The fungi that causes powdery mildew when present will begin to show up on cucurbit leaf surfaces as light yellow or white spots. Upon further invasion on leaf surfaces, the disease will begin to look like a white powdery substance as the name indicates. While powdery mildew does not harm cucurbit fruit in a direct manner, it does indirectly diminish fruit size and quality by destroying cucurbit plant leaves. As leaves are attacked they will begin to turn yellow and dry out providing no more benefit to the cucurbit fruit.
Powdery mildew fungi is believed to overwinter in diseased plant debris from warmer Southern U.S. climates and progress into cooler climates during the summer season. The fungi thrive in higher humid conditions in ideal temperatures of around 70° to 80° F. While the disease does grow and spread in higher humid conditions, the disease does not live on plant surfaces where is water present. In plant micro-environments, among plant leaves and vines, where there is less air-circulation and sun light penetration, the disease grows and spread very fast.
Powdery mildew control begins well before the growing season starts. Rotate cucurbit crops on to different planting locations yearly if possible especially if located in warmer U.S. climates and if the disease is known to be a problem in the area. Remove all cucurbit debris after harvest to prevent overwintering of diseases in fields, whether or not powdery mildew infection is known (there are likely other cucurbit diseases present). Allow a few feet of spacing between full size cucurbit plants to allow adequate air circulation between plants later in the season, utilizing trellises if necessary to save space.
After the cucurbit plants have been seeded there are several options you can use to for the natural and organic control of powdery mildew. First, make sure the plants are healthy, with the addition of beneficial health nutrients (in addition to just N-P-K fertilizers) and biological fungicides; healthy plants are less vulnerable to attack. Prior to disease infection, apply ongoing applications of natural and organic fungicides that consist of neem oil, sulfur and / or copper ingredients. You can also use an old home remedy liquid solution that consists of mixing 1 part low fat milk to 10 parts water and apply every 1 – 2 weeks. With these fungicides, its best to fully coat all leaf surfaces (top and bottom) and maintain a regular spraying schedule starting early on as it is easier to prevent the powdery mildew disease than to stop it.
RESOURCES / REFERENCES:
Hansen, Mary Ann, Extension Plant Pathologist, Department of Plant Pathology, Physiology and Weed Science, Virginia Tech. Powdery Mildew of Cucurbits, Sheet 450-710. Virginia Cooperative Extension, Virginia Tech and Virginia State University.
Retrieved From: http://pubs.ext.vt.edu/450/450-710/450-710.html
Langevin, Don. (1998). How To Grow World Class Giant Pumpkins, II. Norton, MA: Annedawn Publishing.
Nunez-Palenius, Hector G., Graduate Assistant, Cantliffe, Daniel J., Professor and Chair, Horticultural Sciences Department, Hopkins, Donald, Professor and Center Director, Mid-Florida REC-Apopka. Powdery Mildew of Cucurbits in Florida, Publication #HS1067. Original Publication date June 2006, reviewed August 2009. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Retrieved from: http://edis.ifas.ufl.edu/hs321