A Genetic and Cultural "Package" for Management of Tomato Spotted Wilt Virus in Peanut

Diseases caused by thrips-vectored tospoviruses cause severe problems in many of the world's agroecosystems. Spotted wilt, caused by tomato spotted wilt tospovirus (TSWV), has become one of the most serious and complex disease problems in production of peanut (Arachis hypogaea L.) and other crops since 1985 in the southeastern U.S. (11). Spotted wilt is now common across the peanut growing areas of Georgia, Florida, and Alabama. In 1997, losses to spotted wilt in peanut were estimated at $40 million for Georgia alone.

TSWV and its vector TSWV present a perplexing challenge in epidemiology and disease control. Tobacco thrips, Frankliniella fusca Hinds (Sakimura) and western flower thrips, Frankliniella occidentalis (Pergande), are confirmed vectors of TSWV and both infest peanut plants in Georgia and the southeastern U.S. (15,16). Since the emergence of spotted wilt as an important problem in peanut production in the U.S., numerous studies have generally failed to show that chemical control of thrips affords a reduction in incidence of spotted wilt (15,16). An exception to these findings was the discovery that in-furrow application of phorate suppressed epidemics of the disease (4,15).

Unlike the situation with many of the fungal disease and insect problems that are controlled by fungicides or insecticides, there are no viricides for control of this disease in peanut. Although several factors have been shown to provide some suppression of spotted wilt in peanut, no single measure alone has been effective in field situations of heavy disease pressure.

A team approach in research and extension has been employed to improved management of spotted wilt in Georgia and Florida. This team has succeeded in identifying and combining some critical management inputs that collectively minimize the risk of losses to spotted wilt. The most important inputs employ genetic resistance and other cultural practices. Initial experiments to elucidate the effects of these inputs were done with only one or combinations of two factors. Results from intensive cooperative research have shown that the cumulative effects of multiple management practices can have a huge impact on epidemics of spotted wilt, whereas one input alone resulted in marginal suppression of the disease.

Cultivar Selection

The single most important factor in management of spotted wilt in the southeast was cultivar selection. Cultivars are now available and widely planted with moderate levels of field resistance to spotted wilt. Research in Texas during the mid-1980's found that the cultivar Southern Runner had a moderate level of field resistance to spotted wilt (2). Subsequent, epidemiological studies in Georgia corroborated these findings on Southern Runner (5,6,9). Intensive screening of cultivars and breeding lines has identified several sources of moderate resistance, and resulted in release of the runner-type cultivars, Georgia Browne (5), Georgia Green (9), UF MDR 98 (11), and UF 84x9B (now C-99R) (11) with levels of resistance similar to that of Southern Runner. Virugard, a runner-type cultivar from Agratech (Ashburn, GA), also has a level of resistance similar Southern Runner. None of the cultivars released to date has a high level of resistance, and may suffer significant damage during extremely intense epidemics. Typically, the incidence and severity of spotted wilt in these cultivars has been approximately half that observed in susceptible cultivars such as Florunner and Georgia Runner, which were popular before spotted wilt epidemics became severe.

During the 1998 growing season, over 75% of the peanut acreage in Georgia was planted to Georgia Green in response to the concern for losses to the spotted wilt disease. In 1999, as much as 95% of peanut acreage in Georgia was planted to this cultivar. This shift to a moderately resistant cultivar was the "cornerstone" of our disease management regime. No currently available cultivar has a high level of resistance to TSWV, and even moderately resistant cultivars can suffer severe damage in fields with heavy pressure from spotted wilt. Therefore, combining the use of moderately resistant cultivars with other disease-suppressive factors has been important for all fields in which spotted wilt is a threat.

Planting Date

Planting date has been reported to be an important factor in spotted wilt in Texas (14). Likewise, planting date studies in the southeastern U.S. have indicated that planting in the first 2 wk of May typically resulted in the lowest incidence of spotted wilt, whereas planting in early April or June resulted in much greater incidence of disease. Surveys of spotted wilt infestations in production fields in Georgia have corroborated these findings (3). However, the effects of planting dates have not been as consistent as cultivar effects. The trend toward greater infestations of spotted wilt in early planted peanuts compared to peanuts planted in early to mid-May has been consistent. Although the planting date "window" for minimizing spotted wilt incidence may vary as much as a week from year to year, planting in the first two weeks of May has usually given good disease suppression.

Explanations for the differing effects of planting date have been based on circumstantial evidence and remain quite speculative. A common explanation has been that thrips populations are often variable across planting dates. Over multiple-year investigations, greatest numbers of F. fusca occurred on April-planted peanuts, whereas peanuts planted in May had smaller populations. The population dynamics of thrips in non-crop plants or volunteer peanuts early in the season have been hypothesized as a reason for these effects since these plants may serve as reservoirs for TSWV. However, proof of this mechanism has not been reported.

Although manipulation of planting date offers a viable tool for suppressing spotted wilt, it does have limitations. Because of the size and number of fields to be planted, limitations on equipment and labor, and the uncertainty of weather, most growers cannot plant all of their acreage in the "optimum" planting window. However, in the 1998 growing season, as of 26 April, only 5% of Georgia's peanut crop had been planted, compared to the five-year average of 21% for that same date. As of 3 May 1998, 19% of the crop had been planted, compared to 40% of the crop planted by that date across the previous 5 years.

Plant Population

Infection of an individual peanut plant with tomato spotted wilt virus is of greater probability among sparse plant populations than among dense populations (13). Establishing higher plant populations does not appear to reduce the number of infections in a particular field, but likely reduces the percentage of plants that are infected. Field surveys in Georgia in 1992 indicated a substantial reduction in the percentage of plants infected with spotted wilt as plant density increased from <2 to 2-4 to >4 plants/ft. of row. Gorbet and Shokes (13) found corresponding increases in incidence of spotted wilt as plant populations decreased in Florida.

While some inconsistencies in plant population effects have been reported (3), the overall consistency of plant population effects and the magnitude of those effects have increased as spotted wilt epidemics in Georgia became more severe. The establishment of uniform stands of >4 plants/ft. of row has been recommended for minimizing losses to spotted wilt. The actual seeding rate required to achieve this population has been a function of seeding rate in conjunction with seed quality, soil moisture, soil temperature and planting depth.

Phorate Insecticide

Although TSWV is vectored by thrips, the use of insecticides to control thrips in general has been ineffective for suppressing spotted wilt. While providing good control of thrips damage to plants, insecticide applications have been ineffective in preventing plant feeding and inoculation by viruliferous thrips that have migrated from areas outside the field. Despite the overall disappointing results with insecticides, phorate has provided consistent, low-level suppression of TSWV. The mechanism of disease suppression has no direct link with thrips control, since phorate typically offers no better control than other insecticides. Phorate is phytotoxic, and often causes marginal chlorosis and necrosis on peanut leaves. This effect on young plants may induce a host defense response, or serve in some other way to inhibit virus replication or movement. The effect of this systemic insecticide may be worthy of investigation in the future as being a potential inducer of systemic acquired resistance. Regardless of the mode of action, in-furrow application of phorate at planting provides control of thrips and a low to moderate level of suppression of spotted wilt in peanut.

Row Pattern

The planting of twin rows spaced 7-10 in. apart at the same seeding rate per acre as single rows has become increasingly popular in Georgia. The twin row patterns often result in earlier plant coverage of the ground and less competition from weeds. Research to compare single and twin row plantings of irrigated peanut has shown a tendency for higher yields and improved grades with the twin row pattern (1). In addition, reductions in spotted wilt incidence have averaged 25-30% in twin rows compared to the single row pattern (1). The reason for this reduction in spotted wilt incidence is not fully understood, but may involve visual interference with the ability of migrating thrips to recognize host plants.

Changing from single to twin rows requires considerable effort and expense. Twin rows require adjustments in cultural practices such as cultivation, and the digger-inverter at harvest. Furthermore, cultivars that lack a prominent main stem and produce excessive vine growth may be extremely difficult to manage and harvest when planted in a twin row pattern. While growers are not encouraged to change to twin row production just for suppression of spotted wilt, this planting pattern does provide some suppression of the disease.

Tillage Systems

Minimum tillage and no-till systems have become very attractive as conservation measures and a means to reduce the cost of crop production. Interest in minimum tillage practices in peanut has increased dramatically in recent years across the southeastern U.S. Many of the benefits and disadvantages of minimum tillage in terms of disease management remain to be determined in peanut production. However, a growing database indicates that use of minimum tillage in peanut results in lower incidence of spotted wilt as compared to conventional tillage. The change to minimum tillage presents a myriad of new interactions for the host, vector and environment which probably account for a reduction in spotted wilt incidence. Lower thrips populations have been reported in no-till peanuts compared to conventional tillage peanuts (3), and less feeding injury by thrips has been reported for peanut produced with minimum tillage compared to conventional tillage (3). As with a twin-row pattern, the general change in architecture of the crop and environment due to the presence of stubble and/or debris from the previous crop may interfere with visual detection of host plants by migrating thrips.

As with row patterns, the decision to produce peanuts with a minimum tillage or no-till system should not be based solely on suppression of spotted wilt epidemics. However, these tillage practices can help to minimize incidence of spotted wilt where they are compatible with the grower's production system and economically feasible.

Spotted Wilt Risk Assessment Index

The adoption of genetic and cultural practices for management of spotted wilt of peanut has been enhanced greatly by the development and use of a Spotted Wilt Risk Assessment Index (4). The index has become an educational tool by which growers can assess the relative risk of spotted wilt in a particular field and identify the combination of disease-suppressive factors that best apply to their situation. Currently, the index has a maximum of 150 points based on cultivar selection, planting date, plant population, in-furrow insecticide, disease history, row pattern and tillage practices. The index is dynamic in that it has been improved annually since inception in 1996. The relative weights of the various factors are based on research findings in Georgia and may require adjustment for cultivars, cultural practices and environmental conditions before application in other regions. Each index has been systematically validated by factorial experiments that examined the factors in various combinations. The index has also been validated by on-farm surveys that assessed the correlation between predicted relative risk and the observed incidence of spotted wilt. For the 1999 season, the risk index can be calculated based on the protocols in Table 1. Major changes from the 1998 protocol were the addition of row pattern and tillage practices as factors. The risk assessment index has been very well received in Georgia, and has played a major role in the rapid adoption of integrated measures for management of spotted wilt of peanut. In 1998, estimated losses to tomato spotted wilt in peanut in Georgia were approximately one fourth as high as the $40 million dollar loss estimate for 1997. This was due largely to: 1) changing to the new moderately resistant cultivar Georgia Green; 2) delaying the planting date to the low risk period of early to mid May; 3) establishing higher plant populations; and 4) adopting phorate insecticide for suppression of the virus. The shift to planting over 75% of the peanut acreage to Georgia Green represented the greatest sudden change in cultivars since the 1970 introduction of the cultivar Florunner. This cultivar had been the industry standard, runner-type peanut for over 20 years. In addition, there has never been a case in Georgia when such changes in genetic and cultural practices were made simultaneously for any purpose, especially disease control.

Management in the Future

While the impact of the management package on spotted wilt in 1998 was encouraging, the disease continues to pose a serious threat to peanut production. Concerted interdisciplinary, multi-state efforts in research and extension must be continued to sustain progress in understanding the factors that contribute to epidemics of the disease and developing improved strategies for disease control. New cultivars with greater resistance to spotted wilt appear to have the most potential for improving disease management. Several breeding lines from the University of Georgia, University of Florida, USDA, and private industry have shown greater field resistance to the disease than Georgia Green. Across tests in two locations in 1997 and 1998, advanced breeding lines, such as GA 942007 (7,8) and UF 84x9B (10,12), have been equal to or better than Georgia Green in resistance to TSWV. UF 84x9B has just been approved for release as a cultivar but is yet to be named and only a limited supply of seed will be available in 1999. A few lines have been identified with even greater levels of resistance. Under severe disease pressure at two locations in 1997 and 1998, Florida breeding lines from crosses of F 86x43 and F 84x47 exhibited levels of disease that were significantly lower than those of Georgia Green (10,12). In 1998, several late-maturing breeding lines showed levels of spotted wilt that were less than half that in Georgia Green. These included the USDA line CC11-2-39 which across two locations had average final intensity ratings of 11.9% compared to 57.9% in Georgia Green and 85.6% for Georgia Runner (12). To date, this line appears to possess the highest level of field resistance to spotted wilt in peanut. For the first time, we also found medium maturity breeding lines with more resistance than Georgia Green (8). From tests conducted in 1998, the University of Florida breeding line F 90x7-3-5-1-b2-B was among the most resistant, medium maturity lines evaluated. It is not known at this time whether these lines are suitable for release as cultivars, but they do provide genetic material for further improvement of disease resistance. There has been no indication that reduced incidence and severity of spotted wilt in advanced breeding lines or cultivars has been due to lack of preference by thrips or reduced suitability for thrips reproduction (5,6,9,11).

Progress in developing peanut lines with resistance to spotted wilt through a transgenic approach has lagged behind that in other crops. Transformation of peanut has been more difficult than transformation of other crop plants. In a preliminary field evaluation, spanish-type peanut lines transformed to encode for the TSWV coat protein gene had higher incidence of spotted wilt than the non-transformed standard. Nonetheless, as progress is made in adapting this technology to peanut, transgenic resistance has potential to aid greatly in development of improved cultivars with disease resistance. A combined effort of biotechnology and traditional breeding may further enhance opportunities for development of disease resistant cultivars that meet the market requirements for peanut in the U.S.

At this time, it seems unlikely that any single control measures can provide adequate control of spotted wilt in peanut. Consequently, an integrated "package approach" of moderately resistant cultivars, and cultural practices offers the most practical approach for management of the disease in the southeastern U.S.

Literature Cited