OCCURRENCE OF MOSAIC-VIRUSES IN MELONS IN THE CENTRAL VALLEY OF CALIFORNIA

Three melon-growing regions in the Central Valley of California were s urveyed for the incidence and severity of watermelon mosaic virus 2 (W MV2), zucchini yellow mosaic virus (ZYMV), cucumber mosaic virus (CMV) , and papaya ringspot virus-watermelon strain (PRSV-W), during 1988 an d 1989. WMV2 was the most prevalent virus in all three regions (Yolo/S utter/Yuba counties, Stanislaus County, and Merced/Fresno counties), b oth in the number of sites with infected plants and in the proportion of symptomatic plants within each site. CMV and ZYMV were detected in fewer sites and generally infected 20% or fewer symptomatic plants wit hin a site. In Stanislaus County, ZYMV, which had not previously been reported in the Central Valley, was detected in a higher number of sit es and a higher proportion of plants per site in 1989 than in 1988. Th e increased incidence of ZYMV is of concern because this virus is seve rely pathogenic. PRSV-W was detected in low levels in the growing area s of Stanislaus and Merced/Fresno counties. This study suggests that m anagement strategies for dealing with virus diseases in the agricultur ally diverse Central Valley will be specific to each particular region .