Fusarium Wilt of China Aster

Kenneth F. Baker.

The fusarium wilt disease stem rot, black stem, aster wilt caused by Fusarium oxysporum f. callistephi, is generally the most serious disease of China aster (Callistephus chinensis) in commercial and home-yard plantings over most of the world.

The China aster was introduced from China into France about 1731 and into the United States before 1800. Commercial aster culture had developed in this country by 1890. The first definite report of fusarium wilt was made by B. T. Galloway from several places in northeastern United States in 1896, but it probably was present and unnoticed before then. The fungus was widely distributed during the next 25 years, probably with the seed.

In many places it is the factor that determines whether asters can be successfully grown. Soil once infested with the fungus is essentially ruined for growing asters. Growers in California, therefore, avoid land that is known ever to have been used for the crop. Some home gardeners have found asters so unreliable because of disease that they are replacing them with zinnias.

Losses result from three phases of the disease: Damping-off of the seedlings at any time from emergence to time of transplanting; the typical wilt of mature plants; and the decay of flowers in storage. Wilt of mature plants usually causes the greatest loss, but damping-off is important because seedlings frequently are infected in the seedbed and fail to show the disease until much later, when they have already infested the land with the fungus.

THE SYMPTOMS of fusarium wilt may appear at any stage of plant growth. Following emergence, seedlings may fall over from damping-off caused by the fungus. The warmer the soil (up to about 80 F.) and the greater the soil infestation, the sooner that will occur. The stems of tiny seedlings may rot near the soil surface and the cotyledons wither. Symptoms may be delayed at lower soil temperatures or lesser soil infestation, or with somewhat resistant plants.

Diseased seedlings about 2 inches high may show nothing more specific than the death and browning of basal leaves, stunted plant growth, drooping of leaves during the heat of the day, and perhaps poor green color. The typical diseased plant 4 inches or more high has a black streak up one side of the stem, and all leaves originating from the streak are dead. A single branch may be killed.

Plants grown in heavily infested soil or at high soil temperatures, however, usually have dead leaves on a blackened stem base, and do not have unilateral development. In a dry climate, as in southern California, such stems may be harder and woodier than those of healthy plants. In a humid area that symptom may be lacking, but just above soil level there is often a pink crust consisting of masses of mycelium and spores of the fungus. The roots usually have few or no rotted areas unless they are growing in very wet soil. When infected stems are cut longitudinally, one can see brown or black streaks that extend upward through the woody tissues. The black color of the basal stem may extend into the pith.

All those symptoms do not necessarily appear in any one plant, but most of them may be observed in a large planting. The one-sided development, the black streak in the stem, and the pink spore mass at soil level are the most dependable symptoms.

Other diseases may be confused with fusarium wilt.

Verticillium wilt causes much the same symptoms but lacks the pink basal spore layer.

Phytophthora root rot may cause death of tops, but the roots are then discolored and decayed, whereas fusarium wilt plants have nearly clean roots.

Phomopsis stem canker in humid areas causes a basal stem canker, in which tiny black fungus fruiting bodies are seen; the pink spore crust, the one-sided development, and dark streaks in the wood are lacking.

A stem rot caused by Fusarium roseum produces pink spore crusts on the cankers, but it does not extend up one side of the stem nor become vascular. Symptoms progress from outside inward, in contrast to wilt that develops outwardly from the inner tissues.

The spotted wilt virus causes dead tan areas in leaves and brown surface blotches on stems. Although symptoms may be unilateral, no black streak or pink spore crust is produced.

Root aphids sometimes cause wilting of the plants, but can be recognized by the insects on the roots.

Stem miners can produce wilting of aster plants in California. The tiny tunnels formed in the pith differentiate this trouble from fusarium wilt.

Flowers produced on plants infected by Fusarium under cool conditions may not show obvious external symptoms. The fungus has been found by W. C. Snyder to grow out from infected stems of such flowers and cause them to decay in warm storage.

The host range of the pathogen of fusarium wilt is not known to include plants other than China aster, nor does it even include all varieties of the single species, Callistephus chinensis. Varietal differences in susceptibility have been noted by many workers.

Resistant varieties have been developed several times, only to be lost when introduced into commerce. After 1925, efforts to develop resistant varieties by selection on heavily infested land were conducted by L. O. Kunkel at Yonkers, N. Y., L. R. Jones and R. S. Riker at Madison, Wis., E. R. Honeywell in Indiana, and D. J. MacLeod at Fredrickton, New Brunswick.

Mr. Kunkel's selections were turned over to Jones and Riker, who supplied material to seed companies in California and Illinois in a cooperative arrangement from 1929 to 1932.

The resistant varieties first offered to the trade in 1931 were American Branching Mary Semple, A. B. Azure Blue, Ostrich Feather Deep Rose, Crego Deep Rose, Royal Azure Blue, and Heart of France. More recently, P. E. Tilford at Wooster, Ohio (1940-42) and K. F. Baker at Los Angeles (1940-44) selected resistant plants under more favorable conditions for elimination of susceptible plants.

In the latter instance, only 0.75 percent of 78,000 plants of nearly 100 varieties survived in 1942, while 38.3 percent of 13,500 plants grown from 1942 selections survived in 1943. We have considerable evidence that satisfactory resistance can be obtained in three to five selection generations in heavily infested warm soil. Mr. Tilford's selections were marketed by the Ohio Florists' Association until 1950, when they were discontinued. Mr. Baker's selections were released to several seed companies.

Despite those and other efforts, one seedsman wrote in 1948: "Asters today are at their lowest ebb . . . for petalage and susceptibility to stem rot. Little is usually done except order `rot resistant' strains, and regardless of where they come from, all too frequently they are disappointing."

Resistance was lost through one or more of the following factors:

(1) Making single-plant selections for flower type under conditions of very low wilt losses and correspondingly great risk of selecting susceptible plants. Selections thus made in commercial seed fields where soil was either poorly infested or too cool for severe wilt have lost resistance in a single generation.

(2) Planting in cool or poorly infested soil in all or in most years. Resistance to this disease is not stable or homozygous, and continuous selection is necessary to maintain it. The variable low percentage of natural crossing among even highly double varieties in seed fields contributes to this instability.

(3) Expense of maintaining resistance, which involves special procedures and single-plant lines. To develop or maintain resistant lines one must be able to sacrifice plants and seed yield. That in turn means more costly seed. Resistance consequently is basically an economic problem.

(4) Use of alternative plants, such as zinnia, in home yards and the disinfestation of soil and seed in commercial plantings have reduced the demand for resistant varieties.

The China aster is peculiar among hosts of wilt Fusaria in that it is the sole representative of the host genus and probably was developed from plants cultivated from antiquity by the Chinese. Indeed, it is not certain that the true wild progenitor is even represented in herbaria. It is possible that the full complement of potential resistance factors of the genus is not represented in the present commercial aster, and that if such factors were incorporated into commercial varieties a stable high-level resistance could be maintained.

Present commercial resistance seems to be best explained as the result of more than one genetic factor, perhaps several, which may be cumulative in effect. There is no correlation between resistance and flower color, but single varieties such as Single Chinensis and Rainbow types often, but not always, are quite resistant.

THE CAUSAL FUNGUS is Fusarium oxysporum f. callistephi. Other Fusaria (F. roseum, F. lateritium, F. solani, and F. episphaeria) have also been suggested ascausal, but it is now known that some of them (F. roseum in particular) produce stem decay under moist conditions, rather than a vascular wilt of China aster. In earlier literature the wilt Fusarium has gone under such names (now synonyms of F. oxysporum f. callistephi) as F. conglutinans var. callistephi and F. conglutinans var. majus. Likewise the existence of several strains of forma callistephi has been postulated in explanation of the "breakdown of resistance" in commercial varieties since 1930. The assumption here that the level of resistance of aster varieties has not changed since 1930 is unwarranted, however.

Riker and Jones in 1935 presented evidence for the existence in Indiana and ,Japan of fungus strains of greater virulence to aster than those from Germany, Canada, and other sections of this country. It is not known, however, whether they were dealing with a greater degree of virulence or with a different type of pathogenicity.

Different isolates of the aster wilt Fusarium, as well as those from other hosts, are known to vary from weakly to strongly parasitic. In practice this affects severity of injury but not the host range, because a plant truly resistant to a virulent isolate under the most favorable conditions will not be susceptible under any circumstances. This differs greatly from type of pathogenicity (selective pathogenicity), in which the parasite has the ability to attack a new host, or against which a new set of genetic factors is necessary for resistance. This latter situation is clearly known among vascular Fusaria in garden pea and tomato, but is thought to be uncommon.