‘From Race 1 to Race S’
As more and more sources of resistance to melon powdery mildew, the number of races has multiplied exponentially. In part, this is due to the fact that single melon differential lines, such as PMR 45 or PI 414723 or PI 313970, may contain several distinct genes for resistance. Although this leads to the identification of multiple races, the benefit is in the discovery of multiple resistance genes in some differentials that can be incorporated in new melon cultivars. Pyramiding of such genes can lead to the development of more durable resistance to mildew and other pathogens.
Powdery mildew on melons first came into prominence in the Western US in 1925 in the Imperial Valley of California. Cantaloupes were inundated with mildew caused by what we now know is Podosphaera xanthii. In 1926, powdery mildew reduced cantaloupe yield by some 30 percent, and in some succeeding years losses were as much as 50 percent. To combat mildew, a plant breeding program was initiated using a resistant plant from India crossed with the variety Hale’s Best. By 1935, a new powdery mildew resistant cultivar was developed: Powdery Mildew Resistant No 45 (PMR45). PMR45 was similar horticulturally to Hale’s Best, but a little later in maturing. By 1937, the majority of the plantings in Imperial County were of PMR 45. In 1938, a new race, distinct from the now prevalent Race 1, and this Race 2 overcame resistance in PMR 45. In searches for resistance to Race 2, resistance was found in the Indian line that was used to develop PMR 45. PMR 5 and PMR 6 are resistant to Races 1 and 2.
In 1976, an additional race, Race 3, was identified in Texas. PMR 45, PMR 5 and PMR 6 were susceptible. PI 124111, PI 124112 and Edisto 47 are resistant to Races 1, 2 and 3. The discovery in 2003 in the Imperial Valley of Race S, which can overcome multiple sources of resistance has increased the urgency of finding new sources of resistance such as PI 313970, since the mildew population has also developed resistance to many of the site-specific fungicides in current use.
This database represents a new beginning in providing plant breeders with a vital tool, namely in the availability of a range of pathogenic strains stored under liquid nitrogen, to be employed in the selection of resistance sources to be incorporated in new melon cultivars.
HISTORY OF RACES IN CALIFORNIA
Jagger, I. 1926. Powdery mildew of muskmelons in the Imperial Valley of California in l925. Phytopathology 16: 1009-1010. (Established race 1)
Jagger, I.C. and G.W. Scott. 1937. Development of powdery mildew resistant cantaloup No. 45. USDA Circ. 441:6. (Described ‘PMR 45’, first melon bred for resistance to powdery mildew race 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. Rptr. 22:275–276. (Defined race 2 on melon)
Whitaker, T.W. and D.E. Pryor. 1942. Genes for resistance to powdery mildew in Cucumis melo. J. Amer. Soc. Hort. Sci. 41:270-273.
Thomas, C.E. 1978. A new biological race of powdery mildew of cantaloups. Plant Dis. Rptr. 62:223. (Defined race 3)
McCreight, J.D., M. Pitrat, C.E. Thomas, A.N. Kishaba, and G.W. Bohn. 1987. Powdery mildew resistance genes in muskmelon. J. Amer. Soc. Hort. Sci. 112:156–160. (First report of variation in races, i.e., different reactions to the same race at widely separated locations)
Pitrat, M and D. Besombes. 2008. Inheritance of Podosphaera xanthii resistance in melon line ‘90625’. Cucurbitaceae 2008.
McCreight, JD and Coffey, MD. 2011. Inheritance of race and non-race-specific resistance in melon PI 313970 to cucurbit powdery mildew incited by Podosphaera xanthii race S. HortScience. 46(6):1-3. (Defined race S)