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1. IntroductionCurly Top is a virus disease caused by Beet Curly Top Virus (BCTV). The virus is presumably indigenous to the Mediterranean basin, and was introduced into North America before the start of the 20th century. As do most viruses, BCTV has a vector that moves it from plant to plant. The beet leafhopper is the vector for BCTV in North America. Today, BCTV occurs throughout the western United States, including the eastern slope of the Rocky Mountains, Texas panhandle, south-western Canada and in Mexico.
The virus causes severe symptoms on sugarbeet. These symptoms develop on young leaves after infection. No symptoms develop on leaves that mature at the time of infection. Soon after infection by BCTV, the leaf veins become roughened on the lower side and often produce swellings and spine-like outgrowths (see Image #1). Infected plants develop dwarfed tap roots, and will be woody and brittle with lateral roots becoming twisted and deformed. The malformed roots are less functional and plants may wilt during the heat of the day (see Image #2). A clear liquid may appear in the petioles of infected plants. This liquid will usually oxidize to a dark color. Cross sections of diseased tap root usually exhibit dark vascular bundles that appear as concentric rings throughout the tap root. Sections of petioles from infected plants exhibit darkened vascular bundles as well. Click image for full page view.
3. Viral DescriptionBCTV belongs to the virus group called Gemini-viruses. These viruses are small spherical particles composed of a single strand of DNA surrounded by proteins. The particles are very small (approximately 0.8 millionths of an inch in diameter), always exist in pairs and can only be observed with an electron microscope. The viruses only infect and replicate in the phloem tissue of the plant. The phloem is part of the vascular system of the plant and its function is to move carbohydrates throughout all the plant's tissues. A leafhopper inserts its stylet into the phloem as it feeds on a plant. Infected leafhoppers inject virus particles into the phloem during this feeding process. The virus particles replicate in the phloem tissue and are distributed throughout the plant in this conducting tissue.
4. Viral SpreadThe beet leaf hopper (Circulifer tenellus) is one of the most important insect pests of sugarbeets in the western United States because it is the vector of BCTV. The beet leafhopper is small (1/8 inch long) and pale green to gray in color. The insect over-winters on biennial and perennial plants. The winter rains bring about wild mustard and other winter annuals that occur on uncultivated fields and range lands. The insect spends the winter and spring as adults, feeding on these winter hosts. In the spring, females deposit eggs on these hosts, and the first generation of leafhoppers hatches. As the temperatures warm, and the winter annuals mature, the first generation leafhoppers migrate out of the range lands to sugarbeet fields, carrying the virus with them. Leafhoppers produce several generations each year, which migrate through susceptible crops spreading the virus. As the crops mature and dry, the leafhoppers move back into the over wintering areas in search of the winter host. Leafhoppers are capable of long distance movement. In some areas, leafhopppers may migrate 400 miles or more, from the winter breeding areas to cultivated fields. Leafhoppers moving from breeding grounds in southern Arizona are known to have carried the virus to sugarbeet crops growing near Grand Junction, Colorado. Curly Top sporadically occurs east of the Mississippi River, indicating a very long dissemination from western breeding grounds. Leafhoppers acquire BCTV by feeding on infected host, either the winter host or crop plants. Leafhoppers are able to acquire the virus during very short feeding times. The leafhopper retains the ability to transmit BCTV for a month or more after acquisition. The vector may maintain the virus during its over-wintering period.
Curly top is being controlled by a complex integration of various approaches. Today, variety resistance is the backbone of curly top control. All major companies involved in sugarbeet variety development for the West and the intermountain region have active Curly Top resistance programs. Most varieties approved for planting in these areas must meet or exceed Curly Top resistance standards set by local variety evaluation committees. Sugarbeet seed companies continue to search for improved varieties with Curly top resistance. In addition to variety resistance, Curly Top is being controlled by other approaches. Planting date is a very important factor in dealing with Curly Top. Resistance in sugarbeet to damage and infection increases with plant age. Young seedlings are very susceptible to infection and damage. Early spring planting, or in California, fall planting, limits infection and damage due to BCTV. The early planted beets achieve significant size before the leafhopper flights and incur less damage than smaller seedlings. Reduction to leafhopper breeding areas is another approach at curly top control. In California and Wyoming, The Curly Top Virus Control Program monitors the populations of leafhoppers and the incidence of BCTV in the over-wintering areas. Elimination of host plants in the over-wintering areas reduces the leafhopper population. The program uses insecticide sprays to reduce leafhopper populations before the leafhoppers migrate to cultivated areas in the spring of the year. The California Curly Top Virus Control Program publishes advisories and makes recommendations to growers on how to limit the disease. Similar programs exist in other states. The development of systemic insecticides has aided in Curly Top control. The insecticides only affect the leafhoppers after they feed on the plants, so BCTV infections still occur, but these chemicals limit additional spread of the vector and the virus. In furrow treatments of Phorate are very effective for curly top control, apparently acting as a repellent, keeping the leafhopper out of the sugarbeet fields, thus preventing virus infection.
6. ConclusionThe Curly Top disease of sugarbeet has been a menace since the inception of the sugarbeet industry in the western United States. A malady that at one time nearly eliminated sugarbeet production west of the Rocky Mountains is now mostly under control through the cooperation of growers, plant breeders and regulatory agencies. We haven't eliminated the disease, but we have learned to live with it and to continue the successful cultivation of sugarbeets in spite of it. Perhaps curly top control represents on of the first and most successful applications of integrated pest management.
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