Host Plant Resistance to Cucurbit Yellow Stunting Disorder Virus and Bemisia Tabaci in Melon: Status and Prospects

The combination of Cucurbit yellow stunting disorder virus (CYSDV), a newly introduced virus and heavy feeding pressure by its insect vector, sweet potato whitefly (Bemisia tabaci) biotype B (SPWF-B) severely impact melon production in California and Arizona. The appearance of the aggressive SPWF-B in 1990 significantly reduced the Fall melon crop directly through feeding damage; numbers of SPWF-B immatures (eggs + nymphs) was 1500-fold greater than in melon producing areas of Texas and Florida. CYSDV, which is spread by SPWF-B and other biotypes, appeared in California in Fall 2006, and the combination of CYSDV with high SPWF-B populations quickly eliminated fall melon production. Moreover, CYSDV is a potential threat to spring melon production when SPWF-B survives the winter in sufficient numbers to spread the virus. No commercial melon cultivars are available with resistance to either SPWF or CYSDV anywhere in the world. Three non-sweet, vegetable type melons from Africa and India are resistant to CYSDV. Two of them, TGR-1551 (Zimbabwe) and PI 313970 (India), have low-level resistance to SPWF that is inadequate for reducing the adverse biological and economic impacts of whitefly feeding and virus transmission in the low desert of California and Arizona. Single plant selections from six of 100 Indian melon accessions exhibited putative resistance to CYSDV in during Fall 2011 tests under severe virus pressure: PI 116482, PI 123496, PI 123689, PI 124107, PI 124431, PI 145594. Two of these (PI 123689, PI 123496) also performed well during Fall 2012 under similar conditions. All produced large plants that were in good condition at the end of the 2012 test though they were variable for expression of CYSDV symptoms and virus titer.  Numbers of SPWF-B (nymphs through 4th instar; seven weeks post-planting) on two selections, PI 124107, PI 123689 were significantly lower than on TGR-1551 and PI 313970, which suggested they might be exhibiting resistance to feeding by SPWF-B. Fruit of the six accessions have several fruit characteristics of sweet, dessert type melons and may be better donor parents for use in breeding CYSDV-resistant muskmelon, honeydew and mixed melons adapted to California. Combined resistances to CYSDV and SPWF-B would be important components of an integrated pest management program to reduce, if not eliminate, applications of insecticides to control this insect, and reduce the adverse impact of the virus on plant growth, and consequently on fruit yield and quality.