Control of Three Ills of Chrysanthemum

A. W. Dimock.

The chrysanthemum, one of the most popular and profitable of our common flowering plants, is subject to serious attack by a diverse array of disease organisms.

The common and serious diseases include, for example, a powdery mildew (Erysiphe cichoracearum), a rust (Puccinia chrysanthemi), two fungus leaf spots (septoria obesa and S. chiysanthemella), a leaf nematode disease (Aphelenchoides species), a fungus wilt (Verticillium alboatrum), two petal blights (Mycosphaerella ligulicola and Botrytis cinerea), and a virus stunt disease.

Despite the importance of chrysanthemums as garden plants and commercial cut flowers, little progress toward the solution of the important disease problems was made before 1940. It is my aim to discuss here the rapid progress made since 1940 in the control of three of the most important diseases septoria leaf spot, leaf nematode disease, and verticillium wilt.

SEPTORIA LEAF SPOT could be found almost every season in field or cloth-house plantings of chrysanthemums throughout the eastern part of the United States before 1940. In dry seasons it caused only minor damage, but in wet seasons it frequently reduced the quality of the crop so seriously that a profit could not be realized. Pathologists could easily identify the cause of the trouble but could suggest only the then current panacea, bordeaux mixture. Some times it seemed to help, but often the results were inadequate.

To one better-than-average chrysanthemum grower the situation was not at all satisfactory. He wanted to know not only how to control the disease; he also wanted to know why he was not obtaining control with bordeaux mixture. A visit by pathologists confirmed that he was spraying with bordeaux mixture and that no control was resulting. More important, it was noted that although the upper surfaces of the leaves were thoroughly coated with the bluish spray deposit, the lower surfaces were not covered. Experiments were conducted at once. They showed that nearly all infection with septoria leaf spot occurs through the lower surfaces of chrysanthemum leaves. Once this was ascertained, a simple shift of spraying practices to cover the lower leaf surfaces and forget the upper surfaces enabled the grower to obtain excellent control of the disease with the same spray material bordeaux mixture.

That still did not satisfy the grower, because bordeaux mixture leaves an unsightly deposit, which detracts from the sales appeal of the product and causes some reduction in growth. At that point, current progress in the development of new fungicides provided the answer. A newly recognized fungicide, ferric dimethyl dithiocarbamate, now known as ferbam, was being supplied for trial by a manufacturer of agricultural chemicals. Careful tests showed it to be highly effective for septoria leaf spot control, to lack objectionable residue when properly utilized, and to be non-injurious to the plants. It could safely be used as a preplanting dip for the rooted cuttings, a procedure that eliminated infection of the lower leaves, which could not be adequately protected by field sprays.

The results with the new material were good. After two seasons of use, septoria leaf spot could not be found in the 3 acres of chrysanthemums grown by the producer. The program was soon adopted generally by chrysanthemum growers throughout the United States and has been so successful that the septoria diseases, which had once been considered constant and uncontrollable hazards of chrysanthemum culture, are now rarely seen in commercial plantings.

THE LEAF NEMATODE DISEASE has been a running mate of septoria leaf spot. It is present over much the same geographical range, causes similar symptoms, and is, or was, encountered about as frequently. During wet seasons, however, it often was even more destructive than septoria leaf spot; sometimes it caused death of 100 percent of the foliage on infected plants. Although the gross effects on the plants is similar to that produced by the septoria disease, which is caused by a fungus, the nematode disease is caused by microscopic worms, which invade the leaves and feed on the cell contents. Unfortunately, the ferbam spray, which proved so effective against septoria leaf spot, had no perceptible effect against the nematode.

Investigations, initiated about 1940, confirmed that the nematodes cannot spread from infected leaves to healthy ones except in splashed water or in a film of water on the plants. Thus it appeared that if the parent plants were grown in greenhouses and if all splashing were eliminated, terminal cuttings taken from the plants should be completely free of leaf nematode infection.

That, indeed, proved to be the case. A solution was at hand to the problem of producing nematode-free young plants. It was further found that the nematodes cannot survive the winter in dead, infected leaves, but can survive in infected suckers or leaves that are sufficiently hardy or well-protected to come through the winter without being killed. If the old plants were either pulled up or plowed under rather deeply in the fall so that there was no over-winter survival of infected plants, the healthy plants brought out from the greenhouse would not become infected in the field. Careful adherence to this program has resulted in complete elimination of the disease in many plantings.

But the need for a chemical means of control in the field remained, because the practices I just mentioned cannot always be employed in the culture of the garden chrysanthemums.

At about that time, W. E. Blauvelt, of Cornell University, was developing the use of sodium selenate soil treatments for the control of insect pests of ornamental plants. In this method of control, the poisonous selenate was taken up through the roots of the plants and permeated the tissues of the leaves. Sucking insects feeding on such tissues got lethal doses of the poison.

Because leaf nematodes also were known to feed on the cell contents, it was reasoned that they also might be controlled by sodium selenate applied to the soil. Just so: The leaves of chrysanthemums grown on selenate-treated soil were essentially immune to attacks of leaf nematodes. The method was recommended primarily for the treatment of parent plants from which cuttings were to be taken; it proved to be a good means of producing nematode-free young plants, particularly in situations where splashing of the parent plants could not easily be avoided. It has also been used for field and garden plantings.