Consequently the search was continued for safer methods of field control that would be as effective as the selenate treatment. Once again industry and the entomologists provided the answer. Shortly after the end of the Second World War a material known as parathion, which had been developed by the Germans, was brought to this country to be tested. Research workers found that it was highly effective as an insecticide and it could be absorbed to some extent by the sprayed foliage. Nearly perfect control of leaf nematodes was achieved with about four spray applications. Parathion also proved to be compatible with ferbam, so that a single combination spray could be employed to control leaf spot, rust (against which ferbam had proved to be effective), foliar nematodes, and most of the more important chrysanthemum insects.

UNLIKE septoria leaf spot and the leaf nematode disease, which are caused by organisms that make a direct attack on the leaves, verticillium wilt is caused by a soil-inhabiting fungus that invades the water-conducting tissues of the plant, grows upward in them, and eventually causes the leaves to wilt. It may attack through the roots of healthy plants set in infested soil, or it may be introduced to healthy soil in cuttings taken from diseased plants.

Before 1940 the disease was to be found in nearly all greenhouses in which chrysanthemums were grown, its severity varying with variety. With the more susceptible ones it was common to see entire crops rendered essentially worthless. Many such varieties, otherwise highly desirable, were discarded because of the seeming impossibility of freeing them of wilt.

The difficulty in obtaining adequate control lay in the fact that the disease is readily transmitted through cuttings or divisions taken from diseased plants, and, further, that the disease is rarely obvious in the parent plants at the time cuttings are taken.

Control procedures consisted of roguing out obviously diseased parent plants in the fall, when symptoms are most evident; taking terminal cuttings from rapidly growing, vigorous shoots; and planting in new or sterilized soil. Those procedures provided a measure of control and still yielded results far better than mere chance selection, but at best the degree of control left much to be desired. A more satisfactory solution to the problem was developed by the adaptation of a common laboratory procedure of plant pathology to the needs of the industry.

As one phase of an investigation on the verticillium disease it was desired to determine how closely the growth of the fungus within the plant kept pace with the growth of the shoots during the normal season of propagation. Laboratory cultures were made on nutrient agar of segments of the shoots at various distances back of the growing point. If the fungus grew from the segments it was obviously present within the water-conducting tissues at that point. If the fungus did not grow out, it was felt to be a safe assumption that the fungus growth had not advanced to the particular point in question. The results confirmed the reliability of the method and further showed that a fair percentage of the shoots from diseased parent plants either were free of the fungus or were infected only at the base. A most significant fact was that culturing provided the only means of distinguishing between healthy and infected shoots superficially they were indistinguishable.

What possibility was there that this procedure of culturing from the base of prospective cuttings might provide a means of reliably selecting verticillium-free cuttings to be used as parent plants for subsequent propagation of wilt-free flowering stock? Although it was felt that such a process might not be adaptable to commercial practice, a test was set up in which a large block Of cultured plants of a very susceptible variety were compared with a similar block of plants propagated in the usual manner from the same parent plants. The results were outstanding a high percentage of the noncultured plants were severely diseased, but nearly 100 percent of the cultured cuttings remained healthy.

A representative of a large chrysanthemum-plant-producing concern saw the test. He started at once to develop a program of culturing of the parent stocks of the 300-odd varieties offered by his firm. Many problems had to be solved, many cultural practices had to be changed, and a staff had to be trained in specialized techniques before the process could be put on a mass-production basis. But through the efforts of the firm's own scientists and cooperation with experiment station workers, a satisfactory program was developed. The cost was high, but the results were satisfactory.

The benefits of this undertaking by a single firm have been reaped by nearly all growers of chrysanthemums throughout the country because that firm supplies much of the planting stock of all the commercial chrysanthemums grown. Serious cases of verticillium disease have been rare in commercial greenhouses since the program was developed, and a number of the fine varieties that had been discarded because of their susceptibility to the disease have been reintroduced. In fairness it must be stated that the full value of cultured stock is realized only in greenhouse culture, where the soil can be sterilized to free it of the wilt fungus. In field plantings, where adequate sterilization is as yet impractical, susceptible varieties may still suffer seriously even though the planting stock is free of the fungus.

The stories of the conquest of these three diseases of chrysanthemums might well be joined by those of equally successful attacks on two other major diseases virus stunt and mycosphaerella blossom blight. Mastery of these diseases, paralleled by similarly effective development of insect control procedures and improvement in cultural practices, has eliminated the major hazards in chrysanthemum cultivation and has put the industry on a sound, reasonably predictable, and profitable basis.

A. W. Dimock, professor of plant pathology at Cornell University, has specialized in the study of the diseases of ornamental Plants since 1937.