Aster Yellows

L. O. Kunkel.

Aster yellows is a disease you see almost everywhere. It causes conspicuous symptoms and affects many different kinds of plants.

It occurs on buckwheat, red clover, tomato, carrot, lettuce, onion, parsnip, salsify, and spinach; on such common weeds as daisy fleabane, dandelion, horseweed, plantain, ragweed, and wild lettuce in grasslands, forests, and waste places; and on such popular flowers as calendula, Centaurea, China aster, chrysanthemum, Clarkia, cockscomb, coreopsis, cosmos, gaillardia, marigold, nemesia, petunia, phlox, Scabiosa, snapdragon, Statice, straw-flower, and Veronica in gardens even penthouse gardens throughout the country.

It is exhibited, unintentionally, at the great flower shows. Not infrequently the Paris daisy that wins first place in a flower show has aster yellows in one or more of its branches.

Aster yellows was the first virus disease shown to infect a large number of species in many different families of plants. By 1931 I had taken the disease experimentally to 184 species belonging in 38 families, all of which were Dicotyledones. Aster yellows since then has been found to infect about 300 different species belonging in 48 families, three of them Monocotyledones and 45 Dicotyledones. The known host range extends from the onion, a representative of the Liliaceae, to the China aster, a representative of the Compositae, and thus most of the evolutionary range seed plants.

SYMPTOMS OF ASTER YELLOWS vary greatly in plants of different ages growing at different rates, in different species, and under different conditions. The chief effects, more or less general in many host species, are stunting, production of numerous slender secondary shoots, chlorosis in foliage, virescence in flowers, sterility, and an upright habit of growth. In the twining plant Thunbergia alata the disease destroys the plant's ability to climb.

The earliest symptom in young China aster plants and in plants of many susceptible species is a clearing of veins in immature leaves usually on one side of one or two leaves. Leaves produced during chronic stages are more or less chlorotic throughout. They are dwarfed and somewhat narrower than comparable leaves on healthy plants. Also, they are sometimes deformed as a result of unequal growth in the two halves. They tend to take an upright position instead of laying down as do leaves of healthy plants. Old leaves may develop a slightly reddish, brownish, or purplish tinge in the late stages. The main stems usually have shortened internodes. Secondary shoots produce an abnormally large number of side branches that are slender and yellowish.

Aster plants affected while young produce no flowers. Plants affected after they have reached an advanced stage of development produce stunted, malformed flowers. Flowers affected after they have started to mature produce seeds of normal size or seeds that are either smaller of larger than normal size. All such seeds are sterile, but seeds from unaffected flowers in the same heads may be normal. Aster yellows virus is not known to be passed in the seeds of any plant.

One of the most striking effects on diseased flowers is phyllody. Stamens and Ovules may develop into leafy structures. Buds often are produced in the stigmas of flowers. They give rise to secondary flowers, which in turn may give rise to tertiary flowers by the same process. In that way flower chains are produced. The chains usually consist of only two or three flowers, but occasionally they may be composed of as many as six flowers each. The flower chains frequently tend to turn green.

Aster yellows can be identified by symptoms only if it can be observed in several different kinds of plants. The disease cannot be identified by symptoms alone in any one species, for there are many other diseases of the yellows type that cause like symptoms.

AN INSECT, the leafhopper Cicadula sexnotata, later renamed Macrosteles divisus and still later, M. fascifrons, spreads aster yellows. Other insects that feed on the China aster, including several other leafhoppers, do not spread the disease. Newly hatched aster leafhoppers, even those from parents that carry the virus, are invariably virus-free. They cannot spread the disease until they obtain virus from a diseased plant and, surprising as it may seem, they are never able to transmit immediately after first feeding on such a plant. A rather long period, usually 9 to 14 days, has to elapse between the time when the virus-free insects first feed on a diseased plant and the time when they are first able to transmit. Once this period (which has come to be known as the incubation period of the virus in the insect) is completed, the insect usually is able to transmit continuously as long as it lives without again feeding on a diseased plant.

Strong colonies of the aster leafhopper have been maintained continuously since 1924, when we learned that it spreads the disease. The availability of the insect, its high efficiency as a vector, and its capacity to live and breed on many different species of plants made it suitable for use in studies of virus transmission. The sensitivity of the virus of aster yellows to moderately high temperatures made possible its destruction in whole or in part in both diseased plants and virus-bearing insects without undue harm to either, and thus gave an excellent experimental approach for studies on virus concentration in both plants and insects. Those characteristics of the aster leafhopper and the virus combined to give them a special suitability for use in work on the insect-vector relationship.