Cucurbit Yellow Stunting Disorder

Infected cucurbit plants initially show a chlorotic (yellow) spotting, which eventually develops into a striking interveinal chlorosis (yellowing) in which the veins remain more or less green but the rest of the leaf turns bright yellow. Leaves will often roll upward and become brittle. Fruit on infected plants may appear normal but often have reduced levels of sugars; this results in poor marketability and economic loss. Symptoms of Cucurbit yellow stunting disorder virus infection can be confused with abiotic factors, such as nutrient deficiency.

Cucurbit yellow stunting disorder is a primarily a disease of cucurbits (e.g., melons, watermelon and squash) and is caused by a plant virus named Cucurbit yellow stunting disorder virus CYSDV; genus Crinivirus, family Closteroviridae). This virus was first detected in southern California and Arizona in fall of 2006, infecting cantaloupe and honeydew melon, watermelon, and various types of squash. Other crops it infects include alfalfa (Medicago sativa), lettuce (Lactuca sativa), and snap bean (Phaseolus vulgaris), as well as several weed species.

Cucurbit yellow stunting disorder virus is spread from plant-to-plant exclusively by the whitefly vector, Bemisia tabaci. All biotypes of B. tabaci known to exist in North America can transmit the virus efficiently, including biotypes A, B and Q. Cucurbit yellow stunting disorder virus is not transmitted by the greenhouse whitefly (Trialeurodes vaporariorum). Whitefly transmission is entirely responsible for virus spread over short distances (e.g., within and between fields). The virus is not transmitted mechanically (by touch) nor is it seed-transmitted. Consequently, the disorder is almost always associated with whiteflies; it does not take many insects to spread the virus. For more information, see WHITEFLIES.

Cucurbit yellow stunting disorder virus is spread over long distances through the movement of infected plants, especially cucurbit transplants. As it can take 3 to 4 weeks for disease symptoms to develop following infection, infected symptomless plants can be unknowingly transported. The virus also can be moved long distance by virus-carrying whiteflies that may accompany transported plant material such as cucurbits and other susceptible infected plants with or without symptoms. Finally, the virus can be maintained in infectious form within whiteflies for up to 9 days. Because Bemisia tabaci whiteflies can move long distances, especially with high winds, the virus may be transported over long distances in this manner as well.

Cucurbit yellow stunting disorder virus has the potential to cause serious damage to cucurbit production, particularly in areas where B. tabaci populations become well established during the growing season. In addition to California, the virus is presently an economic problem in many countries that have Mediterranean climates (e.g., countries in the Middle East and southern Europe).

Although Cucurbit yellow stunting disorder virus has a relatively narrow host range, it was able to overwinter in California and Arizona in 2006-07. Although the incidence of CYSDV was relatively low in spring-planted melons in 2007, it was high in fall-planted melons in the Imperial and Yuma Valleys in both years. Thus, it appears that the virus is now established in southern California. It remains to be seen if it will primarily be a problem in fall-planted cucurbits or if it will increase in incidence in spring-planted melons as well.

Monitoring for the incidence of the virus in California to document the establishment of the virus and the pattern of infection of cucurbits in the desert growing regions of the southwestern United States is ongoing.

Rapid and precise tests for the virus are available at UC Davis and the USDA-ARS in Salinas. These tests can be carried out in less than 24 hours. Anyone finding cucurbits with CYSDV-like symptoms can contact their county farm advisor, Robert Gilbertson at UC Davis (telephone: 530-752-3163) or William Wintermantel at USDA-ARS (telephone: 831-755-2824). Another virus, Cucurbit aphid borne yellow virus (CABYV), which is found in California and also causes yellowing in cucurbits, can be confused with Cucurbit yellow stunting disorder virus (for more information, see CABYV). Molecular tests are needed to differentiate the two viruses.

Currently, a number of management strategies are recommended to minimize the chance of this virus becoming established throughout California and/or causing significant losses to cucurbit production in southern California.

Before the Growing Season

• Although the virus is not seed-transmitted, it is important to use pathogen-free, high quality seed.
• Use virus- and whitefly-free transplants.
• DO NOT import cucurbit (or any potential whitefly host) transplants from areas known to have the virus (Texas, Florida, or Mexico).
• Manage whiteflies on transplants; for more information, see WHITEFLIES.

During the Growing Season

• Plant immediately after any cucurbit-free period (either an arranged regional host-free period or true winter season).
• Avoid planting new fields near older fields (especially those with plants confirmed to be infected by the virus).
• Apply a soil application of a neonicotnoid insecticide (imidacloprid, dinotefuran, or thiamethoxam) at transplanting. Recent research results indicate that dinotefuron may slow the spread and development of the disease, whereas imidacloprid and thiamethoxam may be less effective (see WHITEFLIES for more information).
• Monitor whitefly populations throughout the growing season and implement insecticide application as needed. Rotate insecticides with different modes of action Group numbers to minimize development of insecticide resistance.
• If feasible, cover seedbeds with floating row covers of fine mesh (Agryl or Agribon) in mid-bed trenches of 12-inch depth for fall planting season (row covers need to be removed for pollination, but they prevent early infections).
• Practice good weed management in and around fields to the extent feasible.

After the Growing Season

• Sanitation is very important; remove and destroy old crops/volunteers on a regional basis (e.g., plowing/physical removal).
• In areas lacking a true winter season (i.e., temperatures low enough to prevent crop cultivation and whitefly survival), implement a voluntary or enforced regional cucurbit -free period to eliminate the virus from the cropping system.