Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
I recently visited a grower that grows both tomatoes and bedding plants. The plants are grown in a high tunnel-like setting, i.e., with heat. The grower was having problems in his tomato plants, but not in his bedding plants. The tomato plants looked like they had tomato spotted wilt virus (TSWV). The symptoms were found on most of his tomato plants, which would be unusual, as most of the time only an occassional plant here and there would be infected with the virus. The grower thought that is what he had as did an alert County Educator—and they were right. The grower unfortunaetly had a perfect storm in his operations that produced high percentages of TSWV infection in his tomato crop, but not his bedding plants.
Tomato spotted wilt virus is an obligate parasite; it must have a living host and must be moved from one plant to another by thrips or through cuttings or possibly seed. TSWV is transmitted most efficiently by Western flower thrips (WFT) (Frankliniella occidentalis), and less so by Onion thrips (Thrips tabaci), Tobacco thrips (Frankliniella fusca) and a few other thrips species. It is not transmitted by Eastern flower thrips (Frankliniella tritici).
WFT completes its life cycle in about 10-18 days. Eggs are laid in the leaf or tomato fruit. When WFT oviposit into tomato fruit they often cause a deeper dimple (black arrows Fig. 1) than other thrips species and very often the dimple is surrounded by a halo of white tomato tissue (white arrow Fig 1). This is how I could tell the grower had WFT present at one time (when I visited I could find no live thrips) at fairly high levels by the ovipositioning marks on his tomatoes. Larvae hatch in about three days and immediately begin to feed and in so doing pick up the virus. After four days, they pupate in the soil, and in a little over three days, the pupae become adults. Only immature thrips can acquire the virus, which they can acquire within 15 minutes of feeding, but adults are just about the only stage able to transmit the virus. Adults can transmit the virus for weeks.
TSWV infected leaves may show small, dark-brown spots (Fig. 2) or streaks on stems and leaf petioles (we found one prickly lettuce weed with such a symptom). Growing tips are usually affected with systemic necrosis and potentially stunted growth. Tomato fruit will have mottled, light green or yellow rings usually with raised centers (Fig 3).
Weed hosts function as important virus reservoirs for TSWV and can survive in and around greenhouses or even high tunnels through the winter. Some of these weeds include prickly lettuce, chickweed, (Fig. 4) spiny amaranth, lambquarters, black nightshade, shepherd’s purse, galinsoga and burdock. This grower unfortunately had a good crop of prickly lettuce at one end of his high tunnel.
The grower had been able to control his thrips populations with spinosad, but western flower thrips are notorious for developing resistance and sure enough have developed resistance to this insecticide in many greenhouses. So populations of WFT increased and with the weeds that were around and in the high tunnel some of which tested positive for TSWV, but negative for INSV, it was a perfect scenario for an outbreak of TSWV. I should note here that we tested for both INSV and TSWV on the tomatoes, weeds and impatiens. Only TSWV was found in the tomato and the weeds. No INSV was found in any sample. Although both viruses are transmitted by the same thrips species these viruses tend to infect either bedding plants (INSV) or tomato/pepper plants (TSWV). The grower threw out all his infected tomato plants and is in the process of killing his weeds in and around his high tunnel. He was able to get control of his thrips in his bedding plants using combinations of pylon and pyrethroids. One variety of tomato the grower was growing that did not show any symptoms of TSWV, even though it was right next to the other infected varieties was Mountain Glory.
Fig 1 Tomato fruit with WFT ovipositor marks
Fig. 2 Tomato leaves with TSWV symptoms and positive immunostrip (two black arrows; Agdia, Inc)
Fig. 3 Tomato fruit with TSWV symptoms
Fig. 4 Two common weed hosts of TSWV; prickly lettuce and chickweed