Temporal Dynamics of Two Genetically Diverse Downy Mildew Mitochondrial Clades on Cucumber Cultivars in South Carolina and Florida

Downy mildew (DM, Pseudoperonospora cubensis) is an important disease that can significantly reduce the yield of field-grown cucumbers in the United States. Cucumber cultivars with resistance to DM were widely grown for more than 30 years. However, resistance broke down in 2004, resulting in severe DM epidemics. Large plot trials were conducted to characterize pathogen virulence and population dynamics on DM-tolerant cultivars Peacemaker and Expedition and susceptible SMR-58 in Charleston, SC, and Fort Pierce, FL, in 2016 and 2017. Rapid DM development on SMR-58 was observed at both locations in 2016 and 2017. Disease progress on Expedition and Peacemaker were slow as compared with SMR-58 in Fort Pierce, FL, in 2016. Very low to no DM was observed on Peacemaker or Expedition in Charleston in 2016. In 2017, all cultivars were affected in Charleston, but only SMR-58 had severe DM (95 to 100%) in Fort Pierce. Pathogen population analysis using DM-specific mitochondrial clade (MC1 and MC2) markers showed differences between the P. cubensis populations across location and years on the cucumber cultivars. Both MC1 and MC2 were detected in cucumber plots in South Carolina and Florida. The most prevalent DM mitochondrial clade in South Carolina during June 2016 was MC1, primarily on SMR-58 (95 to 100%). However, in July 2016, that population shifted toward MC2 (45 to 77%). This suggests that the DM populations can change relatively quickly in a suitable environment. Mixed populations of MC1 and MC2 continued to be observed in Charleston, SC, in 2017. In Fort Pierce, FL, a complete shift was observed from MC2 in 2016 to MC1 in 2017. Because both MCs are present in the Southeastern United States either at the same time or possibly different times during the production year, cultivars with combined resistance to P. cubensis populations will be needed to reduce reliance on fungicides to manage DM.