J. W. Todd

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

Field Response of New Peanut Cultivar UF 91108 to Tomato Spotted Wilt Virus

40 million. Since 1997, annual losses have sharply declined to approximately

Disease progress of tomato spotted wilt virus in selected peanut cultivars and advanced breeding lines

10 million in 2000 (60) and

Molecular Diagnosis of Tomato Spotted Wilt Tospovirus Infection of Peanut and Other Field and Greenhouse Crops

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

High Levels of Field Resistance to Tomato spotted wilt virus in Peanut Breeding Lines Derived from hypogaea and hirsuta Botanical Varieties

Epidemics of spotted wilt, caused by tomato spotted wilt tospovirus (TSWV), were monitored in field plots of the new runner-type peanut (Arachis hypogaea) cv. UF 91108, in advanced breeding line F 84 × 9B-4-2-1-1-2-b2-B, in runner-type peanut cvs. Southern Runner and Florunner, and in Virginia-type cv. NC-V11 at two locations in 1994 and 1995. Epidemics of spotted wilt were suppressed in UF 91108 compared to the standard runner-type cv. Florunner. Final disease incidence, standardized area under the disease progress curve values, and final disease intensity ratings were lower in UF 91108 than in Florunner and were similar to those in the moderately resistant cv. Southern Runner. Results indicate that new cv. UF 91108 represents a new potential tool for management of spotted wilt in peanut production areas of the southeastern United States. UF 91108 is the first peanut cultivar in which an oil composition of approximately 65% oleic acid is combined with a moderate level of field resistance to TSWV. Epidemics of spotted wilt also were suppressed in breeding line F 84 × 9B-4-2-1-1-2-b2-B. Across the four tests, the effects of NC-V11 on epidemic development, final incidence, and spotted wilt intensity ratings were not consistent relative to the other genotypes. Use of final disease intensity ratings provided separation of the genotypes similar to use of final incidence of spotted wilt. Assessment values by these two methods were highly correlated. In three of four experiments, final disease intensity ratings were more closely correlated with pod yield than was final incidence. The new intensity rating method described in this paper requires much less time and effort than determining disease incidence and may be a practical alternative to individual plant assessment for characterization of genotype responses to TSWV.

Differential Response of Selected Peanut (Arachis hypogaea) Genotypes to Mechanical Inoculation by Tomato spotted wilt virus

Epidemics of spotted wilt, caused by tomato spotted wilt tospovirus, were monitored in field plots of advanced breeding lines of peanut (Arachis hypogaea) GA T-2846, F 84X9B-1-1-1-b3-B, and F 84X1A-7-2-1-1-b2-B, and in runner-type peanut cultivars Georgia Browne, Southern Runner, and Florunner in one location in 1993 and in three locations in 1994. Across all tests, final incidence of spotted wilt and standardized areas under the disease progress curves were lower in the three breeding lines, Georgia Browne, and Southern Runner compared to standard runner-type cultivar Florunner. With some exceptions, numbers of tobacco thrips (Frankliniella fusca), western flower thrips (F. occidentalis), and larvae of Frankliniella spp. were similar for all peanut cultivars and breeding lines. There was no evidence that differences in disease progress among the six entries were due to differences in preference by thrips or to suitability for thrips reproduction. Results indicate that all three advanced breeding lines evaluated in this study represent potential tools for management of spotted wilt in peanut production areas of the southeastern United States.

Effect of new peanut cultivar Georgia Browne on epidemics of spotted wilt

Tomato spotted wilt tospovirus (TSWV) is a major disease constraint to peanut, tomato, pepper, and tobacco production in Georgia. Rapid molecular diagnosis of TSWV infection in peanut and its molecular studies were severely hampered by the lack of practical and rapid procedures for the extraction and amplification of the genomic nucleic acid. To circumvent this technical constraint, we adapted an immunocapture-procedure (ICP) for enriching the peanut tissue extracts for TSWV, and combined the ICP with a single-buffer, one-tube reverse transcription polymerase chain reaction (RT-PCR) to achieve rapid and reliable amplification of TSWV sequences from peanut. Both leaf and root tissue of peanut provided PCR-quality templates. Immunocapture, RT, and PCR were done in the same tube, allowing higher throughput. The technique was applicable to a wide range of TSWV-susceptible crops such as tomato, pepper, tobacco, gloxinia, and impatiens. Primers derived from the nucleocapsid protein gene as well as from the large RNA of the viral genome were able to amplify the target sequences in a highly specific and reproducible manner. This approach facilitated rapid molecular typing of natural populations of TSWV in Georgia.

First Report on the Multiplication of Tomato Spotted Wilt Tospovirus in Tobacco Thrips, Frankliniella fusca

Tomato spotted wilt, caused by Tomato spotted wilt tospovirus (TSWV) is a major problem in peanut (Arachis hypogaea L.) producing areas of the southeastern U.S. The integrated program used to manage spotted wilt relies heavily on cultivars with field resistance to TSWV, and finding new sources and greater levels of resistance to TSWV is highly desirable. Field tests were conducted in 2003 and 2004 in Marianna, FL and Tifton, GA to compare three peanut breeding lines, F NC94022-1-2-1-1-b3-B, C 11-2-39, and C 11-186 to that of standard moderately resistant cultivar Georgia Green for field response to TSWV. F NC94022-1-2-1-1-b3-B was of particular interest because it was developed from a cross between lines of A. hypogaea subsp. hypogaea var. hirsuta Kohler and A. hypogaea subsp. hypogaea var. hypogaea. In all tests, final spotted wilt ratings for breeding lines F NC94022-1-2-1-1-b3-B, C 11-2-39, and C 11-186 were lower and pod yields were higher than for Georgia Green. In three tests, final spotted wilt intensity ratings did not differ among F NC94022-1-2-1-1-b3-B, C 11-2-39, and C 11-186. At Marianna in 2004, spotted wilt intensity ratings were lower and pod yields were higher in F NC94022-1-2-1-1-b3-B than in any other entry. The high level of field resistance to TSWV in F NC94022-1-2-1-1-b3-B is presumably derived from its hirsuta type parent, PI 576638.

First Report of Natural Infection of Peanut (Groundnut) by Impatiens Necrotic Spot Tospovirus (Family Bunyaviridae)

Screening of peanut germ plasm for resistance to Tomato spotted wilt virus (TSWV) has been largely inefficient due to the lack of a screening technique based on mechanical transmission of the virus under controlled environmental conditions. We have studied the reaction of three peanut cultivars (Georgia Green, Georgia Runner, C-99R) and one breeding line (C11-2-39) using a highly efficient mechanical inoculation procedure. The disease response was studied at two temperature regimes, 25 to 30°C (low temperature) and 30 to 37°C (high temperature). Based on percent transmission, symptomatology, distribution of TSWV, and relative levels of TSWV nucleocapsid (N) protein, Georgia Runner and Georgia Green were found to be susceptible, whereas C-99R and C11-2-39 were resistant. Of the four genotypes tested, C11-2-39 had the highest level of resistance to TSWV. The results correlated with the field performance of the genotypes except in the case of Georgia Green, which could not be distinguished from TSWV-susceptible Georgia Runner. Exposure of the inoculated plants to higher temperature (30 to 37°C) resulted in a better resistant response as reflected by reduced systemic infection, localized symptom expression, restricted viral movement, and reduced levels of TSWV antigen. To our knowledge, this is the first report of differential response of peanut genotypes to TSWV using mechanical inoculation. The four peanut genotypes should be useful as reference standards for the initial screening and identification of sources of TSWV resistance in peanut germ plasm.

Field Evaluations of Leaf Spot Resistance and Yield in Peanut Genotypes in the United States and Bolivia

Epidemics of spotted wilt, caused by tomato spotted wilt tospovirus, were monitored in plantings of groundnut cultivars Georgia Browne, Southern Runner and Florunner in 5 tests during 1990-93 at Attapulgus, Georgia, USA, and in plantings of the 3 cultivars plus Marc I and AT-127 in 3 tests in Colquitt County, Georgia, in 1993. Final incidence of spotted wilt and area under the disease progress curve values for Georgia Browne were similar to those for Southern Runner but lower than those for Florunner, Marc I and At-127. Pod yields for Georgia Browne were higher than those for Florunner in all tests at both locations and higher than those for the 4 other cultivars in Colquitt County in 1993. Pod yields were similar for Georgia Browne and Southern Runner in 5 tests in Attapulgus. Numbers of tobacco thrips (Frankliniella fusca), western flower thrips (F. occidentalis) or larvae of undifferentiated Frankliniella spp. that colonized the cultivars were similar in most cases. Differences among the cultivars in incidence of spotted wilt could not be attributed to differences in thrips population.