Aster yellows is a disease caused by a mycoplasma-like organism which attacks a wide range of plants. Plants may be stunted or with numerous secondary shoots. Foliage is yellow and seeds are usually sterile. Plants have an upright habit of growth. In many plants the veins of immature leaves are clear. Affected leaves are somewhat narrower than healthy leaves. Old leaves may develop a slightly reddish, brownish, or purplish tinge in the late stages. The main branches will be shortened. Flower parts may develop into leafy structures. In lettuce, the head leaves fail to fully develop and they have pink to tan spots. There is a curling and twisting of inner leaves. Infected plants may fail to head. In carrots, the tops become yellow, stunted, and bunchy. Many small rootlets are on the carrot. Onion leaves are twisted, yellow, more numerous and dwarfed. Small, purple, terminal leaves and auxiliary tubers develop on potato plants. Aster yellows affects 300 different species that represent more than 40 families of plants. The ones listed below are the most important.
Crops: broccoli, buckwheat, cabbage, carrot, cauliflower, celery, endive, flax, lettuce, onion, parlsey, potato, parsnip, pumpkin, red clover, salsify, spinach, strawberry and tomato.
Flowers: aster, anemone, calendula, Centaurea, China aster, chrysanthemum, Clarkia, cockscomb, Coreopsis, cosmos, delphinium, daisies, Gaillardia, hydrangea, marigold, Nemesia, Paris daisy, periwinkle, petunia, phylox, Scabiosa, snapdragon, statice, strawflower, veronica, and zinnia.
Weeds: cinquefoil, daisy fleabane, dandelion, horseweed, plantain, ragweed, thistle, wild carrot, and wild lettuce.
The mycoplasma overwinters in leafhoppers on perennial host plants. Leafhoppers can spread the mycoplasma 9 to 21 days after feeding on diseased plants. The mycoplasma multiplies in the insects and leafhoppers can spread the disease for 100 days or more after becoming infective. The ability of leafhoppers to transmit the organism is reduced when temperature is over 90°F. Overwintering of the mycoplasma occurs more often in some plants than in others because leafhoppers prefer to feed on those host plants. Symptoms show in plants in 10 to 40 days after insect feeding. The disease can be serious when dry weather forces leafhoppers to migrate from wild weeds to irrigated fields of susceptible plants. The six spotted leafhopper is one of the most common vectors in Texas, but at least twelve different species of leafhoppers may transmit the organism to healthy plants.
Control recommendations include the following:
(1) Obtain healthy seed, cuttings and plants.
(2) Early control of leafhoppers on lettuce and carrots.
(3) Spray weeds surrounding field with insecticide according to current recommendations.
(4) Apply insect control before cultivation, weeding, and other field operations.
(5) Control weeds during the growing season in the field, on irrigation ditch banks and in surrounding areas.
(6) Avoid rotations where one susceptible crop follows another.
(7) Destroy volunteer overwintering plants and avoid planting near established diseased crops.
(8) Destroy affected plants in small areas as soon as they appear to be diseased.
(9) Screen small plantings with wire mesh to exclude leafhoppers if practical.
(10) Always keep in mind the relationship between cultivated hosts, insect vectors and wild or alternate hosts and practice proper sanitation and good husbandry throughout the year to insure optimum growth of crop, plants and proper control of weeds and insects.