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  • Author or Editor: Michael D. Coffey x
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A new strain of cucurbit powdery mildew incited by Podosphaera xanthii (Castagne) Braun & Shishkoff (formerly Sphaerotheca fuliginea), designated race S, is virulent on all the commonly used melon (Cucumis melo L.) powdery mildew race differentials and first appeared on melon in Yuma, AZ, and Imperial Valley, CA, in 2003. Melon PI 313970 (C. melo var. acidulus) was resistant to P. xanthii race S in 2003 and subsequent years. Inheritance of resistance to P. xanthii race S was studied in two naturally infected, replicated field tests in Imperial Valley in 2005 and 2009 using the parents and F1, F2, and reciprocal backcross generations from crosses of PI 313970 with powdery mildew-susceptible ‘Top Mark’. Resistance to race S was recessive: all F1 and BCTM individuals were susceptible. One recessive gene, designated pm-S, conditioned resistance to race S in the F2 and BCPI. The relationship of pm-S to the previously reported recessive and codominant genes in PI 313970 for resistance to P. xanthii races 1, 2, 2U.S., 3, 3.5, 4.5, and 5 remains to be determined.

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Races 1 and 2 of Podosphaera xanthii (syn. Sphaerotheca fuliginea) were defined in Imperial Valley, Calif. 1938 when P. xanthii overcame genetic resistance in `PMR 45'. Race 3 was first observed in the U.S. in 1976 in Texas; 15 additional races of P. xanthii have been reported in the literature since 1996. Races 1 and 2 have been common in Arizona and California based upon the effectiveness of the powdery mildew resistance genes in commercially available melon cultivars grown in these states. Field data from 11 commonly used melon P. xanthii race differentials in 2001 and 2002 indicated the presence of race 1 in the Imperial Valley and San Joaquin Valley of California, and Yuma, Arizona. In spring 2003, the powdery mildew race situation changed. The first evidence was the occurrence of a severe and widespread infection of powdery mildew in a commercial cantaloupe field. The 11 powdery mildew race differentials were susceptible to powdery mildew in a nearby replicated field test. PI 313970, a melon from India, was resistant to this apparent new race of powdery mildew.

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