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

Authors: MANDAL, MIHIR - Orise Fellow; Turechek, William; Ikerd, Jennifer; WALLACE, EMMA - North Carolina State University; QUESDA-OCAMPO, LINA - North Carolina State University; Kousik, Chandrasekar

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2024
Publication Date: 4/11/2025
Citation: Mandal, M., Turechek, W., Ikerd, J.L., Wallace, E., Quesda-Ocampo, L., Kousik, C.S. 2025. Temporal Dynamics of Two Genetically Diverse Downy Mildew Mitochondrial Clades on Cultivated Cucumber Cultivars in South Carolina and Florida. Plant Disease. https://doi.org/10.1094/PHP-01-24-0003-RS.
DOI: https://doi.org/10.1094/PHP-01-24-0003-RS

Interpretive Summary: Cucumber is an important vegetable crop grown in most states in the U.S.A. Many diseases and pests attack cucumber plants and reduce their yield resulting in monetary loss for growers. One such disease, known as Downy mildew can infest cucumber plants in open fields and greenhouses and can cause reduced vigor or death of plants and result in significant yield reduction. Downy mildew devastated the cucumber industry around 2004 and beyond. Growers were used to growing cucumbers with resistance to downy mildew for many years prior to 2004. However, in 2004, the available resistance in cucumber broke down forcing the growers to spray expensive pesticides to manage this disease. This was due to a new strain of downy mildew. ARS scientists conducted studies in Charleston, SC and Fort Pierce, FL using downy mildew tolerant and susceptible varieties to determine the prevailing strains of downy mildew. The study suggested that using cultivars with resistance to the old and new downy mildew strains will be needed to manage this disease. The information will be useful to growers, seed company plant breeders, University researchers, extension agents and USDA scientists

Technical Abstract: Downy mildew (DM) caused by Pseudoperonospora cubensis can significantly reduce yield of field-grown cucumber across the United States. Cucumber (Cucumis sativus) cultivars with resistance to DM have been widely grown for over 30 years. However, after a breakdown of host resistance in 2004, intensive fungicide programs are now needed to prevent severe epidemics. In this study, large field plot trials were conducted to characterize pathogen virulence and mitochondrial clade (MC) occurrence on two DM tolerant cultivars ‘Peacemaker’ and ‘Expedition’ and the susceptible cultivar ‘SMR-58’ in Charleston, SC and Fort Pierce, FL during the growing seasons in 2016 and 2017. Rapid DM development on ‘SMR-58’ was observed at both locations in 2016 and 2017. Disease progress lagged behind ‘SMR-58’ on both ‘Expedition’ and ‘Peacemaker’ in Fort Pierce, FL in 2016, but disease incidence reached 100% by the end of the season. No DM was observed on ‘Peacemaker’ or ‘Expedition’ in Charleston in 2016. In 2017, both tolerant cultivars and ‘SMR-58’ were affected in Charleston, but only SMR-58 had severe DM in Fort Pierce. Molecular marker analysis using DM specific mitochondrial clade (MC) markers showed population differences between the two P. cubensis populations collected from FL and SC and provided evidence of genetic shift in the two populations in 2017. The most prevalent DM mitochondrial clade at Charleston during June 2016 was MC1 (95-100%). However, in July 2016 that population began to shift towards MC2 (45-77%) and was predominately MC2 in October 2016 (59-100%). Mixed populations of MC1 and MC2 continued to be observed in Charleston in 2017. In FL, a complete shift was observed from MC2 in 2016 to MC1 in 2017. Since both MCs are present in the southeastern United States and can infect cucumber, cultivars with combined resistance to these diverse P. cubensis populations will be needed to manage downy mildew.