The 2012 ASHS Annual Conference

Many physiological races of the cucurbit powdery mildew pathogen (CPM) Podosphaera xanthii (Castagne) Braun & Shishkoff have been reported on melon (Cucumis melo L.). Melon accession PI 313970 is the only reported source of host plant resistance to race S, which first appeared in Imperial Valley, CA in Spring 2003. Race SD, which overcomes resistance in PI 313970, occurred in single spore isolates from collections of CPM from Imperial Valley and on melons in a greenhouse at Salinas, CA following many years of growing PI 313970 year-round in the greenhouse. Variants of races S and SD, designated SW and SDW, infect watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai]. Four hundred thirty melon accessions were evaluated for new host plant resistance to races 1, S, SW, SD and SDW in controlled-inoculation greenhouse tests at Riverside, CA in 2008 and 2009. Twelve of the accessions, which were collected from five sites in Madhya Pradesh, India, exhibited high-level resistance to one or more races. All 12 accessions were identified as members of C. melo ssp. agrestis. Two accessions were further classified as C. melo ssp. agrestis var. flexuosus (PI 614543, PI 614576); seven were further classified as C. melo ssp. agrestis var. momordica (PI 614524, PI 614527, PI 614528, PI 614544, PI 614545, PI 614546, PI 614577); and the varietas classifications of three accessions were not designated (PI 614525, PI 614526, PI 614529). In subsequent open field tests at Holtville they were all resistant to a CPM population that could have been race 2 in 2010 (‘PMR 45’ failed to germinate), and only PI 614543 was susceptible to race 1 in 2011. Four accessions (PI 614525, PI 614543, PI 614544, PI 614576) were susceptible to a Salinas isolate of race S in a growth chamber. Only three accessions (PI 614524, PI 614527, PI 614545) were resistant to a Salinas isolate of race SD in a greenhouse. The discrepant reactions of these 12 accessions among the respective tests of CPM isolates may have resulted from three potential sources of variability: (1) heterogeneity within the accessions (they are open-pollinated), (2) genetic variation among identical CPM isolates based on phenotype characterization by the melon CPM race differentials, and (3) environmental variation. Regardless of the source of variation, these 12 C. melo ssp. agrestis accessions are potentially useful sources of genes for host plant resistance to several races of CPM.