Reports on Plant Diseases

RPD No. 609 - Tulip Fire or Botrytis Blight

March 2000

Tulip fire or Botrytis blight is caused by the fungus Botrytis tulipae. It attacks all parts of the plant and is by far the most common and serious disease wherever this popular flower is grown. Once a tulip bed is infested, fire or Botrytis blight generally becomes more serious in succeeding crops. The disease commonly follows frost or hail injury. If not controlled, tulip fire can cause an almost complete loss of flowers and greatly reduce the yields of bulbs during cool, wet spring weather. Attacks are confined to the tulip (Tulipa gesneriana), other Tulipa species, and hybrid tulips. All commercially grown tulip cultivars and species are susceptible to some degree.

Symptoms

The first evidence of disease in the spring is usually the appearance of scattered stunted shoots, called primary infectors or "fireheads", that emerge with their leaves twisted, tightly rolled, and blighted (Figure 1).

The weakened shoot often collapses and dies. In damp, overcast weather a dense grayish mold develops on these primary infectors. The mold is largely composed of tremendous numbers of microscopic spores (conidia) of Botrytis. If the blighted leaves and shoots do manage to unfold, they are "ragged" and partly withered with a fuzzy grayish mold forming on diseased parts in moist weather (Figure 2).

Minute spots soon appear on the leaves of other nearby tulip plants (Figure 3). These spots are oval to round and turn yellow to gray-brown, each with a dark, water-soaked border. Similar, more elongated spots appear on the stems. In wet weather some leaf lesions enlarge rapidly and merge, turn whitish gray with a brownish tinge, and cover a large part or all of a leaf (Figure 4). In dry weather, invaded leaf tissue becomes brittle and is often split and torn by the wind. Leaf infections may spread into the stem where gray to brown, depressed, and often zonate spots are formed. Weakened stems often collapse and die.

Flower buds are spotted. Badly blighted ones fail to open and become covered with the dense gray mold of Botrytis. Small, whitish, somewhat blistered spots develop on colored tulip flowers, with light yellow to tan lesions forming on white petals within 10 hours after infection (Figure 5). If the weather continues to be damp, the lesions soon enlarge, turn a deeper brown, and merge. Within a few days, a flower may become completely blighted. During or following wet weather, blighted flowers are covered with the typical gray mold.

Shiny, black bodies (sclerotia) the size of a pin head of Botrytis tulipae develop on or under the outer husk which may be discolored and split. Dark yellow to brown, round, slightly sunken "scabby" spots commonly form in the outermost flesh bulb scale. The lesions are usually on the side of the bulbs, but may occur at the nose or base of the bulb. The minute black sclerotia may form in the larger scab-like bulb lesions (Figure 6), in tulip leaves and flower parts rotting on the soil surface, and on flower stems. Infected stems and/or bulbs may cause leaves to turn reddish to purple without any spotting.

Click on image for larger version

Figure 3. Botrytis blight
or fire of tulip (British
Ministry of
Agriculture photo).

If damp, the Botrytis fungus may penetrate deeply into the bulb and rot it completely. Under drier conditions, the bulb lesions become more or less dormant and the bulb survives until planting time. When planted, such infected bulbs may rot without growing a shoot, produce a stunted blighted plant (Figures 1 and 2), or give rise to a healthy plant.

Back to Top

Disease Cycle

The most common source of infection each spring is the stunted and blighted tulip plants (primary infectors) that have grown from diseased bulbs accidentally planted with the crop. Other sources of infection arise from sclerotia of Botrytis tulipae germinating in the soil, decaying tulip stems, and other plant parts. If tulips are planted in contaminated soil within two years, there is considerable risk that they will become infected.

Plants that emerge after being infected in the soil are usually less seriously affected than those arising from infected bulbs, but they also serve as primary infectors. When an infected bulb produces a healthy shoot, normal development of daughter bulbs occurs, and the Botrytis fungus can spread to these from the mother bulb. By late spring, the outermost scale of the daughter bulbs is white and fleshy, later turning brown and dry.

If the fungus penetrates beneath the outer scale at this stage, bulb lesions may form and remain hidden, unless the brown husks are removed. Daughter bulbs thus infected often escape detection and are replanted in the fall. The Botrytis fungus can survive in lesions on the outermost white bulb scale and can resume growth when the bulb is planted. The fungus persists longer under cool and moist conditions, than when they are stored where it is warm and dry

Control

1. Purchase only the largest, blemish- and disease-free bulbs available. Buy from a reputable nursery or garden supply store.

2. Plant tulips in the same location no oftener than every third year. The spot should be sunny where air circulation and soil drainage are good. Remove the outer brown husks, and discard all spotted, damaged, or moldy bulbs. Avoid a wet mulch, overwatering and high rates of nitrogen fertilizer. Fertilize based on a soil test. Keep tulip beds weeded to increase circulation.

3. Dig the bulbs in dry weather and not later than three weeks after the petals have fallen. Remove the stems and handle the bulbs with care. Infection occurs more easily on bruised and cut bulbs than on undamaged bulbs.

4. Dry and clean the bulbs promptly after digging and before storing in thin layers in a dry, well-ventilated location. Commercial concerns usually store their bulbs after receiving them in early autumn at 77° to 81° F (25°-27° C) until November 1, and thereafter at 63° F (17° C).