Occurrence of mosaic viruses in melons in the Central Valley of California

Three melon-growing regions in the Central Valley of California were surveyed for the incidence and severity of watermelon mosaic virus 2 (WMV2), zucchini yellow mosaic virus (ZYMV), cucumber mosaic virus (CMV), and papaya ringspot virus-watermelon strain (PRSV-W), during 1988 and 1989. WMV2 was the most prevalent virus in all three regions (Yolo/Sutter/Yuba counties, Stanislaus County, and Merced/Fresno counties), both 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 within a site. In Stanislaus County, ZYMV, which had not previously been reported in the Central Valley, was detected in a higher number of sites and a higher proportion of plants per site in 1989 than in 1988. The increased incidence of ZYMV is of concern because this virus is severely pathogenic. PRSV-W was detected in low levels in the growing areas of Stanislaus and Merced/Fresno counties. This study suggests that management strategies for dealing with virus diseases in the agriculturally diverse Central Valley will be specific to each particular region.