Cucurbit Genetics Cooperative Report 7:58-59 (article 26) 1984

The Importance of Monitoring Races of Powdery Mildew on Muskmelon

Thomas, C.E., A.N. Kishaba, J.D. McCreight, and P.E. Nugent

USDA/ARS, U.S. Vegetable Laboratory, Charleston, SC 29407 (first and fourth authors), USDA/ARS, Boyden Entomological Laboratory, Riverside, CA 92521 (second author); and USDA/ARS, U.S. Agricultural Research Station, Salinas, CA 93915 (third author)

Powdery mildew is a widespread and often production limiting disease of muskmelon (Cucumis melo L.). Resistance against it is one object of many breeding programs. For many years the incitant fungus was considered to be Erysiphe cichoracearum DC ex Merat. All recent reports, however, indicate Sphaerotheca fuliginea (Schlecht. ex Fr.) Poll. as the incitant. Sitterly (2) presents an excellent review of this situation. Three races of the powdery mildew pathogen have been demonstrated to occur in the U.S.A. (2, 4). The importance of monitoring these races, especially to breeding programs, is explained below.

In 1980 the authors began a cooperative research effort to determine which races of powdery mildew occurred on muskmelon in different geographic areas of the U.S.A. The differential cultivars listed by Thomas (4) and other cultivars with known reactions to individual races have been evaluated repeatedly for their reaction to powdery mildew in both the greenhouse and field at Brawley, Salinas, and Riverside, California; Charleston, South Carolina; and Weslaco, Texas.

Thus far we have confirmed a shift in the natural population of powdery mildew at Brawley, California, from a predominance of race 2 in 1982 to race 1 in 1983 and several shifts back and forth from race 3 to race 1 at Weslaco, Texas from 1980 through 1982. In 1983 we detected a shift in our greenhouse culture of powdery mildew at Charleston, South Carolina from race 3 to race 1. This shift occurred within a period of a few weeks and would have gone undetected for some time had we not maintained differential cultivars in the greenhouse.

Due to the population shifts that we and Sowell (3) have detected and the rapidity with which these shifts can occur, we recommend that races of the powdery mildew pathogen on muskmelon be monitored in research efforts, especially breeding programs. Table 1 lists the minimum recommended cultivars and their reactions to races 1, 2, and 3 of powdery mildew to accomplish this monitoring. The senior author will furnish small quantities of seed of these cultivars to interested researchers who are willing to share with us the reactions of these cultivars in their areas.

Table 1. Reactions of cantaloup cultivars to races of powdery mildew incited by Sphaerotheca fuliginea.
Cultivar	Reaction
	Race 1	Race 2	Race 3
Hale's Best Jumbo	susceptible	susceptible	susceptible
PMR 45	resistant	susceptible	susceptible
PMR 6	resistant	resistant	susceptible

Literature Cited

1. Jagger, I.C., T.W. Whitaker and D.R. Porter. 1938. A new biologic form of powdery mildew on muskmelons in the Imperial Valley of California. Plant Dis. Reptr. 22:275-276.
2. Sitterly, W.R. 1978. Powdery mildews of cucurbits. In: The Powdery Mildews. D. M. Spencer (ed.), Academic Press, New York. pp. 359-379.
3. Sowell, G., Jr. 1982. Population shift of Sphaerotheca fuliginea on muskmelon from race 2 to race 1 in the southeastern United States. Plant Dis. Reptr. 66:130-131.
4. Thomas, C.E. 1978. A new biological race of powdery mildew of cantaloups. Plant Dis. Reptr. 62:223.