Development of a method of risk assessment to facilitate integrated management of spotted wilt of peanut

Tomato spotted wilt virus (TSWV) is one of a growing list of tospoviruses adversely affecting agriculture worldwide (36,38). Spotted wilt, the disease caused by TSWV, was first observed in peanut (Arachis hypogaea L.) growing regions of the southeastern United States in 1986 (41), and its importance steadily increased in Georgia through 1997 to the extent that annual losses exceeded $40 million. Since 1997, annual losses have sharply declined to approximately $10 million in 2000 (60) and $3.3 million in 2003 (15). The only known means of virus transmission is via vectors belonging to a few species of thrips (36,50). Only first instar larvae of Frankliniella occidentalis (Pergande), one vector species, can acquire the virus from an infected plant (57). After acquisition, the virus replicates in the vector and the viruliferous thrips is capable of transmission for the duration of its life (50,56,59). In Georgia, the primary vectors are tobacco thrips, F. fusca (Hinds), and western flower thrips, F. occidentalis (53). Most spotted wilt in peanut is thought to be the result of primary transmission, but some secondary transmission probably occurs as well (22), mostly by F. fusca, which readily reproduces on peanut (53,54). Peanut plants may initiate symptom expression at any stage of plant development (6), but severe stunting (Fig. 1) typically occurs on younger plants, whereas the initiation of symptoms on older plants typically results in generalized yellowing and/or localized leaf mottling (Fig. 2). Linear regression indicates that both number of seed and seed yield per infected plant increase with time from planting until symptom expression (27). Incidence and severity of tomato spotted wilt of peanut have been extremely variable in Georgia peanut fields (11). Research programs conducted since 1990 have identified several contributing factors affecting disease severity and have provided estimates of their relative importance. Some factors are more important than others, but no single factor can be manipulated to control the disease (26). Using available research data, a spotted wilt risk index was developed as a means of helping peanut growers to assess risk levels associated with specific combinations of production practices and thereby to avoid high-risk situations. Pertinent production practices were assigned point values that were weighted relative to their influence on final spotted wilt severity. As new research data became available, the index was modified and validated with extensive small plot trials and on-farm surveys. This article describes how the index was developed and modified and reports the results of on-farm surveys used to validate the index for peanut grown under a wide array of production practices. Factors Affecting Severity of Tomato Spotted Wilt in Peanut Peanut cultivar. Soon after spotted wilt became established in Georgia, the cultivar ‘Southern Runner’ was noted to have about half the incidence of that seen on the more widely grown ‘Florunner’ (5,28). This serendipitous discovery was followed by the discovery of a series of other varieties, some of which were produced from crosses with ‘Southern Runner’, having similar levels of resistance. ‘Georgia Browne’ (25), ‘Georgia Green’ (29), ‘Florida MDR 98’ (32), ‘ViruGard’ (31), ‘C99R’ (30,40,58), and several advanced breeding lines (29,30) have been shown to exhibit lower incidence of spotted wilt than ‘Florunner’ and other equally susceptible cultivars. The mechanism of resistance exhibited by these cultivars 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 (25,28–31). Planting date. 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 (47), 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 Corresponding author: Steve L. Brown E-mail: bugbrown@arches.uga.edu