The Beet Leafhopper

J. R. Douglass, William C. Cook.

The beet leafhopper is the only known carrier of curly top, a destructive virus disease of sugar beets, beets, beans, tomatoes, spinach, melons, other crops, ornamental flowering plants and many weeds. The insect favors and and semiarid localities of the western United States, northern Mexico, and southwestern Canada. Its breeding grounds are abandoned and overgrazed lands on which weed hosts occur. Such areas are also reservoirs for the virus.

Curly top has been given many common names on sugar beets it has been called California beet blight, western blight, blight, curly leaf, and curly top; on tomatoes, tomato blight, yellow blight, summer blight, western blight, western yellow tomato blight, tomato yellows, and tomato curly top; on beans, bean blight.

How serious it can be is shown in records for southern Idaho, where growers of sugar beets in 1924 abandoned 11,442 out Of 22,418 acres they planted. In 1934 they abandoned 18,635 out of 21,389 planted acres. The average yields of the harvested fields were 5.51 and 4.88 tons an acre for 1924 and 1934 far below the 16-ton average in years of little leafhopper exposure. Factories were dismantled and moved to other places, only to be dismantled and moved again when it was found that they had been relocated in areas infested by the beet leafhopper.

A leafhopper can pick up the virus from a diseased plant and transfer it to a healthy plant in 4 hours. Once a leafhopper has become infected with the virus of curly top, it remains infective, but it cannot transmit the virus through the eggs to its progeny.

Serious losses to cantaloups and muskmelons have been reported in Arizona, California, Idaho, and Utah. In 1945, N. J. Giddings, a specialist on curly-top virus, found that flax from the San Joaquin Valley, Calif., was infected with curly top. Later tests indicated the possibility of serious injury to flax during seasons of high infestations.

THE BEET LEAFHOPPER, commonly called the whitefly in the West, is slightly more than one-eighth inch long. It is gray to greenish yellow. It is a sun-loving, dry-climate insect and often breeds on many species of introduced weeds established on nonagricultural and deteriorated range lands. It feeds by sucking juices from its host plants. Rarely does it become numerous enough to cause great direct damage by its feeding. It is important then only because it carries curly top.

The virus of curly top survives the winter in both the beet leafhopper and its winter host plants. The leafhopper transmits it during feeding. It is carried from the winter hosts to other weed hosts and cultivated susceptible crops, principally during the spring movement. Some of the crops in their seedling stage are very susceptible to the disease. Infected plants often die. The percentage of the spring-generation leafhoppers carrying the virus has varied from year to year, with a low of 4 percent and a high of 80 percent.

Varieties of sugar beets that are resistant to curly top have been developed. We have no commercial varieties of tomatoes that are resistant. Serious losses occur during years of leafhopper outbreaks in parts of California, Colorado, Oregon, Utah, and Washington. In southern Idaho and some other areas, tomatoes are not grown for commercial use, because the crop is practically a complete loss in years of drastic exposure to curly top.

Most varieties of snap beans are susceptible. Southern Idaho produces approximately 80 percent of the national requirement of garden seed beans. The area is free from bacterial blight and other seed-borne bean diseases. During years when large spring movements of hoppers coincide with the "crookneck," or seedling, stage, field after field of the most susceptible varieties of beans in southern Idaho have been so seriously damaged by curly top that it has been necessary to plow them under. Losses to less susceptible varieties have also been high. In the past, serious losses occurred in most of the varieties of the field, or dry, beans. The Idaho Agricultural Experiment Station has developed resistant varieties of Great Northern and pinto beans.

THE EARLIEST VISIBLE SYMPTOMS of curly top in beets are the clearing of the tiny veinlets and the inward rolling of the lower and outer margins of the youngest leaves. As the disease gets more severe, the curling and distortion of the leaves increase, vein swelling occurs, and numerous papillae, or wartlike bumps, appear on the under sides of the leaves. A general stunting often ends in the death of the plant in the most severe cases. The diseased leaves are dark, dull-green, thick, crisp, and brittle. The roots show marked symptoms. The disease causes the death of the lateral rootlets and the beet then sends out a large. number of new lateral rootlets, which look hairy or woolly. A cross section of a diseased root often shows dark concentric rings alternating with light circular areas. A longitudinal section shows the dark discoloration extending lengthwise throughout the beet.

In beans the first symptoms are the Most pronounced on the trifoliate leaves, which become slightly puckered, curl downward, turn yellow, and die. They and the primary leaves are thicker than normal and brittle and break off readily. The infected young plants soon die. Plants infected later in the season may drop their blossoms, become chlorotic and die. Affected plants are decidedly dwarfed and have short internodes, which give them a bunchy appearance. Plants infected late in the season do not always develop typical symptoms of the disease and generally grow to maturity.

In tomatoes the first reliable symptom is a general drooping, but not wilting, accompanied by yellowing of the young leaves and purpling of the veins. The plant is abnormal; often silvery in color. The leaves thicken and become leathery and brittle. The entire plant turns yellow and usually dies. In seriously diseased plants the blossoms may drop and no more fruit is set. Fruits that are already formed turn yellowish red, ripen prematurely, and are stunted and of poor quality.

In squash the symptoms are somewhat similar to those in the other susceptible commercial crops. If the young seedling is infected by virus-carrying leafhoppers as soon as it emerges above the soil, it may die before its true leaves appear. In the older infected plants, new growth is stunted, internodes are shortened, and the leaves may roll upward at the margins. An upward bending of the tip of the runner is characteristic. Blossoms may drop and not set fruit, and the fruits already formed are stunted. There are no distinctive color differences by which a diseased plant may be identified. Wilting is not characteristic of curly-top infection in squash plants.

Infected cantaloups show no reliable symptoms. Infected seedlings become severely stunted and usually die. In the older infected plants, new growth is stunted, internodes are shortened toward the end of the runners, and the leaves may become puckered with the margins turned down. The flowers become dwarfed and often become dry before the petals expand. Yellowing occurs in severe cases.

Spinach affected by curly top undergoes stunting, shows crinkling and curling of the leaves, and acquires a more leathery texture and, in severe cases, yellowing of the leaves.

Considerable variations may occur in the symptoms of the infected plants, depending on the strains of the virus. The length of the incubation period and the severity of the disease that develops depend on the age and condition of the plant, its resistance, virulence of the virus, temperature, relative humidity, and light intensity. High temperature, low relative humidity, and high light intensity all increase the severity of the disease and the rate of development. Irrigation does not check the disease. Serious epidemics of curly top are dependent on several contributing factors magnitude and time of the movement of spring-generation leafhoppers, percentage of leafhoppers carrying the virus of curly top, size and condition of susceptible plants at the time of infection, and weather conditions.

The first variety of beets resistant to curly top, U. S. 1, was developed by the Bureau of Plant Industry, Soils, and Agricultural Engineering and was released to growers in 1934. Since then, other resistant varieties have been released. Each is an improvement in resistance and adaptation. The development of varieties of sugar beets resistant to curly top has greatly lessened the losses to the crop. Eubanks Carsner and F. V. Owen have told the story in Science in Farming, the 1943-1947 Yearbook of Agriculture, of the research that made this possible.

The development of resistant varieties of sugar beets has made it profitable to grow beets again in areas of the western part of the United States that are affected by the beet leafhopper. Even the resistant varieties are susceptible to curly top in the early stages of growth, however, although they are far more resistant to injury than the nonresistant varieties previously grown, such as Old Type. Although the threat of failure to the beet crop has been greatly lessened, curly top has not vanished, as serious losses from curly top have occurred in California, Idaho, Nevada, and Utah in recent years when large spring movements of beet leafhoppers have coincided with the susceptible seedling stage of the plant. Although the losses have been local in extent, they have dealt hard blows to growers in affected areas. There is also the possibility that new and possibly more virulent strains of the virus of curly top will have to be dealt with.

Since the beet leafhopper survives only in a dry climate, it has probably reached the limits of its economic distribution in North America. The general climatic condition, rather than its host plants, is the limiting factor in restricting further geographical distribution of this insect, as its summer, winter, and spring breeding host plants are found growing abundantly outside of its economic range.

In only two instances has the insect been reported in the Eastern States. D. M. DeLong found it reproducing on purslane sesuvium (Sesuvium portucastrum) at Miami, Fla., in 1921. In 1936 he and K. J. Kadow collected it from horseradish at Collinsville, Ill.

An occasional outbreak of the insect and the curly top disease may be expected in areas removed from its normal range. Such outbreaks have occurred in the past in the Big Horn Basin, Wyo., and near Billings, Mont. Those outbreaks evidently followed long-distance migrations of the leafhopper into the areas, followed by favorable weather conditions for the insect for a few years. Because the disease depends on the vector for its spread, it is limited to the region infested by the beet leafhopper.