Tomato spotted wilt spreads in Western San Joaquin Valley

Tomato spotted wilt virus continued to invade California processing tomato fields in 2006, mostly on the west side of the San Joaquin Valley, and University of California researchers are now forging IPM strategies to manage it.

Michelle LeStrange, farm advisor for Tulare and Kings counties, says she and others are evaluating controls of host plants that support the virus or its thrips vectors, direct controls of thrips themselves, use of resistant tomato varieties, and combinations of the above.

At a recent processing tomato production meeting in Modesto, LeStrange said the virus has a wide genetic variability, enabling it to evolve many races up and down the state and defy single control practices.

“We have seen it here and there in fields for the past few years, but incidence of the virus is becoming more severe,” she said. “We wanted to find out where it's hanging out and what its reservoirs are, whether in weeds, or annual or perennial crops.”

The virus, one of the tospovirus group increasing in California, is detected on tomatoes as bronzing of young foliage that later develops into necrotic spots. The leaves may cup downward, and some dieback may be seen. On ripe fruit, it causes chlorotic spots and blotches, often with concentric rings.

Pepper and lettuce

It also goes to peppers and lettuce and is known to occur on some 800 crops, weeds, and ornamental plants around the world.

Spread by thrips, mostly western flower thrips but also onion and chili thrips, the virus is acquired by the vectors only during their first and second instars and then distributed by them as adults. It is not transmitted by virus-free adults moving from infected plants.

LeStrange collaborated with virologist Brice Falk and plant pathologist Mike Davis at UC Davis in a survey from Kettleman City to Five Points in 2005-06 to locate the virus in tomatoes, onions, potatoes, lettuce, and radicchio.

“We found hot spots of it around Firebaugh, but incidence was not uniform. You could see one field with 50 percent incidence and another across the road with 10 percent. We saw it early in the season and we saw it late in the season,” LeStrange said.

Fall harvested lettuce and other crops in western Fresno County, she said, supply a constant reservoir of the virus “to live happily along with the thrips,” and a host crop-free period might be needed to break the life cycle.

The survey showed no correlation of infection with direct seeding, transplants, varieties, or time of planting.

The research team, LeStrange added, now plans to redirect its observations of the virus using the findings from the University of Georgia, where 20 researchers are working on managing it in peanuts. Data is being collected there on thrips flights, host plants, and resistant varieties. The team is sorting out what might have promise in California fields.

IPM approach

“The control strategy for us to consider now is a total IPM approach. We have some processing tomato varieties with resistance being developed, but it will take time for them to have all the requirements of processors and growers,” she said.

Among other elements to consider is a trap crop such as buckwheat, where thrips could accumulate and be destroyed with the stand. Planting dates to avoid the insects on a regional basis, although difficult in practice, might help. Keying thrips flights to more selective insecticide applications might have a place.

Some growers in the Huron area have been trying rouging of infected plants. Reflective mulches like those used in peppers might help reduce thrips. Tomato plant populations might have some influence. One source of encouragement is California's recent cold weather, which could have an impact on the virus, host weeds, and thrips.

“Even conservation tillage has had an effect in reducing the virus in other parts of the country,” LeStrange said.

Reporting on his recent research with multiple plants in processing tomato transplant plugs, Gene Miyao, farm advisor for Yolo, Sacramento, and Solano counties, reported encouraging results, although not in all cases.

His project with LeStrange in Fresno County and Mike Murray, farm advisor for Sutter, Colusa and Yuba counties, revealed “very substantial increases in yields by increasing the number of plants in a plug,” in Fresno County, he said.

The economic advantage was as much as twice, and the multiple plants performed best in fields where the stand was thin. That was the case in all locations except those in Yolo County, he added.

Multiple plug plants

Miyao said the practice of multiple plants per plug began when nurseries found greater efficiency by doubling or tripling the plant populations in plugs to avoid blanks. Later field research showed no loss in quality from the multiple plants, along with yield increases of nearly 15 percent when three seeds were planted to each plug.

Subsequent trials in Fresno County suggested that multiple plant plugs tended to fend off beet leafhopper vectors of curly top virus, since the insects were drawn to thinner stands.

The double-seeded plugs in the Fresno trials were very competitive and more robust than the single-seeded and led to increased yields. However, Miyao found that additional vigor and early vegetative growth did not translate to larger canopy cover and significant yield increase in his Yolo County trials.

He said across the overall Colusa, Fresno, and Yolo counties trials, significant yield increases, or an average of 1.4 tons per acre ($84 based a crop price of $60 per ton), were recorded in six of 22 plots.

Considering seed prices approaching $10 per 1,000 seed, planting about 7,000 additional seeds per acre is $70 per acre more in seed costs, leaving a $14 per acre increase in returns.

Precision sprayer

David Slaughter, professor of agricultural engineering at UC, Davis, detailed his latest research with a precision herbicide sprayer for processing tomatoes.

Used with direct-seeded stands at the first- through third-leaf stages of the crop, the device's hyperspectral imaging system can distinguish between the leaf colors of tomato plants and several common weeds. Its grid of tiny nozzles sprays at a height of one-half inch to eliminate drift.

Slaughter said the computer-linked, scanning unit showed in his 2006 trials an accuracy of 95 percent for tomato plants, 95 percent for black nightshade, 95 percent for pigweed, and 99 percent for purslane.

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