BATON ROUGE, La. — Tomato spotted wilt virus is causing losses to many tomato growers. “It appears that we will have a lot of problems if this continues,” says Tom Koske, LSU AgCenter horticulturist.
Symptoms of the disease typically include a cupping and rolling of the upper leaves which usually turn purple/brown along the veins. An internal spotting or browning of leaf tissue (spotted wilt) occurs frequently but is not always evident.
If the fruit are set prior to infection, they may be distorted or develop blotchy orange ring spots as they ripen.
Peppers, cucurbits and certain ornamentals also can be infected by a virus.
LSU AgCenter plant pathologist Ken Whitam explains that TSWV is transmitted by thrips and is not easily mechanically transmitted by rubbing. Juvenile thrips acquire the virus from infected weed hosts. They disperse it after becoming winged adults which feed on host and target plants.
Weed hosts identified as potential virus carriers include spiny Amaranthus, wild lettuce, pasture buttercup, Solanum sp. and sowthistle.
“These are all abundant in our state at the time tomatoes are in the field,” says LSU AgCenter commercial vegetable expert Jimmy Boudreaux. Surveys show higher incidence of the virus in parishes that border rivers in the state. This is probably because the weeds grow best in wet or moderately wet locations.
“Spotted wilt is difficult to control,” Whitman says, adding, “Unfortunately there is not much that anyone can do once the plant becomes infected.” He notes it is heartbreaking that TSWV shows up after most of the work on the tomato crop, such as tying, pruning and staking, has been completed.
The pathologist advises to spray early for thrips, recognize symptoms and remove problem plants and weed hosts from around the fields. He notes that the LSU AgCenter has been working with TSWV for the last several years. The easiest way to prevent this virus is to grow TSWV-resistant varieties.
“It is unfortunate that most of the TSWV tomato varieties are not real good eating tomatoes,” Boudreaux says, explaining that they are commercial varieties, which produce a lot of large, very firm and crack-resistant fruit.
“Seed companies are making progress, however, on better quality TSWV-resistant tomato varieties,” Boudreaux says, identifying the top two resistant tomato varieties in AgCenter demonstration plots as BHN 640 and Amelia.
BHN 640 is a medium-early variety with good yields of medium to large fruit with good fruit color. Amelia is a medium-early variety that produces medium to large fruit with good fruit color.