THE LIFE HISTORY of the fungus is similar to that of other wilt Fusaria. The organism persists in the soil, even in the absence of aster plants, for many years and reduces the usefulness of the land for the crop. The disease potential may decline after 5 years, but it builds up rapidly with return of the crop. The fungus is carried from place to place on tools and workmen's shoes, in irrigation water, and perhaps with soil particles by the wind.

Conidia formed on the stems are sticky and probably not airborne, although readily spread by water. These spores can germinate at once or remain alive under dry conditions for several months. The fungus can persist in plant refuse as dormant, thick-walled chlamydospores and be spread through the medium of the compost pile. These spores are more resistant to drying, heat, and fungicides than are the Conidia or mycelium. The fungus also is commonly distributed with transplants from the seedbeds, where it may or may not have had time to produce disease. An infested seedbed may therefore introduce the disease to several fields or home yards.

The more delicate type of spore (conidium) is carried on the surface of the seeds. Presumably it gets there during threshing operations, either from dust or from the pink masses on stem bases. Spores are also carried on debris mixed with the seed. Whether a crop grown from contaminated seed will show wilt depends in part on soil temperature; it may do so at 77 F. but appear tardily or not at all at 60 . In either case the soil becomes infested. The fungus establishes most readily in soil freed (i. e., pasteurized) of other competitive organisms, and will produce heaviest losses in such soil. This seed-borne fungus is initially most destructive in steamed soil under warm conditions, and is thus most rapidly damaging in glass houses and seedbeds.

Plants are susceptible to infection at any time from germination to maturity. Infection generally occurs through uninjured roots, but inoculations may be made into stem wounds. Infection in most instances is between the cells of the root cap and between the epidermal cells in the region of elongation.

In susceptible plants the fungus develops copiously in the xylem of the root, from which it spreads upward through the xylem of the stems. From the xylem it may spread outward into phloem and cortex in roots or stems. Stem cankers thus arise in moist, warm weather. In resistant plants there is little development of mycelium in the root tip, and it remains localized, though still alive, in tiny lesions. Resistance is of the physiological type, as in cabbage and tomato fusarium wilts, rather than of the morphological type. It is reported that lower losses result from seeding in place than from transplanting, but the results in all instances may have been due to higher soil temperature in the seedbed than in the field, rather than to transplanting injuries.

The damage to the plant results from toxins formed in the water-conducting tissues and carried to the leaves and stems, where cells are injured or killed. Insufficient mycelium is formed to block the xylem vessels and interfere seriously with water movement.

ENVIRONMENTAL CONDITIONS are important in determining the severity of the disease. Soil temperature is of such significance as to determine largely whether. the disease occurs or not. Thus A. B. Jackson found in 1927 that parallel series in naturally infested soil at 63 -68 F. and 68 -77 developed 12.5 and 100 percent wilt, respectively. The optimum for growth of the fungus in culture is about 80 -86 , and no growth is produced at 39 or 95 . No symptoms other than slight loss of weight develop on asters grown in infested soil at 54 . Slight wilt develops at 61 . The optimum for wilt is about 80 , and the maximum is 90 .

Thus, even in heavily infested soil at about 55 , plants will sustain little loss from fusarium wilt, and only in heavily infested soil will symptoms develop at about 60 . On the other hand, lightly infested soil (such as clean soil planted with infested seed) will produce wilt plants if temperatures are 77 to 80 . To a certain extent, then, temperature and amount of fungus in the soil are interchangeable factors an increase of one will offset a decrease of the other.

A certain length of time is required for wilt symptoms to appear. The lower the soil temperature or soil infestation, the longer that interval will be. Other things being equal, the shorter the exposure to heavily infested warm soil, the less will be the disease. However, winter planting of asters in coastal California has led to excessive losses from botrytis gray mold and rhizoctonia stem rot. The losses in a given field grown continuously to asters will vary greatly from year to year, depending on soil temperatures. Asters grown in the cool soils along the coast of California consistently have less wilt than those grown in the warm soils of the interior valleys of that State and are undesirable for a wilt-selection program.

A new commercial area planted to asters normally may pass through several stages. In the first year scattered diseased plants will appear through the field, their number depending on whether spread is restricted by seed sown in place and whether infected transplants were used and on the prevailing soil temperatures. The next year the disease will appear more uniformly through the field; if the soil is warm, severe losses may occur. By the third season a profitable crop will be produced only if the soil remains very cool. Usually the venture is then abandoned as uneconomic.

THE CONTROL of fusarium wilt is made difficult by the unavailability of commercial asters of high resistance, although advertisements may claim the contrary. Should truly resistant varieties again become available they would provide the ideal solution for home-yard and commercial plantings. In the meantime some benefit may be gained by using seed saved from the few plants surviving in heavily infested warm soil.

The protection of uninfected soil from introduction of the wilt fungus is highly important. Treatment of seed is essential in this, but one should recognize that it is done to protect the soil from infestation from the seed, rather than the seedling from the soil-borne fungus.

The best treatment at present is a mercuric chloride (corrosive sublimate) soak. A glass jar is filled about one-third full of seed and filled up with corrosive sublimate (1 to 1,000, or one 7 1/2-grain tablet per pint of water), unheated and without any spreading agent. The lid is replaced and the seed intermittently shaken for 30 minutes. Then the fungicide is poured out through a cheesecloth held over the top of the jar. Three separate changes of water are used in rinsing. The jar is shaken each time. The seed is then spread out to dry in a warm place. The treatment lowers germination somewhat, particularly of seed whose cracked coats permit the mercury to reach the embryo. Heating the solution above room temperature is highly injurious. W. O. Gloyer found that seed treatment permitted the growing of asters in the same field in New York for five consecutive years without wilt.

To avoid infestation of land it is further necessary to prevent carrying the fungus on tools, machinery, shoes, and such. Formaldehyde solution (1 part commercial formaldehyde in 15 parts of water), may be prepared for dipping infested tools, for applying to shoes, and for spraying machinery before it is used on clean land.

Once soil becomes infested, three courses are open if resistant lines are not available: It may be abandoned for asters; it may be used once every five or more years for this crop, being rotated with any other in the interim; the soil may be steamed or treated chemically to free it of the organism.

The third is imperative for seedbeds as a routine procedure. If steam is used, the soil should be heated to at least 180 F. and held at this level for 30 minutes. A formaldehyde drench (diluted 1 to 40) has been recommended; enough of it is applied to penetrate the soil to at least an 18-inch level. The soil is covered with canvas for 2 days, and then aerated for at least a week before planting.

Old infected plant parts, in which the fungus can survive for at least a year, should be burned. They should never be used in a compost pile unless it is to be steamed before use.

Because of the restrictive effect of cool soil on the occurrence of wilt, losses may be reduced by shading if shading keeps soil temperatures near 60 F.

KENNETH F. BAKER, a plant pathologist in the California Agricultural Experiment Station, Los Angeles, has been engaged in studies on diseases of floricultural crops for more than 15 years. He has conducted pathological investigations in Washington, Wisconsin, Nebraska, Hawaii, New York, and California.