Powdery mildew has been a concern to farmers for years, and some management techniques such as sulfur have been around for just as long. In fact, this pathogen was among the first studied plant pathogens that helped to bring the importance of studying plant diseases to public awareness, and subsequently bring the field of plant pathology to fruition.

On Sept. 22, 1847, observations of powdery mildew disease and control measures were first published by E. Tucker in Gardeners 9 Journal. Tucker was an uneducated gardener of the Margate, Kent, England grape vineyards who noticed the leaves in the vineyards were overcome by a white fluffy growth. At the time, light microscopes were very new and popular, and it was common for citizens to own and use microscopes for leisure and pastime. Tucker looked at the organism under his microscope and decided that this growth was the cause of the disease. Tucker had previously tried sulfur and lime for control of mildew on peach trees, and tested it successfully on this new mildew disease he had found in the vineyards. Following this discovery by Tucker, the fungal organism causing powdery mildew of grapes was identified and named by Rev. J. Berkeley as Oidium tuckeri in 1847.

Although primarily identified as causing powdery mildew problems on grapes in the 1800s, various powdery mildew pathogens have been identified since then. There are many different types of plant parasitic powdery mildew fungi that can cause problems on various crops, including cucurbits.

Pathogen, occurrence, and symptoms

Powdery mildew of melon (Cucumis melo) is caused by Podosphaera xanthii (syn. Sphaerotheca fuliginea), and occurs most commonly in early to mid summer on cucurbits in the Imperial Valley. P. xanthii is less common in watermelon than on other cucurbits. Currently, P. xanthii races 1, 2 US and S have been identified in the lower deserts of California and Arizona. Various locations within California were tested for presence of fungicide resistance in 2006. P. xanthii isolates were found to have resistance to different classes of fungicides: Topsin (benzimidazole), Rally (DMI) and Flint (strobilurin).

In 2006 and 2007, field samples from Fresno and Merced counties were identified to have fungicide resistance to both Flint and Rally. When race S was identified in Imperial Valley in 2003, commercially available hybrid varieties such as Impac with race 1 and 2 resistance were susceptible to race S.

Current studies are focused on characterizing resistance to race S in a vegetable type melon from India and transferring resistances to race 1, 2 US and S to Western shipping type cantaloupe.

Optimal conditions for powdery mildew growth are temperatures between 60 F to 80 F and shady conditions. P. xanthii prefers dry conditions, with relative humidity of 50 percent to 90 percent for spore production. Although relative humidity doesn’t often reach those levels consistently in the desert, dense foliage and irrigation may create microclimates that favor powdery mildew development. However, presence of water on the plant surface may wash off and/or inhibit the germination of the fungal spores. P. xanthii grows on the outside of leaves, first appearing as a small white growth, eventually becoming a layer of white fluffy mycelium and spores on the upper or lower side of leaves. P. xanthii does not grow systemically (able to spread through the inside of the entire plant), instead this fungus prefers to produce mycelium on the outer surface, and penetrate the leaves with specialized feeding cells called haustoria. The fungus will then produce spores in the absence of free water; spore production gives the leaves the characteristic powdery appearance. The spores are spread to new infection sites by wind. After the fungus has extracted nutrients, the plant cells die, causing the leaves to become brown and papery. Fruit produced by the plant may accumulate less sugar, have reduced yield, and shortened production times.

Management options

There are commercially produced varieties that are bred for resistance that may be planted. However, although there are commercially available varieties of powdery mildew resistant melons, they are only available against races 1 and 2. Currently, there are no known U.S. commercial varieties with resistance to race S.

Protective fungicide treatments are very successful at preventing the development of powdery mildew on melon. Sulfur is a treatment that has been used for hundreds of years, in fact since the first identification of the causal disease pathogen in grapes in 1847. If sulfurs are used, it is important to apply sulfur when temperatures are below 85 F to avoid burn and damage to leaf tissue. Some varieties of cucurbits are more susceptible to sulfur injury than others, therefore, make sure to check varietal sensitivity prior to applying sulfur.

Should plants develop powdery mildew symptoms, eradicant fungicides should be used after an infection has established. Regardless of the fungicide used, it is imperative to rotate fungicide classes to prevent the development of resistant races. Additionally, be aware that fungicide resistant races exist in California as well as new races; therefore a treatment or cultivar that has worked in the past may now be ineffective.