Tomato spotted wilt virus (TSWV) is causing losses to many commercial and home tomato growers. It appears that we will continue to have a lot of problems most years unless resistant cultivars are developed and adopted. Symptoms of the disease typically include 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 before infection, they may be distorted or develop blotchy orange ring spots as they ripen.
Peppers, cucurbits and certain ornamentals can also be infected by TSWV. The virus is transmitted by insects called thrips. It is not easily mechanically transmitted by rubbing. Juvenile thrips acquire the virus from infected weed hosts. They disperse it after becoming winged adults that feed on host tomato and other target plants.
Weed hosts identified as potential virus carriers include spiny Amaranthus, wild lettuce (Lactuca sp.), pasture buttercup (Ranunculus sp.), Solanum sp., and sowthistle (Sonchus sp.). These are all abundant in our state when tomatoes are in the field.
Surveys show higher incidence of the virus in cool spring in parishes that border rivers. Dr. Jimmy Boudreaux and I have recently seen a very high incidence of TSWV in the Lafourche and Terrebonne area. This is probably because the weeds grow best in wet or moderately wet locations such as bayous and other waterways.
Spotted wilt is difficult to control. Unfortunately there is nothing to do but pull the plant once the plant becomes infected. It is also heartbreaking that TSWV shows up after most of the work on the tomato crop, such as tying, pruning and staking, has been completed. You should spray early to control for thrip insects. Recognize the symptoms and remove problem plants and weed hosts from around the fields.
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 cultivars. It is unfortunate that most of the TSWV tomato cultivars are not very tasty tomatoes. They are commercial cultivars that produce a lot of large, very firm and crack-resistant fruit; however, the seed companies are making progress on better quality TSWV-resistant tomato cultivars.
The top two resistant ones in LSU AgCenter demonstration plots were BHN 640 and Amelia. BHN 640 was a medium early cultivar with good yields of medium to large fruit with good fruit color. Growers commented that the fruit was smoother and had better flesh color than BHN 444.
Amelia was seen at only two locations. It is a medium early cultivar that produces medium to large fruit with good color. Growers commented that the taste of Amelia was better than BHN 444. Another new resistant cultivar entry in 2005 is Top Gun; it is listed as having some heat tolerance and is being evaluated.
Question: I have sprayed with Malathion as per label direction and still have thrips in my vegetable garden. Why am I not achieving positive results?
Answer: According to LSU AgCenter entomologist, Dr. Dale Pollet, water pH must be adjusted to 5.5 to 6.0 for insecticides to have optimum knock down and control. Most treated water has a pH of 7.0 or higher and will neutralize the effectiveness unless the pH is buffered. If you can catch rainwater and use when spraying insecticides, you will get better results and have to spray less often.