Leonard P. Gianessi Cressida S. Silvers Sujatha Sankula Janet E. Carpenter

Transcription

1 Plant Biotechnology: Current and Potential Impact For Improving Pest Management In U.S. Agriculture An Analysis of 40 Case Studies June 2002 Viral Resistant Peanut Leonard P. Gianessi Cressida S. Silvers Sujatha Sankula Janet E. Carpenter National Center for Food and Agricultural Policy 1616 P Street, NW Washington, DC Phone: (202) Fax: (202) ncfap@ncfap.org Website: Financial Support for this study was provided by the Rockefeller Foundation, Monsanto, The Biotechnology Industry Organization, The Council for Biotechnology Information, The Grocery Manufacturers of America and CropLife America.

2 3. PEANUT Viral Resistant Production The United States produced approximately 3.3 billion pounds of peanuts in 2000, 40% of which was produced in Georgia [1]. Georgia is the number one state in peanut acreage, production and value. The state s 2000 crop was planted on 540,000 acres with a value of $355 million and a volume of 1.3 billion pounds per year [2]. Tomato Spotted Wilt Virus Tomato spotted wilt virus (TSWV) has been a disease in Georgia peanuts since It has a host range that includes more than 500 plant species, including many crops, and it is present in most Georgia peanut fields [3]. Georgia is at greater risk than other peanut states because several TSWV susceptible crops are grown in close proximity to peanuts (tobacco, tomatoes and other vegetables) [24]. Incidence levels vary with region and year. In severe infestations, incidence levels have exceeded 90% [22]. By 1989, peanut plants showing symptoms of spotted wilt were found in almost every peanut field surveyed in Georgia, and by 1993, incidence of symptomatic plants approached 60% in some fields [20]. Symptoms of TSWV in peanuts vary. They include white ring spots developing on leaves, and stunting of the plant [4]. Symptoms such as stunting can be more severe if infection occurs early in the growing season [5]. Diseased plants can be severely weakened or killed. A field that is 50% infected will lose 1,000 to 2,000 pounds of yield per acre [23]. Researchers do not yet understand fully the process of disease development once the virus gets into the plant [23]. Tomato spotted wilt virus infection reduces pod size and numbers, especially if the plants are infected early. Seed produced on infected plants may be reduced in size and malformed, and have discolored (red) seed coats [4]. 2

3 Infection with TSWV dramatically reduces seed yield in peanut plants. Reduction in yield is due to fewer seed produced, as well as lower average weight of the individual seed [18]. TSWV is transmitted only by thrips, a group of tiny, slender, winged insects (1/32 to 1/16 inch in length) that damage crops through feeding. Females can deposit 50 to 60 eggs within the plant foliage [3]. Of the several thrips species known to transmit TSWV, two of them, Western flower thrips and tobacco thrips, are found in Georgia peanuts. There may be over 5 million thrips per acre in peanut fields [17]. Young thrips pick up TSWV particles while feeding on infected plants, but they must feed for several minutes, up to thirty, in order to acquire the virus. Once acquired, the virus circulates and replicates within the thrips body and may remain there for the rest of the insect s life. Only young thrips can acquire the virus, adult thrips cannot, but both young and adults can transmit it. Adults cannot acquire the virus because a barrier develops that prevents passage from the midgut [21]. Approximately one week after thrips pick up the virus, they are able to transmit it to uninfected plants. Transmission occurs after about fifteen minutes of feeding. [3] Not all thrips carry the virus. In 1996, 8% of tobacco thrips adults and 3% of western flower thrips adults surveyed tested positive for TSWV [21]. There may be extremely high thrips numbers in a field but there may not be a high incidence of TSWV [23]. In addition, differences in the size of the thrips population make disease incidence unpredictable from year to year. There are no pesticides to treat TSWV. One management approach is to control thrips with insecticide applications in order to reduce transmission of the virus. Research has shown, though, that insecticidal control of thrips does not decrease TSWV incidence or severity [6, 7]. If thrips feed on a peanut plant treated with a systemic insecticide (such as aldicarb), the insect dies, but simply by feeding on it, an infected thrips may transmit TWSV to the plant. Thus, the pesticide kills the thrips, but does not prevent TSWV infection [24]. However, use of the insecticide phorate does appear to provide slight TSWV suppression [6]. The mechanism by which phorate suppresses TSWV does not seem to be related to its control of thrips levels, since the level of control it provides is no greater than that of other insecticides tested. Phorate 3

4 provides about 80% control of thrips populations [16]. Phorate may induce a defense response in the peanut plant that allows the plant to resist infections better or inhibits virus replication [6]. Phorate usage reduces the incidence of TSWV by 20-25% [22]. Use of phorate in Georgia peanuts increased from 7% of the acreage in 1991 to 27% in 1999 [8]. Table 3.1 delineates phorate use and cost in Georgia peanuts in The 123,000 pounds of phorate used in 1999 represents approximately 22% of the volume of insecticides used in Georgia peanuts that year. In 1995, researchers at the University of Georgia developed the Tomato Spotted Wilt Risk Index, a system by which peanut growers could assess the risk of their fields developing TSWV [6, 9]. The index works by assigning risk points to production practices a high risk is assigned a high point value. By identifying sources of risk, growers can modify their practices to minimize that risk and prevent losses to TSWV. Among the many factors identified as having an impact on TSWV risk, variety selection has the widest range of points, indicating the importance of planting varieties with some TSWV resistance. Preliminary economic analysis of the Tomato Spotted Wilt Risk Index over four years found that, on average, planting resistant varieties increased per acre net returns by $204 as compared to non-resistant varieties [11]. Since the emergence of TSWV as a limiting factor in Southeastern peanut production, there has been a shift in the cultivars grown in the region. In the late 1980 s and early 1990 s, the predominant cultivar was Florunner (70% of the acreage), which had little resistance to TSWV infections. By 1998, Florunner had ceased to be planted. Currently the predominant variety grown is the moderately resistant Georgia Green (more than 90%) [12, 13]. Research indicated that 23% of Florunner plants developed TSWV symptoms while only 12% of the Georgia Green plant population developed TSWV symptoms [10]. But peanut cultivars that are referred to as TSWV resistant, including Georgia Green, in fact do not have high levels of resistance and may suffer significant damage during severe epidemics [13]. They do provide moderate suppression of TSWV in the field, but the mechanism of this suppression is unknown. Losses due to TSWV are generally not significant, averaging around 5% (Table 3.2), although in 1997 losses to TSWV in Georgia peanut production were estimated at over $43 million. Losses have been reduced in recent years due to the widespread use of the TSWV risk index [9]. 4

5 Viral Resistant Peanut The goal of researchers at the University of Georgia is to use genetic engineering techniques to develop peanut varieties with total resistance, not just tolerance, to TSWV [14, 26]. The peanut variety Marc I is not grown commercially, but it is susceptible to TSWV, and it responds well in tissue culture, which is essential for regenerating plants from transformed cells. Marc I tissue has been transformed using microprojectile bombardment, or gene gun [14, 15]. Small gold particles coated with the genes to be introduced are shot into peanut tissue cells. In this case, the introduced genes were TSWV coat protein genes. Tissue that survived the bombardment was cultured into plantlets, which were screened for positive transformation. The insertion of genetic material from the virus confers resistance to infection by preventing virus replication and spread. Initial field trials (60 replicated plots) of the resulting transformed plants indicate that they are more resistant to TSWV than untransformed Marc I plants, and the resistance is comparable to the resistance shown by Georgia Green [15]. More field trials are being conducted to test how the transgenic plants hold up against TSWV in a variety of locations and environmental conditions. In addition, research on TSWV resistance through biotechnology is striving for higher levels of resistance. One way is to transform commercial varieties such as Georgia Green which already have some natural tolerance to TSWV [25]. Improvements in tissue culture techniques have made this possible. Such stacking of different types of resistance in the same plant may increase both the level and duration of virus protection. Alternative mechanisms of resistance are also being investigated. Researchers are still using viral genes to confer resistance to plants, but the resistance induced in the plant targets the genes for the coat protein rather than the coat protein itself, and is referred to as post transcriptional gene silencing [25, 27]. By making these two changes (transforming cultivars with natural TSWV tolerance and using post transcriptional gene silencing), researchers hope to increase the TSWV resistance of transgenic peanuts. 5

6 Estimated Impacts It is estimated that a transgenic TSWV resistant peanut would be planted universally in Georgia. It is assumed that the transgenic peanut would prevent the loss of 4.5% of Georgia s peanuts with a value of $17 million/year and a volume of 59.4 million pounds. The viral resistant peanut is not expected to affect the use of phorate since it is used to prevent thrips feeding injury irregardless of TSWV [22]. 6

7 Table 3.1: Phorate Use: Georgia Peanuts, 1999 Total Acres 546,000 % Treated 27 Lbs./AI/A/Yr Total Lbs. AI/Yr. 123,000 $/Lb. AI $10.75 Total $/Yr. $1,322,250 Source: [8] Table 3.2: Losses in Georgia Peanuts due to TSWV Year % Reduction in Crop Value Crop Loss ($ million) Source: [19] 7

8 References 1. USDA, Crop Production 2000 Summary, National Agricultural Statistics Service, January USDA, Crop Values 2000 Summary, National Agricultural Statistics Service, February Padgett, G.B., P.F. Bertrand, and J.D. Gay, Spotted Wilt in Georgia s Crops, University of Georgia College of Agricultural and Environmental Services Cooperative Extension Service Bulletin Sherwood, J.C., and H.A. Melouk, Viral Diseases and Their Management, In Melouk, H.A. and F.M. Shokes, eds., Peanut Health Management, APS Press, Garcia, L.E., R.L. Brandenburg, and J.E. Bailey, Incidence of Tomato spotted wilt virus (Bunyaviridae) and Tobacco Thrips in Virginia-Type Peanuts in North Carolina, Plant Disease 84: Brown, S. L., et al, Tomato Spotted Wilt of Peanut, University of Georgia, College of Agricultural and Environmental Sciences, Cooperative Extension Service Bulletin 1165R, Todd, J.W., A.K. Culbreath, and S.L. Brown, Dynamics of Vector Populations and Progress of Spotted Wilt Disease Relative to Insecticide Use in Peanuts, Acta Horticultura 431: USDA, Agricultural Chemical Usage, 1991 and 1999 Field Crop Summaries, separate volumes, National Agricultural Statistics Service. 9. Farmers Fight Back, Peanut Farmer, March Culbreath, A.K., et al., Disease Progress of Tomato Spotted Wilt Virus in Selected Peanut Cultivars and Advanced Breeding Lines, Plant Disease 80: Proven Profits: Using the TSWV Risk Index Returns Net Profit to Your Bottom Line, Peanut Farmer, April, Culbreath, A.K., et al., Reaction of Peanut Cultivars to Spotted Wilt, Peanut Science 27: Culbreath, A.K., et al., Response of Early, Medium, and Late Maturing Peanut Breeding Lines to Field Epidemics of Tomato Spotted Wilt, Peanut Science 26:

9 14. Seeds of the Future: Peanut Researchers Work to build a Better Peanut with Genetic Engineering, The Peanut Farmer, February Engineering Peanuts: Gene Transfer Gives TSWV Resistance, Peanut Farmer, February Bridges, David C., et al., An Analysis of the Use and benefits of Pesticides in U.S. Grown Peanuts: I Southeastern Production region, Crop and Soil Science Department, University of Georgia, Kresta, Kenya K., et al., Survey by ELISA of Thrips Vectored Tomato Spotted Wilt Virus Distribution in Foliage and Flowers of Field-Infected Peanut, Peanut Science, (1995) 22: Culbreath, A.K., et al., Productivity of Florunner Peanut Infected with Tomato Spotted Wilt Virus, Peanut Science, (1992) 19: Georgia Plant Disease Loss Estimates, separate volumes, University of Georgia Cooperative Extension Service. 20. Culbreath, A.K., et al., Effect of New Peanut Cultivar Georgia Brown on Epidemics of Spotted Wilt, Plant Disease, December Todd, J. W., et al., Thrips as Tospovirus Vectors in Peanut In 1992 Proceedings of American Peanut Research and Education Society. 22. Brown, Steve L., University of Georgia, Personal Communication, August Research Paying off in Spotted Wilt Battle, Southeast Farm Press, April 11, TSWV Troubles More Growers in 1990, Peanut Grower, February Ozias-Akins, Peggy, University of Georgia, Personal Communication, February Yang, H., et al, Transgenic Peanut Plants Containing a Nucleocapsid Protein Gene of Tomato Spotted Wilt Virus Show Divergent Levels of Gene Expression, Plant Cell Rep., 17: , Yang, H., et al, Transformation of Peanut with Truncated Nucleocapsid Protein Gene of Tomato Spotted Wilt Virus in Cultivated Peanut Using Particle Bombardment, In Virto, V. 37 pt II, p25,