Tomato spotted wilt virus probed in San Joaquin Valley studies

University of California researchers are continuing to survey tomato spotted wilt virus (TSWV) this summer to learn more about where it comes from and the amount of damage it can inflict in San Joaquin Valley processing tomatoes.

Although long known as a sporadic pest in the Valley, the virus first became a concern in 2003 in Merced County. An outbreak occurred near Coalinga two years later, and since 2006, the UC team has been closely observing the virus along the West Side of the Valley. It has been traced this year from Kettleman City to Firebaugh.

The researchers are Michelle LeStrange, Tulare County farm advisor; Tom Turini, Fresno County farm advisor; and Bob Gilbertson, plant pathologist at UC, Davis. Their work is funded by the California Tomato Research Institute.

They conducted a discussion of what’s been learned about the virus and its thrips vectors recently at the West Side Research and Education Center at Five Points.

LeStrange said incidence of the virus seems higher this year, but assessments of distribution and severity of it and its vectors are still underway. She invited growers and PCAs to submit samples of what they suspect might be TSWV-infected plants for positive identification.

TSWV symptoms early in the season include stunted growth, upcurled leaves, purple veining, bronzing, and necrosis. Later, it shows the above plus necrosis of leaves and stems, and a wilted appearance. Ripe and immature fruit show ring spots, mottling, bumpiness, and necrosis.

The trio is putting together IPM practices, with a foundation of fresh-market tomato growers’ monitoring for thrips, weed control, and field sanitation to help manage the virus.

“We are trying to figure out the sources, the reservoirs of this virus, as it moves up the Valley,” LeStrange said.

When the TSWV was first found on SJV tomatoes, some thought it was brought in on transplants, but, she said, during the past couple of years major greenhouses have been monitored and infections to date “are not a fault of the greenhouses.”

While these trials have shown thrips on greenhouse plants, there has been no evidence of virus.

The researchers are also watching adjacent transplanted and direct-seeded fields to note any differences in development of the disease.

LeStrange said in many fields 10 percent to 15 percent of both TSWV and curlytop virus, whose symptoms are similar, has been detected.

“Since we started the project,” she added, “we’ve been able to point out that TSWV is not going to be an easy virus to solve, because it has so many different host plants. The thrips like a lot of different plants as well.”

She predicted the virus will be a chronic problem for the Valley and said resistant varieties are something to look forward to.

Among the crops hosting TSWV are tomatoes, peppers, lettuce, radicchio, and, elsewhere in the U.S., peanuts. Weed hosts include bindweed, nettle, mallow, and chickweed, and it also goes to ornamentals such as geranium, chrysanthemum, zinnia, and verbena.

Radicchio was earlier believed to be a major reservoir of the virus, but LeStrange said a radicchio field planted this year was well managed for thrips and the disease was minimized. “At least it is not always a reservoir for the virus and thrips,” she added.

She also submitted samples of almond flowers for thrips counts and presence of virus this year. Of the few thrips found, none was found to be carrying the virus.

LeStrange also sampled weeds in Fresno and Kings counties, and samples of 11 species failed to show the virus. However, in Merced County sampling produced the virus on mallow and prickly lettuce. Sampling of many additional weed species is continuing.

Turini said he found TSWV in spring lettuce this year around Five Points and Huron, but not at levels thought to endanger that crop. These may be reservoirs for infection that later is carried by thrips into tomato fields.

Hot spots of disease in processing tomatoes in the Five Points area, he said, showed infection on 18 symptomatic plants per 100 feet of row in mid-July. “Fortunately, these are mainly late-season infections that cause less economic damage.”

Reasoning that specifically-timed insecticide treatments on host plants where thrips reproduce might reduce the spread of the virus, Turini has been applying several materials.

Results of his current insecticide treatments are pending, but he gave some data from his 2007 trials.

The difficulty with insecticides, he noted, is in getting adequate coverage inside flowers and bud tissue where thrips reside, as well as the insects’ ability to develop resistance quickly.

“There were a few materials that reduced the thrips, and the best we got was about a 40 percent reduction, although that 40 percent might prove to be important for thrips that are reproducing in the crop.”

He explained that the virus is acquired from an infected host plant by feeding larvae of various species of thrips, which later transmit it as adults during their 30- to 45-day life.

He said resistance to TSWV exists in fresh market tomato varieties, and his trials in 2007 showed statistically significant differences in wilt symptoms between varieties. For example, the variety AB2 was in the middle range of resistance and AB8058 had virtually no symptoms. He and LeStrange are continuing variety observations this season.

Gilbertson has been evaluating TSWV in processing tomato fields, both transplanted and direct-seeded, in Fresno and Kings counties.

In 2007 he found infections of less than 3 percent from April through harvest. Direct-seeded fields were slightly higher in incidence than transplanted. His monitoring found little of the disease in spring lettuce, although high levels of vectoring thrips and infection were found in a field of radicchio.

Gilbertson has been collecting thrips and infected plants in the field and is using a rapid virus test to determine when thrips are posing the greatest threat to tomatoes in terms of transmitting TSWV.

“Our preliminary results indicate thrips early in the season in Fresno and Merced counties are relatively clean of the virus,” he stated.

“Presumably the thrips are picking up the virus from crop plants early in the season, and then in June and July we are seeing more of it in the thrips. By then there is a large population of thrips to amplify the virus and spread it later in the season.

“That’s why we are pushing the idea that once we see the thrips populations building up early, in late April and May, we need to apply thrips management to prevent the high populations that carry the virus later.”

Gilbertson also noted they are now convinced that, counter to some opinions, thrips do in fact reproduce on tomatoes.

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