Blights of Lilies and Tulips

C. J. Gould.

Whether one grows tulips or lilies for fun or for profit in Brooklyn, Keokuk, or Seattle; in the garden, field, or greenhouse he probably has more grief from botrytis blights than from any other cause.

The blights, or "fire" as they are sometimes called, are world-wide in distribution and most prevalent in cool, moist areas. They are caused by similar, but distinct, fungi. Most common on tulips is Botrytis tulipae. Most common on lilies is Botrytis elliptica.

Tulip blight begins with a diseased bulb or with contaminated soil in which the fungus is living on old remains of the tulip plant. As the young tulip shoot pushes through the ground, it becomes infected if it comes in contact with the fungus and may be changed into a distorted, grayish-brown body covered with powdery masses of spores. Sometimes only part of a leaf is diseased. Often the entire shoot is affected. Soon the leaves of the nearby tulip plants become spotted with small, circular, yellow or brown dots. Many of the spots remain small and dry up, but others, (especially if the weather is cool and moist) enlarge rapidly. Their color becomes a grayish brown or brown, with a dark water-soaked margin. Powdery masses of spores often form in the center. These, when blown to other plants, initiate more spots, which may become visible within 24 hours.

Large spots near the leaf base often cause the leaf to break. Several spots may fuse and decay a leaf completely. Spots on stems are similar and may also cause the stems to break. Flowers are very susceptible and often are made unsalable by white or brown spots covered with spores. Not even the bulb escapes. On its outer fleshy scale the fungus may produce sunken, yellow or brown craters. In them, on the outer bulb husk, and on spots on the upper plant parts, small, hard, black circular masses of the fungus may develop. Such black bodies, the sclerotia, have a function somewhat similar to seeds in that they enable the fungus to live over winter, either on the bulb or on dead tulip parts. Then, when infection from the sclerotia occurs in the spring, the cycle is continued by the spores.

The disease probably occurs wherever tulips are grown, but it is most serious when conditions are cool and moist. Consequently it is prevalent in the Netherlands, southwestern England, and the Pacific Northwest.

Local conditions, however, are important: In western Washington, for example, the disease is more serious in the valleys near Puyallup than it is 90 miles to the north near Mount Vernon, where the plants are exposed to a more or less constant wind from Puget Sound.

The method of planting tulips commercially in beds about 3 feet wide, as practiced in Holland and some parts of the United States, is more conducive to botrytis blight than is the row system used in the Pacific Northwest. Not only is the relative humidity higher in the beds, but there is also a greater opportunity for infection. Likewise the disease is often found in gardens in massed plantings or on plants in shaded places.

Most, if not all, tulip varieties are susceptible, but there is some variation. Among varieties that are very susceptible are the popular William Copland, William Pitt, and Bartigon. Baronne de la Tonnage is supposed to be resistant. This species of Botrytis can also attack bulbous plants other than tulips under certain conditions according to tests by Neil A. MacLean at Washington State College. Other species of Botrytis, including B. cinerea, can sometimes attack tulips.

Although B. tulipae can attack healthy leaves, its entrance is facilitated by injuries from frost, hail, and equipment. The Dutch have tried to reduce frost injury by treating the bulbs to delay growth in the spring until the danger is partly over. This artificial retarding is accomplished by storing the bulbs at approximately 70 F. and delaying planting until October or November.

Much can be done to control the disease in commercial fields and home gardens by proper cultural practices: Digging every year; not replanting tulips in the same location for at least 3 years; planting so as to provide good air circulation and low humidity by proper selection of site, proper spacing, and thorough weeding; removing promptly and destroying diseased shoots, old and infected flowers, and dead plant remains; and planting only healthy bulbs.

Since the original infections arise from diseased bulbs that escape detection, it would seem logical to treat them with a fungicide in order to destroy the fungus. Many treatments have been tested and a few have been recommended, such as Uspulun and Aretan, in Europe. Such treatments have not met with general success, however. Either they injured the bulbs or failed to control the disease adequately.

Early attempts to prevent plants from becoming diseased by using fungicidal sprays were likewise not very successful. Although copper-containing compounds such as bordeaux mixture were found capable of controlling the fungus, they often caused considerable injury to the leaves and flowers. However, in New York in 1940 L. W. Nielson and C. E. Williamson found that silver nitrate sprays compared favorably with the 1.5-4.5-50 bordeaux mixture in fungicidal action. One spray, containing silver nitrate, manganese sulfate, and hydrated lime was adopted by bulb growers on Long Island. Meanwhile tests in 1938 in the Netherlands had shown promising results with thiram, one of the new organic-sulfur compounds. I tested it and related sulfur compounds in comparison with silver and copper fungicides at the Western Washington Agricultural Experiment Station beginning in 1942. Then and later the best results were had with ferbam, another organic sulfur compound. Large-scale tests by growers in 1943 corroborated the experimental trials. Afterwards it came into general use in the Pacific Northwest and the Netherlands.

Four applications of ferbam are usually enough in the Northwest, beginning when the shoots are 2 to 4 inches tall and continuing at 7- to 10-day intervals. The spraying must be accompanied by the prompt removal of infected plants as soon as detected. By combining the proper cultural practices and a spraying program, the disease can now be conquered.

BOTRYTIS BLIGHT OF LILIES, like tulip blight, is an old enemy. Apparently distributed all over the world, it is infectious to all species and varieties Of lilies. Some species, such as Lilium candidum, the Madonna lily, may be killed to the ground if conditions favor development of the blight. The fungus most often responsible is Botrytis elliptica, but a few other species of Botrytis, including the common B. cinerea, can parasitize lilies under certain conditions.

Although B. elliptica may occasionally rot the growing point of small plants, the usual visible symptom is a leaf spotting. The spots at first are small, circular or elongated, and brown or reddish-brown, with a yellowish or water-soaked margin. Under cool, moist conditions they may enlarge, becoming paler in color, and sometimes completely rotting the leaf. The fungus may attack stems near the soil level and so injure them that the foliage above turns yellow.

Spots on the flowers usually are brown and in cool, moist weather rapidly convert the flowers into wet, slimy masses covered with powdery layers of spores. In warm, dry weather the spots on leaves and flowers stop enlarging and dry up. With ample moisture and an optimum temperature near 60 F., however, a new spot may develop within 24 hours and new spores within a few days thereafter. Given suitable moisture and temperature conditions, the cycle may repeat itself every few days throughout the growing season. Meanwhile, hardened masses of the fungus are forming in the diseased portions. These masses, the sclerotia, are first white and later black. They may be rounded, elliptical or irregular, and one-thirty-second to one-quarter inch in size. Under favorable conditions the sclerotia germinate the following spring to produce spores.

The fungus overwinters in at least three ways: It can survive in the form of sclerotia. It can live saprophytically in lily debris. It can survive in the basal rosettes of leaves of such lilies as the Madonna lily. It may also possibly survive in non-lily hosts, since Neil A. MacLean of the State College of Washington discovered that some other plants are susceptible to the fungus. He demonstrated also that the fungus sometimes could rot lily bulbs. That implies another method of over-wintering of the fungus, although it apparently seldom happens in nature.

Moisture is necessary for the germination of spores of the fungus and also for their formation. A saturated atmosphere is required for formation at 80 F., although at lower temperatures a slightly lower humidity permits development. Under favorable conditions spores may form on infected spots and become mature within 9 hours. Germination can only occur in a film of water. Under favorable conditions the fungus can produce visible water-soaked areas within 10 hours after lodging on a leaf. Although the optimum temperature for infection is near 60 once the fungus has entered the plant it then grows best at 70 .

Since a high relative humidity is necessary for both spore formation and germination, it is evident that areas of high rainfall or heavy dews should be favorable for the fungus, provided the temperature is cool enough. Thus the heavy rainfall of the west coast and Gulf States accounts for the severity in those places; according to C. E. F. Guterman, the heavy dews explain the prevalence of the disease in Long Island and Bermuda. The disease is also common in greenhouses, especially during periods of cloudy, wet weather in the fall before heat is turned on.

Although all lilies are apparently susceptible to botrytis blight to a certain extent, they vary in susceptibility. The very susceptible species include L. candidum, L. chalcedonicum, L. humboldtii, and L. testaceum. L. speciosum is somewhat less susceptible. Among the resistant types are L. giganteum and L. willmottiae. The Easter lily, Lilium longiflorum, is classed as susceptible, but F. P. McWhorter has pointed out that the variety Ace is more resistant than the more commonly grown Croft.

CONTROL MEASURES are largely a matter of changing the factors that favor the survival of the fungus. Although not much can be done about rain and dew, it is possible to lower the humidity by planting lilies in locations with good air circulation, avoiding low or shaded spots, avoiding massed plantings, and preventing weed growth.

One should also avoid cultivating or otherwise disturbing the plants when they are wet with dew or rain. In greenhouses the judicious use of heat and ventilation together with proper watering should help keep the humidity down to a point unfavorable for the fungus.

Protection with fungicides is usually a necessary addition to good cultural practice, especially in commercial plantings and in home gardens or, such very susceptible types as the Madonna lily. Bordeaux mixture, with a wetting and sticking agent, gives good results if it is applied before the disease has become extremely serious. A weekly application is necessary in wet weather, but otherwise a spray every other week is usually sufficient.