Tomato Spotted Wilt Virus: TSWV in Vegetable Crops: Tospoviruses

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

Management of TSWV

Thrips Vector in Peanut
Steve Brown

The only known means of virus transmission is via vectors belonging to a few species of thrips vectors (German et al., 1992., Peters et al.,1996). Only first instar larvae of Frankliniella occidentallis (Pergande), one vector species, can acquire the virus from an infected plant (van de Wetering et al.,1996). After acquisition, the virus replicates in the vector and the viruliferous thrips is capable of transmission for the duration of its life (Peters et al., 1996; Ullman et al., 1993, Wijkamp et al., 1993). In Georgia, the primary vectors are tobacco thrips, F. fusca (Hinds), and western flower thrips, F. occidentallis (Ullman et al.,1993). Most spotted wilt in peanut is thought to be the result of primary transmission, but some secondary transmission probably occurs as well (Camann et al., 1995), mostly by F. fusca which readily reproduces on peanut (Todd et al., 1994). Peanut cultivars exhibit a wide range of susceptibility to spotted wilt. The mechanism of resistance is unknown, but since thrips populations on resistant cultivars do not appear to be significantly lower than those on susceptible cultivars, differences in cultivar susceptibility are not thought to be due to differential preference by vectors (Culbreath et al., 1994; Culbreath et al.,1992; Culbreath et al.,1996; Culbreath et al.,1999; Culbreath et al., 2000).

Prior to severe outbreaks of spotted wilt in Georgia, planting date was found to influence the incidence of the disease on peanuts grown in southern Texas (Mitchell et al.,1991), where peanuts planted early and late in the normal planting season tended to have more spotted wilt than peanuts planted in the middle of the planting season, and those planted within a recommended "window" expressed less severe symptoms. Although actual planting dates are slightly different, a similar trend was found in Georgia. Optimum planting dates and the magnitude of the planting date effect vary slightly from year to year, but in general, avoiding early and late planting reduces incidence and severity of spotted wilt in Georgia. Again, the mechanism of the planting date effect is not totally understood, but mid-season planting dates may avoid thrips population peaks (Todd et al.,1996). Although primary infection may occur throughout the growing season, young peanut tissue has been shown to be more susceptible to infection by peanut bud necrosis virus (a tospovirus closely related to TSWV) than more mature tissue (Buiel and Parleviet, 1996). Since planting dates that avoid synchronization of young peanut plants with peak thrips populations appear to significantly reduce TSWV infection levels, plant age may affect peanut susceptibility to TSWV in a similar manner.

An association between low plant populations and high levels of spotted wilt was noted soon after the disease began to impact peanut production in Georgia, and more recently, research has confirmed this observation (Brenneman and Walcott, 2001; Gorbet and Shoke,1994). Brenneman and Walcott (2001) found that 92 percent of the effect of plant population on yield was due to its indirect effect on spotted wilt. They characterized that effect as yield (lbs./A) = 3728-31.5 (TSWV severity) + 176.4 (stand- 2.9 plants/m). Individual peanut plants may have higher numbers of thrips feeding on them in low plant populations than in high plant populations, and therefore may have a higher probability of infection. In many cases, the actual number of infected plants per hectare may be nearly the same in low and high plant populations, but higher plant populations result in more uninfected plants per hectare which help compensate for yield losses on infected plants. Field survey data has also indicated that as populations drop below 13 plants per meter of row, spotted wilt severity progressively increases (Brown et al., unpublished data).

In past reports, the use of insecticides to control thrips vectors has been mostly ineffective in suppressing spotted wilt (Funderburk et al., 1990; Todd et al., 1996). More recent reports in tomato and tobacco suggest that certain intensive early season treatments might be effective (Riley and Pappu, 2004; McPherson et al., 2005). Lowering vector populations with insecticides probably reduces secondary spread, but most infection is thought to be the result of primary infection from overwintering and immigrating thrips (Todd et al., 1997). Despite the overall disappointing results with insecticides in peanut, in-furrow applications of phorate granules have provided consistent, low-level suppression of spotted wilt (Todd et al., 1996). The mechanism of this suppression is not known, but the level of thrips control obtained with phorate is not greater than that obtained with other insecticides. Gallo-Meagher et al. (2001) found oxidative stress in phorate-treated peanuts consistent with systemic acquired resistance mechanisms identified in other plant/pathogen associations (Friedrich et al., 1993). Their study (Gallo-Meagher et al., 2001) identified 22 genes that were turned on and 24 genes that were turned off in phorate-treated peanut plants compared to non-treated plants.

Eighteen to 25-cm twin row spacing (utilizing the same seeding rate per hectare as single row spacing), has become increasingly popular in Georgia. Research has shown a strong tendency for twin row patterns to have significantly higher yields, a one to two point increase in grade (percentage of total in-shell weight attributed to sound, mature kernels), significantly reduced spotted wilt severity and significantly increased net returns per hectare (Baldwin et al., 1999). The reason for reduced spotted wilt is not fully understood, but more rapid ground coverage in twin row patterns may affect the ability of thrips to locate a seedling host.

Peanut growers use a variety of tillage methods, each with its own merits and disadvantages for a given situation. Strip tillage (tillage of a narrow band for the seed furrow, leaving the remainder of the land undisturbed) has some distinct advantages including reduced soil erosion and reduced time and labor required for planting, but in some situations, yields have been disappointing. Previous studies have shown that peanuts grown in reduced tillage systems have less thrips damage (Campbell et al., 1985, Minton et al. 1991). Our on-farm observations during the early 1990s indicated a reduction in spotted wilt as well, and these observations have since been confirmed in replicated research plots (Baldwin et al., 2001; Johnson et al., 2001; Monfort et al., 2004). Wheat straw applied to the soil surface prior to peanut emergence at the rate of 1217, 2437, and 4876 kg/ha to simulate different levels of crop residue resulted in 16, 34 and 51 percent reductions in thrips damage, respectively, and 21, 49 and 65 percent reductions in spotted wilt severity, respectively, compared to that of peanut grown on bare ground (Brown, unpublished data). The cause of this effect is unknown, but ground cover may interfere with thrips ability to visually locate host plants. Slight reductions in spotted wilt do not always justify a change in tillage methods, however, tillage must be considered as a production practice that contributes to the overall variation in spotted wilt severity.

Figure 12. TSWV symptoms on peanut leaves

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