Sugar Beet Performance with Curly Top Is Related to Virus Accumulation and Age at Infection

Beet curly top virus (BCTV), genus Curtovirus, and other closely related curtoviruses transmitted by the beet leafhopper Circulifer tenellus (Baker) have caused significant problems to irrigated agriculture in the western United States for over a century (4). BCTV is known to infect a broad range of crop and weed hosts in many plant families (1). The leafhopper vector also feeds and breeds on an extensive range of plants from different families (6), although sugar beet (Beta vulgaris) is clearly its favorite crop host. C. tenellus transmits curtoviruses most efficiently after a 48-h acquisition-access feed on an infected source plant, but shorter feeding times (2 to 20 min) also result in a low frequency of transmission. Curtovirus transmission by the vector requires a 4-h latent period following ingestion, and leafhoppers can inoculate healthy plants by feeding for as little as a 1-min inoculation access period. Symptoms develop in plants in a minimum of 5 days. Leafhopper vectors retain the ability to transmit BCTV for days to weeks and often for their lifetime (1). Many strains (up to 14) of BCTV were initially distinguished on the basis of differential symptomatology in sugar beet (10). Molecular characterization of BCTV in beet demonstrated that the virus primarily existed as three strains, CFH, Worland, or California/Logan, and genotypic variants of these strains (18). Based on sequence similarity and severity on sugar beet, the three strains are currently designated as separate species with the names Beet severe curly top virus (BSCTV, formerly CFH), Beet mild curly top virus (BMCTV, formerly Worland), and Beet curly top virus (BCTV, formerly California/Logan) (17). Studies conducted over the past 2 years have examined epidemiology of curtoviruses to determine if individual curtovirus species are specifically associated with certain weed hosts, as well as whether localized areas may serve as reservoirs for the more severe curtovirus species. These studies have suggested some clustering of infection centers in California, but it is not yet clear whether such pockets contribute to prevalence of specific virus species (20). The wide host range of curly top and abundance of the leafhopper vector has made managing the virus difficult. The present management strategy focuses on the use of curly top–resistant cultivars when available for a specific crop and large-scale spraying of insecticides to control the leafhopper vector in its overwintering grounds (1,5). The spraying of over-wintering ground is believed to be effective in some years, but effectiveness varies considerably from year to year (13). Losses to curly top are most severe when sugar beet plants are infected as seedlings, and losses decrease with increasing plant age and size at the time of infection (7). Consequently, early planting is recommended to allow significant growth prior to infection, and seedor crop-applied insecticides are recommended to delay infection (9). Curly top–resistant sugar beet cultivars have been grown successfully in California, but currently available curly top–resistant cultivars do not yield as well as susceptible cultivars in the absence of curly top (9). The resistance is a multigenic trait with low heritability that has been difficult to move between breeding and parental lines (15). Field applied insecticides can delay infection and reduce the rate of secondary transmission when applied appropriately (9); however, since the vector needs only a brief feeding interval in which to introduce the virus into a healthy plant, insecticides will not block virus transmission. The current system for evaluation of curly top resistance is based entirely on field evaluation of varieties in industrymaintained disease nurseries. Although these nurseries are effective for curly top resistance testing, the process requires several seasons for evaluation, and effectiveness can vary from year to year depending on environmental influences. Furthermore, field-testing does not account for plants that are not infected; rather, all plants are considered in an evaluation, a fact that has on occasion compromised the quality of variety selection in the field. In an effort to develop a more rapid and conABSTRACT Wintermantel, W. M., and Kaffka, S. R. 2006. Sugar beet performance with curly top is related to virus accumulation and age at infection. Plant Dis. 90:657-662.