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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).
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Figure 1. Tulip Fire--a Primary Infector or "Firehead".
Note distorted, twisting leaves.
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Figure 2. Older, Enlarging Botrytis Lesions on Mature
Tulip Leaf. Note gray mold (photo courtesy G.W. Simone).
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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).
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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.
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Figure 3. Botrytis Blight or Fire of Tulip (British
Ministry of Agriculture photo).
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Figure 4. Older, Enlarged Botrytis Lesions on Mature
Tulip Leaf (photo courtesy, Dr. C.U. Gould, Washington
State Univ.).
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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.
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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.
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Figure 5. Botrytis Spots on Tulip Flower Petals.
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Figure 6. Infected Tulip Bulb with Scab-like Spots
and Black Sclerotia on Outer White Scales (photo courtesy,
Dr. C.J. Gould, Washington State Univ).
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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.
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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
The
tremendous number of microscopic spores (conidia) formed on primary
infectors are spread by air currents and splashing raindrops. The
spores can germinate and infect at temperatures not much above freezing.
Spore production, germination, infection, and the resulting mycelial
growth in the infected plant are all more rapid when the temperatures
are higher provided that the humidity is above 95 percent. These
conditions are quite common in mid to late spring when sharp temperature
drops at night result in heavy dews and when a film of water is
present on aboveground plant parts. Retarding air movement (i.e.,
crowded plantings, excessive weeds, or poor locations) favors infection.
How far the conidia of Botrytis tulipae can be carried by
the wind and still remain capable of causing infection is not known.
It is certain that the spread from one flower bed to neighboring
beds is common. Spores have also been shown to survive for up to
six weeks on the surface of moist soil at 50°F (10°C). The
fungus is active over a fairly wide temperature range with sporulation
occurring between 41° and 81°F (5°-27°C).
Control
- Purchase only the largest, blemish- and disease-free bulbs
available. Buy from a reputable nursery or garden supply store.
- 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.
- 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.
- 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).
- Examine all bulbs carefully before storage and again before
planting. Discard all diseased, bruised, and cut ones.
- When tulips come up in late winter or early spring, carefully
remove all infected plants and plant parts as soon as they are
noticed. Place in a paper bag or other covered container, and
burn them. If possible, these important sanitary measures should
be done in dry weather when the fungus is not producing spores.
- Collect and destroy all leaves, blossoms, and stems as soon
as blooming is over. Entire tops should be cut off an inch or
so below the soil surface, removed, and burned.
- Fungicide sprays are effective as a preventive measure, starting
when the leaves emerge from the soil. Spray several times at 5-
to 10-day intervals and continue until the bloom stage. If the
weather is rainy, spray every five days; if conditions are dry,
every 10 days. The addition of a half teaspoonful of liquid household
detergent or a commercial spreader-sticker (surfactant) to each
gallon of spray helps to ensure that the foliage will be wetted
and that good coverage will be obtained. Fungicide recommendations
are given in the Illinois Pest Control Handbook which is
revised each year, and available at all Illinois Extension offices.
Dipping bulbs in a fungicide has given very good control of fireheads,
as has a soil drench at planting time and again just before emergence.
- When forcing tulips indoors, observe these precautions: water
in the morning rather than in the afternoon; keep water off the
foliage; hold the humidity below 90 percent; and avoid forcing
the tulips at too high a temperature. The temperature should be
kept as uniform as possible to avoid condensation (dew) at night.
Provide for good air circulation. Promptly remove and destroy
all diseased plants and fading flower heads. Disinfect flats in
a fungicide solution and dry before separating. Do not plant susceptible
cultivars close together. Dip bulbs in a recommended fungicide.
Either use fresh soil or pasteurize reused soil. Do not place
flats in wet locations during rooting of the bulbs.
Further
information concerning diseases of ornamental plants can be obtained
by contacting Nancy R. Pataky, Extension Specialist in Plant Pathology
and Director of the Plant Disease Clinic, Department of Crop Sciences,
University of Illinois at Urbana-Champaign.
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