Host Range, Fungicide Resistance and Management of Pseudoperonospora cubensis, Causal Agent of Cucurbit Downy Mildew

COLUCCI, SUSAN JEAN. Host Range, Fungicide Resistance and Management of Pseudoperonospora cubensis, causal agent of Cucurbit Downy Mildew. (Under the direction of Gerald J. Holmes). In 2004, downy mildew (causal agent Pseudoperonospora cubensis) re-emerged as an important foliar disease on cucumber after decades of control with resistant, commercially available cultivars. Because the new strain of P. cubensis had increased aggressiveness on cucumber, it was hypothesized that it may also have a different host range than the previously described populations in the United States. To determine host range, leaf disks of 12 cucurbit host differentials were inoculated with 32 isolates of P. cubensis collected from the eastern United States, including two isolates collected prior to the 2004 epidemic. The result was 32 different host range patterns. Cucumis spp. , including cucumber, were the most susceptible hosts. The least susceptible differentials were Citrullus lanatus, Lagenaria siceraria, Luffa cylindrica, Benincasa hispida and Cucurbita pepo var. pepo. Natural populations of the pathogen were evaluated in field experiments in Florida, South Carolina, Georgia, North Carolina, Delaware, Maryland, New York, Michigan and Ohio. Cucumis and Cucurbita were the most susceptible genera, and Benincasa hispida and Luffa cylindrica were the least susceptible. Because the commercially available resistant cucumber cultivars are no longer effective in controlling downy mildew, fungicides are necessary to manage the disease. A fungicide efficacy trial in 2004 in Sampson County, North Carolina indicated a reduced efficacy of mefenoxam and the QoI fungicides, azoxystrobin and pyraclostrobin. To confirm resistance, the sensitivity of P. cubensis isolates to treatments of mefenoxam (0.01, 0.1, 1.0, 10, 100 μg/ml) and the QoI fungicide, azoxystrobin (0.001, 0.01, 0.1, 1.0, 10, 100 μg/ml) was determined using a whole cucumber plant assay with inoculum derived from 24 single leaf isolates collected in 2004 through 2007 from the eastern United States. Seven additional isolates and four isolates from the original assay were tested with mefenoxam at 1 and 100 μg/ml and azoxystrobin at 1 and 100 μg/ml, as well as positive (fluopicolide) and non-treated control. Insensitivity (less than 25% disease control) to all mefenoxam and azoxystrobin concentrations was demonstrated in 27 out of 31 (87%) of the total isolates assayed. Practical resistance of P. cubensis populations to mefenoxam and pyraclostrobin was evaluated in Florida, South Carolina, North Carolina, Delaware and New York. There was no reduction in disease severity compared to the non-treated control at all locations and marketable yield was significantly reduced in mefenoxam and pyraclostrobin plots in all locations except Florida. Fungicide efficacy trials in Sampson County, NC in 2005, 2006 and 2007 indicate that fungicides with active ingredients famoxadone, cymoxanil, zoxamide, propamocarb hydrochloride, cyazofamid and fluopicolide are the most efficacious at managing downy mildew on cucumber. Treatments involving these fungicides result in lower disease severity and higher yield than the non-treated control. In addition, a locally systemic + protectant fungicide program was compared to a protectant-only program with respect to delay of fungicide application. Results indicated that the locally systemic + protectant program was more effective than the protectant only program when applied before disease detection. If efficacious fungicides are not applied within 2 to 3 weeks of initial detection, downy mildew control was not possible. Host Range, Fungicide Resistance and Management of Pseudoperonospora cubensis, Causal Agent of Cucurbit Downy Mildew. by Susan Jean Colucci A thesis submitted to the Graduate Faculty of North Carolina State University In partial fulfillment of the Requirements for the degree of Master of Science