Physiological races of cucurbit powdery mildew pathogen (Podosphaera xanthii) based on watermelon differentials

Authors: Kousik, Chandrasekar; Ikerd, Jennifer

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2025
Publication Date: 2/19/2025
Citation: Kousik, C.S., Ikerd, J.L. 2025. Physiological races of cucurbit powdery mildew pathogen (Podosphaera xanthii) based on watermelon differentials. Plant Health Progress. .. https://doi.org/10.1094/PHP-11-24-0114-RS.
DOI: https://doi.org/10.1094/PHP-11-24-0114-RS

Interpretive Summary: Watermelon is an important vegetable crop grown in 44 states in the U.S.A. Many diseases and pests attack watermelon seedlings and plants and reduce fruit yield resulting in monetary loss for growers. One such disease, known as powdery mildew, can infest watermelon plants including seedlings and can cause reduced vigor or death of seedlings and also result in yield reduction in the field. ARS scientists in Charleston SC, have developed a set of watermelon lines that will help identify strains among the powdery mildew pathogen populations. These set of watermelon lines known as "race differentials" can be used by private and public sector researchers including plant pathologists, plant breeders, extension agents, and seed company researchers to identify powdery mildew pathogen race. This information will also be useful to seed company plant breeders, University researchers, extension agents and USDA scientists. Knowledge of appropriate powdery mildew race will be helpful to growers to plant appropriate resistant watermelon cultivars.

Technical Abstract: Watermelon is a major cucurbit crop grown across the world and in 44 states in the USA. Powdery mildew (PM) caused by Podosphaera xanthii is an important disease of watermelon that can severely limit fruit production and reduce yield. Extensive work has been done on races of P. xanthii based on melon (Cucumis melo) differential hosts and seven major races of have been identified. However, physiological races of P. xanthii have not been classified for many other cucurbits including watermelon. The objective of this study was to develop a set of watermelon PM race differentials and identify potential races. Ten P. xanthii isolates collected from various states were evaluated using a newly developed set of watermelon (Citrullus lanatus) differentials that included a highly susceptible germplasm line USVL677-PMS, a commercial cultivar Mickey Lee, two resistant germplasm lines USVL608-PMR, PI 189225 (S5 selection) and a resistant pollenizer SP5. The well studied melon (C. melo) powdery mildew differentials Iran H and PMR-45 were also included. All isolates colonized and produced abundant conidia on the melon Iran H, and USVL677-PMS. Watermelon USVL608-PMR was resistant to seven of the ten isolates and displayed intermediate resistance to three isolates. SP5 and PI 189225 were resistant to all isolates. Of the 10 isolates that grew well on USVL677-PMS, only five colonized and produced abundant conidia on Mickey Lee which was highly resistant to the remaining five. Based on reactions of the ten powdery mildew isolates on Mickey Lee and USVL677-PMS, at least two distinct physiological races (0 and 1) can be identified. These two races based on watermelon differentials did not co-relate with race structure based on the melon differentials. As new watermelon cultivars with resistance to powdery mildew are being released it will be important to determine their reaction to P. xanthii isolates as a race structure based on watermelon differentials clearly exists.