Control of Curly Top in Sugar Beet with Seed and Foliar Insecticides

Strausbaugh, C. A., Wenninger, E. J., and Eujayl, I. A. 2014. Control of curly top in sugar beet with seed and foliar insecticides. Plant Dis. 98:10751080. Curly top in sugar beet is a serious problem that is caused by Beet curly top virus and other closely related species and transmitted by the beet leafhopper. In order to find a means of reducing curly top in sugar beet, 15 combinations of insecticide seed (Poncho, Poncho Beta, and Poncho Votivo) and foliar (Asana, Cyazypyr, Lorsban, Mustang, Scorpion, and Sivanto) treatments were evaluated versus an untreated check during the 2012 and 2013 growing seasons. An epiphytotic was created by releasing viruliferous beet leafhoppers 58 to 59 days after planting. The foliar sprays were applied 6 to 7 days before and again 6 to 8 days after leafhopper release. Seed treatments (active ingredient: clothianidin) were able to reduce symptoms by 26 to 42% and increase recoverable sucrose by 16 to 21%. The pyrethroids Asana and Mustang also performed well by reducing symptoms 22 to 56% and increasing yields 13 to 20%. The neonicotinoid seed treatments should be an effective way of supplementing host resistance for early-season (at least 59 days after planting) curly top control in sugar beet. The pyrethroid foliar applications could be used to extend curly top control during the midseason period and provide resistance management. Curly top is a viral disease problem in sugar beet (Beta vulgaris L.) that can lead to severe yield losses in semiarid production areas around the world (3). In the western United States, curly top in sugar beet can be attributed to any of three Curtovirus spp.—Beet severe curly top virus (BSCTV; formerly strain CFH), Beet mild curly top virus (BMCTV; formerly strain Worland), and Beet curly top virus (BCTV; formerly strain Cal/Logan)—although sugar beet production areas outside the United States have been shown to possess additional strains or species (8,22,29,33). Finding more than one of these three species in a commercial sugar beet plant is not unusual (29). Recent taxonomic revisions released by the International Committee on Taxonomy of Viruses (second version 25 March 2013) places the three species mentioned above under the species name BCTV (13). However, there are still discussions as to what strain designations should be used in place of the former species names. Thus, the species names have been used in this article. BCTV is transmitted by the beet leafhopper, Circulifer tenellus (Baker), which disperses to production fields in the spring as weeds become desiccated in desert areas or poorly managed properties (3–5). If infection occurs at an early growth stage, damage is worse than if infection occurs at later growth stages (6,19,32). Curly top almost eliminated sugar beet production in the western United States before resistant cultivars became available in the mid-1930s (3,4). Host resistance is still one of the primary means to manage curly top in sugar beet but this resistance is difficult to maintain in commercial cultivars because it is quantitatively inherited (9,12). When other desirable traits such as resistance to Beet necrotic yellow vein virus (causes rhizomania) and glyphosate were introduced into commercial cultivars, maintaining resistance to BCTV in the parental lines that were used to create the commercial hybrids proved to be problematic (24,28). Currently, resistance in commercial sugar beet cultivars is only low to intermediate (24). These low levels of resistance seem to indicate that growers are willing to gamble on less resistance to maximize yield potential, while relying on other forms of protection such as insecticide seed treatments to control curly top. In an effort to find a way to supplement host resistance, in-furrow, foliar, and seed-treatment insecticides have been investigated (12,14,16,23,25,26,28,30). Based on these investigations, the neonicotinoid seed treatments (active ingredients: clothianidin and thiamethoxam) have shown the most potential to limit curly top and to increase yields (23,25–28). Under low to moderate curly top pressure, the neonicotinoid seed treatments have been shown to increase yields by approximately 20% through providing earlyseason control of the beet leafhopper, the vector for BCTV (25). In some production areas in the western United States, the neonicotinoid seed treatments are now required on all sugar beet seed because of the low level of host resistance in commercial cultivars and the yield increases associated with the seed treatments (28). Though effective against beet leafhopper for at least 50 to 55 days after planting (28), the efficacy of seed treatments is expected to diminish over time. Thus, ways to extend control beyond the early-season period covered by the seed treatments needs to be investigated. In Idaho, the use of foliar sprays in desert areas for beet leafhopper control was no longer justifiable by 1970, because of costs, increasing urbanization near production fields, and influences on nontarget organisms (4). Previous studies with sugar beet would suggest that season-long control with foliar insecticides would not be possible because of their short efficacy period. However, there is a need for continuous control of this chronic problem from spring to fall (30), and extending control provided by seed treatments has not been investigated. Foliar insecticide treatments could potentially be used to extend protection and provide some midseason control after the seed treatments are no longer effective. Moreover, the use of non-neonicotinoid foliar insecticides as a supplement to seed treatments could contribute to insecticide resistance management for neonicotinoid chemistries. Thus, foliar insecticides were investigated with and without insecticide seed treatments to identify potential products that might reduce the influence of curly top in sugar beet through midseason control of the beet leafhopper virus vector. Corresponding author: C. A. Strausbaugh, E-mail: [email protected] Mention of trade names or commercial products in this article is solely for the purpose of providing scientific information and does not imply recommendation or endorsement by the United States Department of Agriculture. Accepted for publication 27 February 2014. http://dx.doi.org/10.1094/PDIS-12-13-1260-RE This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2014.