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

Abstract

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 virulence on cucumber, it was hypothesized that it may also have a different host specialization pattern than the previously described populations in the United States. To determine the host specialization of contemporary populations, 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 specialization patterns. The Cucumis spp., including cucumber, were the most susceptible genera and the least susceptible differentials were Citrullus lanatus, Lagenaria siceraria, Luffa cylindrica, Benincasa hispida and Cucurbita pepo var. pepo. Natural populations were also tested for their host specialization in field experiments in Florida, South Carolina, Georgia, North Carolina, Delaware, Maryland, New York, Michigan and Ohio. Cucumis spp. and Cucurbita spp. were the most susceptible genera and Benincasa hispida and Luffa cylindrica were the least susceptible differentials.

Because the commercially available resistant cucumber cultivars are no longer solely controlling downy mildew, fungicides are necessary to manage the disease. A fungicide efficacy trial in 2004 in Sampson County, North Carolina reported the reduced efficacy of mefenoxam and QoI fungicides, azoxystrobin and pyraclostrobin. To confirm resistance, the sensitivity of P. cubensis isolates to 11 treatments of mefenoxam (0.01, 0.1, 1.0, 10, 100 μg⁄ml) and 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 infected 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 a reduced treatment set of 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 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 protectant-only program with respect to delay of fungicide application. Results indicate that the locally systemic + protectant program is more effective than the protectant only program when applied before disease detection. In addition, if efficacious fungicides are not applied within 2 to 3 weeks of initial detection, downy mildew cannot be controlled.