Citation: McCreight, J.D., Coffey, M.D., Lebeda, A. 2012. Cucurbit powdery mildew of melon incited by Podosphaera xanthii: global and western U.S. perspectives. In: Sari, N., Solmaz, I., Aras, V., editors. Cucurbitaceae 2012, Proceedings of the Xth EUCARPIA meeting on genetics and breeding of Cucurbitaceae, October 15-18, 2012, Antalya, Turkey. p. 181-189.
Interpretive Summary: Lettuce aphid is a major insect pest of lettuce, Lactuca sativa L, in many commercial lettuce productions areas around the world. Resistance to lettuce aphid was first reported in the wild relative of cultivated lettuce, Lactuca virosa L. accession IVT 280 and characterized as complete, i.e., virtually no aphids survived, and genetically dominant to partial resistance in L. virosa accession IVT 273. We previously reported two new potential sources of unique genes for resistance to lettuce aphid in another wild but more closely related Lactuca serriola L. accession PI 491093 and L. virosa PI 274378. Here we report on the genetic and phenotypic nature of resistance to lettuce aphid in these two wild lettuces. Resistance to lettuce aphid in PI 274378 is complete and allelic to complete resistance in IVT 280. Resistance to lettuce aphid in PI 491093 was partial, recessive to complete resistance in ‘Barcelona’ that was derived from IVT 280, but dominant to susceptibility in ‘Salinas’. We propose the revised gene symbols: NrC for complete resistance and NrP for partial resistance, which was originally designated as nr but is now the symbol for susceptibility to lettuce aphid. The dominance relationships among these three alleles are NrC (in IVT 280, ‘Barcelona’) > NrP (in PI 491093) > nr (in susceptible genotypes). Expression of partial resistance in PI 491093 was variable in controlled infestation tests, but in a naturally infested field test provided a potentially useful level of resistance to lettuce aphid. Partial resistance, where complete resistance has not been widely deployed, may either alone or as a component of integrated pest management delay or prevent emergence of lettuce aphid genotypes that overcome complete resistance.
Technical Abstract: Cucurbit powdery mildew (CPM) is a major problem of melon (Cucumis melo L.) production worldwide, that is mostly caused by two fungi: Podosphaera xanthii and Golovinomyces cichoracearum (DC) V.P. Heluta (formerly Erysiphe cichoracearum). The two species may co-infect in some areas of northern Europe, but P. xanthii dominates in warmer climates around the world. Thirty-six races of P. xanthii have been reported on melon based on sets of race differentials that range in number from as few as two to as many as 35. The CPM research community and seed industry are undertaking steps to define uniform sets of CPM race differentials and objective criteria for race nomenclature and designations. Breeders and pathologists must also consider another aspect of CPM that stems from its obligate parasitic nature: race stability as defined by a given set of CPM race differentials. This review summarizes the reported races of P. xanthii largely from the published literature. It also presents annual profiles from 2002 through 2011 of P. xanthii populations in the Central Valley of California, and the lower desert area of Imperial Valley, California and Yuma, Arizona.