The California Department of Food and Agriculture’s Beet Curly Top Virus Control Program has a pesticide use permit to control beet leafhopper (BLH), vector of the virus, on federal rangeland during 2009.
The program, administered by the state since the mid-1940s, has protected tomato, sugar beet, bean, pepper, spinach, and other crops vulnerable to the costly virus.
Guided by levels of BLH populations, it has traditionally sprayed western foothill areas from mid-March to mid-April to reduce BLH numbers migrating into valley cropland as rangeland hosts, such as Russian thistle, dry out.
Historically, when hopper populations are low, up to 80,000 acres of western Fresno, Kings, and Kern counties may need treatment. But in years of less than normal rainfall, the treated area may expand beyond 200,000 acres, including the Cuyama and Salinas valleys as well as western slopes of the Sacramento Valley.
Jim Rudig, supervisor of the program based in Fresno, gave an update during a vegetable crops meeting near Five Points.
“So far in our 2009 winter survey in the San Joaquin Valley,” he said, “the late rains limited germination of winter annual hosts during November and December and the cold and damp conditions limited hopper activity.”
Rudig added that as a result, overwintering BLH populations were not concentrated enough to warrant winter treatments. During 1998 to 2007, winter treatments were needed in only four of the 10 years.
The U.S. Fish and Wildlife Service released in late January an amended biological opinion that allows spring treatments of malathion for BLH as needed on selected rangeland habitat. It was expected to go into effect after a 30-day public comment period.
The permit was issued following talks between state and federal authorities concerning methods of application of malathion, drift analysis, water monitoring, prey insects of endangered species, and habitat of threatened and endangered species.
Included is a buffer extended to one mile between treated areas and sites of the endangered California jewelflower during its blooming period.
Rudig said assessment rates for the program, which is supported by growers of susceptible crops, are being revised according to region, crop value, and crop susceptibility to the virus. Tomato and sugar beet are highly susceptible, and spinach has a low susceptibility.
Meanwhile, University of California scientists are learning more about the curly top virus. In delivering a report compiled by Bob Gilbertson, a UC Davis plant pathologist, Tom Turini, Fresno County farm advisor, said the virus has been detected in three distinct forms: mild, severe, and the original.
Noting that the control program did not spray for BLH in 2008, Turini said detection of the insects in tomatoes during February through July of that year was medium to high in Fresno County, medium to low in Kern County, and low in Imperial County.
A few severe outbreaks of the virus occurred and many fields had economic impact in Fresno County, low incidence in the spring was followed by an outbreak in late-planted tomatoes in Merced County, and incidence was low in Kern and Imperial counties.
DNA tests refined by Gilbertson confirmed in 2008 that 97 percent of central SJV samples of tomato suspected of having the virus did indeed have it. Tests to detect the virus in leafhoppers have also been established.
“In terms of curly top virus management,” Turini said, “there is a need to develop resistant tomato varieties, but nothing is currently available. Vector management is by the Curly Top Virus Control Program and in-field management with various insecticides. Cultural practices are field location, planting time, and plant spacing.”
Turini alerted the assembly of growers and PCAs to another potential disease hazard for SJV and Sacramento Valley tomatoes: tomato yellow leaf curl virus (TYLCV).
Although he said it has not been detected in Fresno County, it is documented in Riverside and Imperial counties, Arizona, Texas, Florida, and Mexico. Symptoms include stunted, yellowed plants, and it is another whitefly-transmitted virus.
Even though TYLCV is more severe in desert areas, he stressed that the tomato industry should nevertheless be proactive to prevent it from becoming established in SJV counties.
Once whiteflies acquire it, they have it for life, although the virus does not replicate in the vector. Infected whiteflies can range five to seven miles, but the disease is thought to move over greater distances when the insects are blown by winds. Disease symptoms appear two to three weeks after inoculation.
It is known to move long distances, especially when transported on infected tomato transplants. It is not seedborne and can infect tomato-related hosts such as peppers and tobacco weed without showing symptoms.
Another serious disease, tomato spotted wilt virus (TSWV), Turini explained, has symptoms that can resemble those of curly top virus initially, although “quick tests” are available for a positive identification in the field in a few minutes.
TSWV can be transmitted by nine species of whitefly, but western flower thrips is the primary vector in the SJV. It is acquired by the larval stage and carried by adults.
Turini’s recommendations for IPM to deal with TSWV are strategic planning of crops, such as avoiding planting of tomatoes near potential sources of the virus, use of resistant varieties in high risk areas, and sanitation with weed control during the season and immediately plowing under crop residue following harvest.
Another step is monitoring of thrips and virus symptoms so that insecticides can be applied at the first indication of the disease.
Trials by Turini and Michelle LeStrange, Tulare County farm advisor, to monitor TSWV symptoms in processing tomato varieties in 2008 showed that the resistant AB 8058 had no or very low symptoms and among susceptible varieties incidence varied significantly.
LeStrange has been identifying hosts of TSWV and traced the spread in crops over the past several years from Merced in 2003 to the west side of Fresno County in 2004 and hot spots as far north as Colusa by 2008.
“It’s been a problem in ornamentals greenhouses in California for a long time. We know that petunias are indicator plants for it, and it has hosts in flowers, weeds, and crops,” she said.
Over the years, growers and plant pathologists speculated that outbreaks were due to wider use of tomato transplants, the increased acreage of radicchio, weeds in fallowed land, and even almonds.
During 2005 and 2006 in western Fresno and Merced counties she tested hundreds of plants as potential reservoirs for TSWV. Among crops testing positive are tomato, potato, lettuce, pepper, and radicchio. Weeds include Russian thistle, groundcherry, black nightshade, and prickly lettuce. Flowers are canna and calla lilies.
LeStrange concluded that pepper and tomato crops can amplify the disease and act as an inoculum reservoir. Lettuce, although a bridge crop for it, is not a big threat because of aggressive pesticide use. Radicchio is a significant threat, but that can be managed with thrips management and crop sanitation. Almond flowers and weeds, she added, do not appear to be a threat.
She found that transplants supported relatively low numbers of thrips, especially in closed greenhouses. She found no evidence of TSWV infection on transplants and no evidence that transplants are a major source of thrips or the virus for processing tomatoes.
However, she said she will be checking greenhouses, almond flowers, and weeds again this season, as she monitors tomato, pepper, and radicchio crops.