Stephen Kaffka, Bill Wintermantel, Michael Burk, and Gary Peterson

Beet curly top virus (BCTV) affects sugarbeets and other important crops in California. One of the first successful plant breeding programs directed against a virus disease was established in the 1920s by the USDA to combat BCTV in California and elsewhere in the western United States by developing resistant cultivars. Up until the last few years, most of the cultivars planted in California included curly top resistance. Recently, increasing amounts of acres are being planted to cultivars with no curly top resistance because of their superior yield. Long term yield limitations that have persisted for several decades seem to have been overcome in recent years (fig.1). BCTV has not been observed at damaging levels in these varieties, but the risk of significant loss remains.

Some control of the vector of BCTV, the beet leafhopper (Circulifer tenellus Baker), is provided by a program in the state of California that controls the insect during the winter by spraying its natural winter nurseries in the Coastal and Sierra foothills. It is not clear, however, how much longer this control program will continue. Given these uncertainties, significant economic loss from BCTV remains a threat to the well-being of the state=s industry, as non-resistant cultivars dominate planted acres.

Recently, Wang et al., (1999) reported on results with a greenhouse trial comparing different insecticides, seedling age, and inoculation intensity on the amount of BCTV infection occurring in sugarbeets and other crops. They compared dimethoate, imidicloprid (Gaucho7), and control treatments. They inoculated seedlings at an early stage, and at one and two week intervals after the first inoculation. Different inoculation intensities (numbers of leafhoppers) were also evaluated.. They found that infection was reduced most by the use of imidicloprid, and that delaying inoculation two or more weeks, resulted in significantly lower infection rates. Imidicloprid has performed well in seed nurseries in Idaho as well (J.R. Stander, per. comm.).

Materials like phorate (Thimet7) that have been proven effective in the past at delaying or preventing curly top infection may be restricted in the future as they go through re-evaluation under the Food Quality Protection Act. The effectiveness of alternative insecticides like imidicloprid at reducing infection among susceptible and resistant cultivars under field conditions where BCTV is likely to be present needs to be evaluated.

One susceptible (cv. SS-Rifle) and one resistant (cv. SS781R) cultivar were planted on March 31, 2000 at a site located at West Hills College in Coalinga, California. This site is located in the overwintering areas of the BCTV vector, the beet leafhopper, and previous curly top trials at this location have been successful at times. In early spring, leafhoppers are likely to be migrating to irrigated areas from the surrounding foothills.

Each cultivar was treated either with imidicloprid as a seed treatment at one of two rates (45 and 90 g per unit of seed--100,000 seeds) or with phorate (Thimet7) at labeled rates applied at planting to the soil, directly beneath the seed line. A control without any seed or soil treatment was also included. There were five replications of each treatment. To insure adequate curly top inoculum at the site, a number of curly top infected plants were produced at the USDA/ARS facility in Salinas and then transplanted at the research site after seedling emergence.

Seedlings and growing plants were evaluated visually for curly top damage and scored for curly top infection at harvest using the scale developed and used by the Beet Sugar Development Foundation in its curly top nursery in Idaho (Table 1). The center two rows of a four row plot were harvested on August 29, 2000. Roots were weighed and analyzed for sugar percent and impurities by the lab at the Spreckels Sugar Company in Mendota.

Cultivars influenced crop performance to a larger degree than did seed or soil treatments (Table 2, and fig. 2). The susceptible variety, Rifle, had a significantly higher infection level than 781R, resulting in smaller roots and lower sugar yields. Sugar percent in roots was not apparently affected by curly top, and reflected differences in cultivar characteristics instead (Table 2).

There were some effects of seed or soil treatments on disease incidence and agronomic performance. The use of imidicloprid at increasing rates seem to protect the susceptible cultivar (Rifle), but had less effect on the resistant line (fig. 3). Similar results were observed for root weight and gross sugar yield (fig. 4, 5). Treatments had no effect on sugar percentage, only on yield. Sugar percentage was a function primarily of varietal differences (fig. 2).

The trial at Westhills College was not well managed. At times, weed growth was excessive, and early season irrigations were uneven. No fertilizer was applied to the plots so plants likely were nutrient limited. Nonetheless, there are some conclusions that can be derived from this study. The plant protection materials used are much less effective at preventing loss to curly top than varietal resistance in the presence of substantial background infection pressure. Resistance remains the most effective defense against loss to this chronic virus disease. This is supported by the strength of the contrasts between cultivars and the high levels of significance observed (Table 2). Despite widespread cultivar susceptibility, however, BCTV has not been reported to be a severe problem in sugarbeet areas of the San Joaquin Valley. Indeed, state wide averages have risen in recent years and passed 10,000 lb sugar per acre in 1999 (Fig. 1). Most of curly top affected areas in the San Joaquin Valley are planted in fall, and develop initially during winter. Fall planting seems to be a defense against early infection. Late infection may still occur, but does not seem to be of much significance in fall planted areas, given increasing yields in recent years. It is not certain, however, that fall planting will always be an effective defense against economic loss.

Imidicloprid did provide some protection for the susceptible line, and may have increased the yield of the susceptible line to levels equal to that of the resistant line without any treatment. Imidicloprid at 45 g appeared to be as effective as at 90 grams in this trial. The use of phorate applied to the soil was not very effective. Gaucho, a more environmentally friendly material, appears to be an adequate substitute for phorate.

Given the management deficiencies of this trial, all these conclusions must be regarded as tentative, except for the relative differences between susceptible and resistant cultivars. The trends identified, but not the magnitude of the differences, seem reasonable. Additional trials will better determine the relative effectiveness of imidicloprid and phorate.

Creamer, R., Luque-Williams, M., and Howo, M. (1996). Epidemiology and incidence of beet curly top gemini virus in naturally infected weed hosts. Plant Dis. 80:533-535.

Duffus, J.E. (1983). Epidemiology and control of curly top diseases of sugarbeet and other crops. Pages 297-304 In: Plumb, R.T., and Thresh, J.M. (eds.). Plant Virus Epidemiology. Blackwell, Oxford.

Wang, H., Gurusinghe, P. de A., and Falk, B. (1999). Systemic insecticides and plant age affect beet curly top virus transmission to selected host plants. Plant Dis. 83(4)352-355.

This research was supported by the California Sugarbeet Research Committee. Robert Lewellen, Bill Wintermantel, Gail Wisler, and Julie Schwenk of the USDA/ARS provided infected plants, advice , and aid with disease ratings. Michael Burke of West Hills College supervised the experiment.

Fig. 1. Recent yields (bars) and sugar percent (line) trends in California.

Fig. 2. Cultivar curly top ratings, gross sugar yields, and sugar percent, West Hills college trial, 2000. Error bars are standard errors.

 Fig. 3. Cultivar curly top rating as influenced by seed or soil treatments. C: control (no treatment), T: phorate at ..., G45: imidicloprid at 45 g a.i. per 100,000 seeds, G90: imidicloprid at 90 g a.i. per 100,000 seeds.

Fig. 4. Individual average root weight by cultivar as influenced by seed or soil treatment.

Fig. 5. Gross sugar yield by cultivar, as influenced by seed or soil treatment.