Relative susceptibility of commercial watermelon varieties to powdery mildew and phytophthora fruit rot

Authors: Kousik, Chandrasekar - Shaker; Ikerd, Jennifer; MIHIR, MANDAL - Oak Ridge Institute For Science And Education (ORISE)

Submitted to: Cucurbitacea
Publication Type: Abstract Only
Publication Acceptance Date: 9/30/2018
Publication Date: 11/12/2018
Citation: Kousik, C.S., Ikerd, J.L., Mandal, M.K. 2018. Relative Susceptibility of Commercial Watermelon Varieties to Powdery Mildew and Phytophthora Fruit Rot.Presented at Cucurbitacea 2018, Davis CA. Cucurbitaceae 2018 conference abstracts, Page 51. https://cucubit2018.ucdavis.edu/ https://doi.org/358779.

Interpretive Summary:

Technical Abstract: Powdery mildew (PM, Podosphaera xanthii) and Phytophthora fruit rot (Phytophthora capsici) have been occurring frequently in recent years in commercial watermelon fields and growers routinely apply fungicides to manage these two diseases. Both these diseases are known to cause significant yield reduction. The current study was conducted in 2014, 2015 and 2016 to determine the relative susceptibility of twenty six watermelon varieties (seeded and seedless) and three pollenizers to PM and fruit rot in South Carolina. USVL677-PMS, which is highly susceptible and USVL531-MDR, which is resistant to PM and fruit rot were included as controls. A randomized complete block design with three replications was used for planting each year. Naturally occurring PM infection on plants were rated on a 0-10 scale of increasing disease severity. Mature fruit were harvested from all the variety plots and inoculated with a 7-mm plug from an actively growing colony of P. capsici. Inoculated fruit were kept on wire shelves in a large chamber (Temperature 26±2 °C) with high relative humidity (=90%) and free moisture to enhance disease development and prevent drying of agar plugs. Five days after inoculation the lesion diameter and sporulation intensity were recorded. During all three years USVL677-PMS was the most susceptible to PM (70%, 3 year mean disease severity) with highest area under disease progress curves. In comparison, USVL531-MDR (2%) was very resistant to PM. The commercial pollenizers, SP5, SP6 and Lion were all resistant to PM (4.2%). Among the red fleshed varieties, Suprema, (seedless variety) was relatively resistant (20%) compared to other seeded and seedless varieties. Most of the seeded varieties evaluated (e.g. Malali, Black Mama, Mickey Lee) were highly susceptible to PM, however, some were relatively less susceptible (e.g. Declaration) under field conditions. Except for the resistant control USVL531-MDR (0.9 cm lesions) and the pollenizer Lion (2.5 cm), all the other watermelon varieties were highly susceptible to Phytophthora fruit rot (7-13 cm). Although resistance sources exist in watermelons, the resistance has not been bred into commercial varieties. There is a critical need to develop varieties with high levels of resistance to PM and/or Phytophthora fruit rot.