Tomato Spotted Wilt Virus: TSWV in Vegetable Crops: Tospoviruses

Tospoviruses In Solanaceae and Other Crops in The Coastal Plain of Georgia

Epidemiology

Ron Gitaitis

Incidence of TSWV in Georgia has increased on pepper and tomato since it was first observed on tomatoes in the state during the late 1980s. Depending on the season, TSWV can be responsible for up to 100 percent loss in individual pepper and tomato fields and was a contributing factor to the decline of the field-grown transplant industry. Infected transplants grown in southern Georgia were implicated in TSWV losses in the northern United States and Canada.

Symptoms on tomato vary considerably with lesions developing on stems, reddish-brown ring spots on leaves (Fig. T1a) that can coalesce and give the appearance of bronzing (Fig. T1b), or larger interveinal areas of necrosis and spotting on leaves (Fig. T2). Depending on the timing and degree of infection, plants can be severely stunted (Fig. T3) or leaves turn a purple hue with cupping and plants show severe wilt stress and die prematurely (Fig. T4). Tomato fruit generally display a ring spot symptom (Fig. T5A) with uneven maturation and development (Fig. T5B). In pepper, symptoms again range from a coalescing of ring spots (Fig. P1) to interveinal necrosis. Frequently, pepper leaves display a fan or V-shaped area of chlorosis at the base of the leaf (Fig. P2). As in tomato, systemic symptoms include severe stunting, wilt and eventual death of the plant (Fig. P3). Fruit symptoms in pepper also can vary from irregular chlorotic blotching to necrotic pits (Fig. P4).

The epidemiology of TSWV in tomato and peppers is not fully understood, but it appears that plant position in the field is correlated with the risk of becoming infected with the virus. It has been documented that the disease distribution frequently is in a pattern of a gradient, with a higher degree of disease incidence along the edge. The presence of a disease gradient may imply that the source of the inoculum is in close proximity to the edge of the field containing the highest incidence of diseased plants. The lack of a gradient generally means that the source of inoculum is of some great distance from the field. Alternatively, the presence of disease gradients may be a result of thrips vectors “preferring” to alight on plants at the edge of the field. It is not known if thrips actively select plants along the edge because of an attraction to the contrasting colors of plants and soil or if it is because of the movement of air currents. Whatever the reason, more disease occurring along the edge because of a greater likelihood of thrips landing on those plants was supported by data in which an “edge effect” was observed in the middle of tomato fields that had bare ground alleys separating blocks of tomatoes. There were higher levels of disease along each new edge despite being in the middle of the field.

The disease gradients from the edge of tomato and pepper fields were observed not to flatten over time. This generally is interpreted to mean that there is little secondary spread within the field and that new infections are a result of either a continual influx of adult thrips carrying primary inoculum or of a staggered incubation period with some plants taking longer to show symptoms if all plants were inoculated 20 Tospoviruses in Solanaceae and Other Crops in the Coastal Plain of Georgia at one time. Additional statistical analysis using methods such as ordinary runs analysis and doublet analysis have confirmed that in pepper and tomato there is little to no spread from plant to plant within a field. Lack of secondary spread and lack of mechanical transmission by farm equipment also was observed in beds of tomato transplants that were clipped with a rotary mower on several occasions over the course of their development. Clipping was a common practice in field-grown transplants and was necessary to promote uniform growth and harvest, to maintain plant size for packaging and shipment, to harden plants for greater survival after transplanting and to de-blossom plants if they developed too fast or had to be held extended periods of time because of inclement weather at their final destination. Although clipping was implicated in the transmission of other diseases, there was no evidence of mechanical transmission of TSWV by clipping of tomato transplants.

Onion is a relatively new member to the list of hosts being damaged by tospoviruses in Georgia. In 2003, TSWV and Iris yellow spot virus (IYSV) were found in onion seedbeds. In other areas of the world only IYSV has been reported on onion; however, in Georgia approximately 50 percent of the infections in onion are caused by TSWV. As with other crops, onions display a range of symptoms when infected with either of these viruses. In some varieties, such as the Japanese bunching types, there are definitive ring spot symptoms and lesions with a “green island” (Fig. O1). In the more traditional Vidalia Sweet onion varieties, symptoms include irregular whitish to gray spots (Fig. O2), sunken bleached areas (Fig. O3), gray, cracked, leathery lesions (Fig. O4), tip burn, leaf dieback and an overall appearance of stress and poor growth. Symptoms on bulbs are rare but can occur (Fig. O5). There are a lot of uncertainties about the long-term impact of IYSV and why TSWV has only recently begun to be a problem in onion even though it has been affecting other crops such as peanut, pepper, tobacco and tomato for close to 20 years. Nonetheless, losses in onion have occurred to both TSWV and IYSV annually since 2003. Although IYSV was first observed in 2003 and thought to be introduced into Georgia, it has since been found in several different weed species more than 100 miles from the Vidalia-onion growing region.

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