ‘Williams 82’. Significant differences (P < 0.0001) were detected in isolate in planta aggressiveness, in vitro growth, and in vitro oxalate production. Furthermore, diverse isolate characteristics were observed within all hosts and locations of collection. Aggressiveness was not correlated to colony growth and was only weakly correlated (r = 0.26, P < 0.0001) to isolate oxalate production. In addition, the host or location of collection did not explain isolate aggressiveness. Isolate oxalic acid production, however, may be partially explained by the host (P < 0.05) and location (P < 0.01) of collection. Using a representative subset of nine S. sclerotiorum isolates and soybean genotypes exhibiting susceptible or resistant responses (determined using a single isolate), a significant interaction (P = 0.04) was detected between isolates and genotypes when SSR severity was evaluated. Our findings suggest that screening of S. sclerotiorum-resistant soybean germplasm should be performed with multiple isolates to account for the overall diversity of S. sclerotiorum isolates found throughout the soybean-growing regions of the United States.
Hop powdery mildew, caused by the ascomycete fungus Podosphaera macularis is a consistent threat to sustainable hop production. The pathogen utilizes two reproductive strategies for overwintering and perennation: (i) asexual vegetative hyphae on dormant buds that emerge the following season as infected shoots; and (ii) sexual ascocarps (chasmothecia), which are presumed to discharge ascospores during spring rain events. We demonstrate that P. macularis chasmothecia, in the absence of any asexual P. macularis growth forms, are a viable overwintering source capable of causing early season infection two to three orders of magnitude greater than that reported for perennation via asexual growth. Two epidemiological models were defined that describe (i) temperature-driven maturation of P. macularis chasmothecia; and (ii) ascosporic discharge in response to the duration of leaf wetness and prevailing temperatures. Podosphaera macularis ascospores were confirmed to be infectious at temperatures ranging from 5 to 20°C. The organism’s chasmothecia were also found to adhere tightly to the host tissue on which they formed, suggesting that these structures likely overwinter wherever hop tissue senesces within a hop yard. These observations suggest that existing early season disease management practices are especially crucial to controlling hop powdery mildew in the presence of P. macularis chasmothecia. Furthermore, these insights provide a baseline for the validation of weather driven models describing maturation and release of P. macularis ascospores that can eventually be incorporated into hop disease management programs.
Hop downy mildew caused by the oomycete Pseudoperonospora humuli is primarily managed with fungicides because commercial varietal resistance is unavailable. Mefenoxam, a phenylamide-class fungicide, is highly effective for systemic disease control but is at high risk for the development of pathogen insensitivity. Due to the recent expansion of hop production in Wisconsin, it was unknown if P. humuli could be managed with mefenoxam because insensitivity has been documented in other regions. During 2015 to 2017, isolates of P. humuli were collected from commercial yards throughout Wisconsin. Isolates were screened for insensitivity using a leaf disk assay with fungicide-amended water agar. Insensitivity was determined using a single discriminatory dose of 25 μg/ml of mefenoxam, and isolates were considered insensitive if they exhibited at least 50% sporulation on amended media relative to nonamended water agar. Over all years, nearly 48% of all isolates were insensitive to mefenoxam. The incidence of insensitivity varied between individual hop yards, with most yards exhibiting a mixture of sensitive and insensitive isolates. At this time, mefenoxam-insensitive populations do not appear to be predominant in Wisconsin. Growers should monitor the effectiveness of mefenoxam applications in their hop yards and be responsive to changes in crop response.
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