D3440: Increasing whitefly populations in the Central Valley of California lead to introduction and establishment of whitefly-transmitted viruses in melon

Whitefly-transmitted viruses (WFTV) associated with high whitefly (Bemisia tabaci MEAM1) populations cause severe economic losses for cucurbit production in the Sonoran Desert of California and Arizona. Cucurbit chlorotic yellows virus (CCYV) and cucurbit yellow stunting disorder virus (CYSDV) (Crinivirus, Closteroviridae), in particular, have led to the loss of most fall production in the desert; however, the large melon production area in the Central Valley of California remained unaffected due to an absence of these viruses and limited whitefly populations until near the end of the melon season. Since 2017, B. tabaci MEAM1 populations have emerged earlier in the summer and achieved higher populations, leading to increased concern for WFTV establishment in the Central Valley. During the summer of 2020 and 2021, melon fields were observed with yellowing symptoms typical of virus infection and elevated whitefly populations. Samples were collected from fields located throughout Fresno County, CA and tested using multiplex RT-PCR and quantitative RT-PCR for CYSDV, CCYV and other viruses. In 2020, CYSDV and CCYV were identified for the first time in the Central Valley in small numbers of plants, but in 2021 CYSDV was found in all plants showing yellowing symptoms that were tested. CCYV was found in one plant each year, and a third WFTV, squash vein yellowing virus, was also found co-infecting plants with CYSDV. The emergence and apparent establishment of CYSDV and CCYV accompanied by increased whitefly populations in the region will necessitate increased efforts for whitefly control in central California.