(This report was compiled by Imperial County UCCE entomology farm advisor Eric T. Natwick, former UCCE Imperial County plant pathology farm advisor Tom Turin who now works in Fresno County and UC Davis plant pathologist Robert Gilbertson)
Several silverleaf whitefly Biotype B-transmitted viruses occur as Squash leaf curl virus (SLCV), Cucurbit leaf crumple virus (CuLCrV), Cotton leaf crumple virus (CLCrV), and Lettuce chlorosis virus (LCV).
Two new whitefly transmitted viruses have been added to that list in the past six months in Imperial County Calif. — Cucurbit Yellow stunting disorder virus (CYSDV) and Tomato yellow leaf curl virus (TYLCV).
It is unknown how these two new viruses were introduced. However, it is possible that they were moved into the valley via transplants; either for commercial production or through retail stores for home gardeners or landscapers. Unfortunately, without more stringent regulations on movement of vegetable and ornamental transplants from other states, introduction of new diseases and insects is a constant threat to agricultural production.
Cucurbit yellow stunting disorder virus
The first of the two new viruses to be found was CYSDV. It can be a devastating disease of melon and squash crops, and caused substantial damage to melons in the Imperial Valley in the fall of 2006. CYSDV was also found in Arizona last fall and even in several melon and squash-growing areas in Mexico.
The symptoms on melons, squash, and cucumbers are expressed first on crown leaves as yellow mottling. The yellow spots soon coalesce into a general yellowing between veins which remain green. Infection with CYSDV early in the plant's life cycle can lead to severe loss of yield through reduced set, and reduced fruit size.
There can also be a quality loss due to reduced sugar in the fruit of melons. In the Imperial Valley, CYSDV is dependent on the silverleaf whitefly for transmission. The virus is not seed-borne, nor mechanically transmitted. The virus is transmitted by whiteflies in a semi-persistent manner.
The whitefly adult must feed for at least two hours to acquire this crinivirus and can remain infectious for seven to nine days. Natural infections have been reported in summer and winter squashes, cucumbers, musk melons such as cantaloupe and honeydew melon, and watermelon. In Texas, where CYSDV was first detected in 2000, host-free periods have been successful in managing the disease. Insecticides cannot protect crops from CYSDV because whitefly adults migrate from field to field spreading the virus. Complete crop destruction followed by a cucurbit host-free period is the only proven control method.
It may be several years before CYSDV-resistant cucurbit cultivars are available to growers.
Tomato yellow leaf curl
TYLCV is the most recent whitefly-transmitted virus to appear in Imperial County. TYLCV threatens commercial tomato production, pepper production, and transplant production of peppers and tomatoes.
It was identified in greenhouse tomato samples from Brawley, Calif. TYLCV is transmitted by adult silverleaf whiteflies and can spread rapidly, but TYLCV is not seed-borne, nor is it mechanically transmitted. The presence of silverleaf whitefly host plants, both cultivated (peppers and tomatoes) or wild hosts (sowthistle, cheeseweed, and nightshade weed) during spring and summer may lead to whitefly migration and spread of TYLCV. During late spring, summer, and early fall, growers need to monitor whitefly populations very closely and destroy whitefly weed hosts and crop residues (melons and cotton).
TYLCV has a broad host range from several plant families including Solanaceae (tomato, peppers, various nightshade weeds, and ornamental plants), Malvaceae (cheeseweed), and Fabaceae (beans).
In tomatoes, typical symptoms for TYLCV include yellow (chlorotic) leaf edges, upward leaf cupping, leaf mottling, reduced leaf size, and flower drop. TYLCV can have a severe impact on tomato production. Plants infected at an early stage won't bear fruit and their growth will be severely stunted.
TYLCV identification based only on symptomatology is unreliable, because similar symptoms can be caused by other viruses or various growing conditions. Proper identification is necessary to control TYLCV:
Use only virus and whitefly-free tomato and pepper transplants. Transplants should be treated with bifenthrin (Capture) or dinotefuran (Venom) for whitefly adults and Oberon for eggs and nymphs. Imidacloprid or thiamethoxam should used in transplant houses at least seven days before shipping. Transplants should be produced in areas well away from tomato and pepper production fields.
Use a neonicotinoid insecticide, such as dinotefuran (Venom) imidacloprid (AdmirePro, Alias, Nuprid, Widow) or thiamethoxam (Platinum), as a soil application or through the drip irrigation system when tomatoes or peppers are transplanted.
After the efficacy of the neonicotinoid insecticide application begins to decline, the secondary spread of whiteflies will need to be controlled. Whitefly populations must be monitored throughout the season, and treated when identified. Insecticide classes should be rotated for insecticide resistance management (IRM). Foliar insecticide treatments used in IRM for whitefly control include: Capture, a pyrethroid; foliar neonicotinoid insecticides dinotefuran (Venom), imidacloprid (Provado), and thiamethoxam (Actara), but do not use if a neonicotinoid insecticide was applied as a soil or drip irrigation treatment; insect growth regulators such as pyriproxyfen (Knack) and buprofezin (Courier); insecticidal soap; and crop oils.
Highly UV-reflective mulches (metalized) and low rates of crop oil (0.25 percent to 0.50 percent) can be used as whitefly repellents to reduce whitefly feeding and virus transmission.
Sanitation is very important for preventing the migration of whitefly adults and the spread of TYLCV. Rogue tomato or pepper plants with early symptoms of TYLCV must be treated immediately (during the first three to four weeks of the season) by placing infected-looking plants in plastic bags. Maintain good weed control in the field and surrounding areas, and prevent the spread of any whiteflies to healthy plants. Tomato and pepper fields should be cleaned up immediately after harvest. Crop residues of melons and cotton should be destroyed immediately after harvest to reduce whitefly migration.