Downy mildew on Watermelon

Pseudoperonospora cubensis

Downy mildew causes angular, yellow to brown lesions on top of the leaves that are restricted by leaf veins. When conditions are favorable, for example early in the morning when moisture is higher, lesions observed on the underside of the leaf may be covered in dark “down”. This downy growth is masses of pathogen spores called sporangia, which can be seen in the field with a 20X hand lens. In the presence of water sporangia can directly germinate or differentiate into swimming zoospores that infect leaves. Identification of downy mildew in cucumber can generally be done in a field setting due to the characteristic angular lesions and pathogen sporulation in the underside of the leaf. However, in crops such as watermelon and cantaloupe where lesions do not always have the characteristic angular shape, sporulation can be scarce and diagnostics can be more difficult.

Diagnostic Resources

• Plant Disease Clinics

APS Diagnostic Guide

• Diagnostic Guide for Cucurbit Downy Mildew
  by: Andres Salcedo, Mary Hausbeck, Stacey Pigg, and Lina M. Quesada-Ocampo
  Plant Health Progress: Vol. 21, No. 3

Disease Control Information

• Better Know a Pest: Downy Mildew of Cucurbits – Smart Lab at Cornell Vegetables (video)
• Cornell Cucurbit Downy Mildew Management Guidelines – Cornell Vegetables (2017)
• Downy Mildew News – Hausbeck Plant Pathology Research Lab at Michigan State University
  Monitoreo y Manejo del Añublo Lanoso de las Cucurbitáceas – Spanish translation
• Downy mildew on cucurbits: Early symptoms – McGrath Lab at Long Island Research and Extension Center, Cornell University
• Downy mildew on cucurbits: Other symptoms and imitators – McGrath Lab at Cornell Vegetables
• The Cucurbit Downy Mildew Forecast ipmPIPE -USDA NIFA (2007 – 2019)
• Resources for commercial growers: Downy Mildew – Smart Lab at Cornell Vegetables
• Vegetable Pathology Lab Factsheets – Quesada-Ocampo Lab at North Carolina State University
  Cucurbit Downy Mildew – NC State Vegetable Pathology Factsheets
  Añublo lanoso en cucurbitáceas – NC State Hoja informativa de patógenos de vegetales