In recent days we have been asked about numerous tomato plants that were looking sick after being transplanted, with leaves turning yellow, and plants very stunted. In some cases the leaves had some purplish-bronze tinge to them. The soil did not seem to be a factor, as some of the adjacent plants were thriving, with lush dark-green foliage. The suspected agent was a virus, as the symptoms did not fit any of the common fungal or bacterial diseases common in South Dakota, and samples were sent to the SDSU Plant Diagnostic Lab. They confirmed that the cause was indeed a virus, Tomato Spotted Wilt Virus (TSWV). This virus can be a problem in both field and greenhouse situations, and most frequently its effects in South Dakota are observed on tomatoes and peppers.
Leaves & Foliage: Symptoms on leaves can be variable. On approaching the plant, one may notice that the newer foliage has a bronze or purplish tinge to it (Fig. 1). Sometimes the plants are stunted, or look wilted. Leaf symptoms may first appear as small, light brown flecks (Fig 2). Closely examine a number of leaves. Sometimes a portion of a leaf (either base or tip) will look normal, while other portions dying back or exhibiting one or more green spots bordered on the outside by brown tissue (Fig. 3, Fig. 4). Spots with normal-looking centers are almost always a sign of a virus. In contrast, fungal and bacterial diseases start from the center and spread outward, so there is no green center with those diseases.
Fruit: The fruit is usually infected also and may exhibit raised circular areas, sometimes ringed with small brown spots (Fig 5). As the fruit ripens, the distinctive spots can quickly become very obvious against the red background. Again, if the spot on the ripe fruit has a red center or red and yellow target pattern, one may be quite sure the cause is a virus. (Fig. 6)
|Fig. 1. Bronzing or purplish cast to leaf may be the first symptom noticed.||Fig. 2. Small flecks may be another early leaf symptom.|
|Fig. 3. Distinctive spots indicate a virus.||Fig. 4. Closeup of Tomato Spotted Wilt Virus leaf spots.|
|Fig. 5. TSWV symptoms may become obvious even on the unripe fruit.||Fig. 6. Distinctive yellow spots indicate a virus.|
Thrips, very tiny, cigar-shaped insects which move from plant to plant are responsible for transmitting the TSWV. Thrips thrive in warm dry springs, thus during hot dry years, we tend to see more thrips and more plants infected with TSWV. Symptoms may appear on plants within a few weeks after infection. Immature thrips feed on infected tissue, and the virus multiplies in their midgut and other organs, and can be subsequently transmitted to whatever the thrips feed on for the rest of their lives. Thus, management of TSWV is dependent on controlling the thrips, or on using TSWV resistant varieties.
- Suspected TSWV infections should be confirmed. As a first step, photos clearly showing infections can be submitted to SDSU Extension specialists. If symptoms are not clear, samples should be submitted for testing. The best material for testing is that which has active symptoms, such as ringspots, and it should be shipped so that tissue remains as fresh as possible. The SDSU Plant Diagnostic Clinic can do this test and can be contacted at 605.688.5545 or by email. See the clinic’s submission guidelines for additional information.
- Whenever available, use plant varieties with resistance to TSWV as a part of any control program. A few of the tomato varieties with TSWV resistance include: Amelia, Bella Rosa, Celebrity, Fletcher, Mountain Glory, Primo Red, Red Defender, Skyway, Tribute, Tribeca, and others. Resistant bell peppers include Declaration, Magico, Heritage, Plato, Procraft, Stiletto, and others.
- Take steps to reduce thrips populations. If insecticides are used, select ones such as those containing the active ingredient Spinosad, which is less likely to kill natural enemies such as minute pirate bugs. Light-colored or reflective mulch may reduce thrips infestations.
- Materials that induce systemic resistance in plants may help reduce infections, but need to be applied prior to feeding by the thrips. These include sprays that contain salicylic acid, harpin proteins, or other substances.
- Remove all infected plant materials. Symptoms of TSWV typically take 10-14 days to develop after the thrips transfer the virus. Remove plants that touch or surround the plants that show symptoms, as the virus may have already been spread by thrips moving from the infected plants and the symptoms have not yet had time to become obvious.
- Many weed species can act as alternate hosts for TSWV and thrips. Thus, weed control is an important part of managing TSWV, especially in and around greenhouses or high tunnels.
- Many herbaceous ornamentals are also susceptible to TSWV infection as well. If these are growing nearby tomatoes or peppers, they may become infected or even act as an initial source of the disease.
For additional information on TSWV, view this helpful Tomato Spotted Wilt Disease publication courtesy of U.C. Davis and U.C. Cooperative Extension.