Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2005
Publication Date: 1/20/2006
Citation: Mccreight, J.D. Melon-powdery mildew interactions reveal variation in melon cultigens and podosphaera xanthii races 1 and 2. J. Amer. Soc. Hort. Sci. 131(1):59-65. 2006. Interpretive Summary: Powdery mildew caused by the fungus Podosphaera xanthii affects yield and quality of melon (cantaloupe, muskmelon, honeydew, etc.) worldwide. Fourteen races of the pathogens that cause powdery mildew on melon, and numerous genes for resistance have been described since 1934. The genetic information about the many sources of resistance is incomplete due in part to the minor crop status of melon, the many places around the world where the numerous reports originated, availability of germplasm for testing, different research objectives and protocols, different populations of the raves used, and lack of any genetic information about the pathogen. The net result has been numerous and sometimes differing or conflicting reports of genes for resistance to powdery mildew. The objective of the present study was to compare the disease reactions of 13 melon cultigens previously reported resistant to races 1 and 2 in Japan, Spain and the U.S. for resistance to U.S. populations of races 1 and 2. Such tests would either confirm their reactions to one population, or reveal new genetic differences for resistance to powdery mildew. These tests would also permit comparisons among P. xanthii populations in Japan, Spain and the U.S. The many differences found among the 13 cultigens for resistance to U.S. populations of races 1 and 2 indicate differences in their genes for resistance to race 1. The results also revealed genetic differences within and among the populations of races 1 and 2 in Japan, Spain and the U.S., and the recognition of 28 races of powdery mildew. Melon breeders and seed companies must verify the reactions of their powdery mildew resistant varieties before growing them on a large scale to ensure effectiveness of their resistance genes.
Technical Abstract: Powdery mildew is a serious disease of melon (Cucumis melo L.) worldwide. Twenty-two melon cultigens have been used to define 22 reported races of the pathogen Podosphaera xanthii (sect. Sphaerotheca) xanthii (Castag.) U. Braun and N. Shish. Comb. nov. [syn. Sphaerotheca fuliginea (Schlecht. Ex Fr.) Poll.]. Discrepancies in the reactions of eight cultigens to populations of P. xanthii races 1 and 2 in California, Japan, and Spain revealed genetic differences among them that can be used to differentiate P. xanthii race 1 and 2 populations in these countries. Implicit in these results is the existence of previously unknown virulence factors in these populations of P. xanthii races 1 and 2 that permit designation of new races of P. xanthii on melon. Synthesis of these results with previous reports resulted in the identification of 28 putative races of P. xanthii on melon that include eight variants of race 1 and six variants of race 2. Six of the cultigens exhibited resistant blisters in response to heavy infection of P. xanthii in field and greenhouse tests.