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How to Manage Pests
UC Pest Management Guidelines
Floriculture and Ornamental Nurseries
Gray Mold
Pathogen: Botrytis cinerea
(Reviewed 1/02,
updated 1/02)
In this Guideline:
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SYMPTOMS
Gray mold is one of the more destructive plant pathogens and attacks a wide
variety of plants. Flower petals and ripening fruits and vegetables are particularly susceptible to
infection, but leaves and
stem tissues also may be
infected, and young seedlings of several crops can be killed. Under conditions
of high relative humidity, the fungus may sporulate on infected tissues and
produce masses of characteristic gray or brownish
spores that become airborne and
are the primary means by which the fungus is disseminated. Spores must have
moisture to germinate and infect.
COMMENTS ON THE DISEASE
Botrytis does not invade healthy green tissue such as leaves and stems
unless (a) an injured or dead area is present, or (b) it grows directly from a
food base such as a fallen petal or leaf. The fungus will first colonize the
food base and then attack healthy tissues. A food base is not required by the
fungus for invasion of flower petals such as African violet, asters, begonia,
carnation, chrysanthemum, cyclamen, cymbidium, gerbera, geranium, gladiolus,
hydrangea, marigolds, orchids, petunia, poinsettia, primrose, ranunculus, rose,
snapdragon, zinnia, and others.
Although the fungus is capable of growth within a wide range of temperatures
from about 28° to 90°F, growth is very slow at the extremes. Optimum
temperature range for growth is 70° to 77°F. The fungus, which is
more active below 70° than it is above 77°F, is particularly
troublesome under conditions of moderate temperature and high humidity.
MANAGEMENT
Refrigeration at temperatures near 32°F will retard but not completely
stop the development of gray mold and when infected tissue is warmed, decay can
proceed rapidly.
Moisture often is more of a limiting factor than temperature. Free moisture is
necessary for germination of Botrytis spores. Moisture is also necessary
for growth within plant tissues, and low humidity may result in arrested growth
of the fungus. However, growth can resume when moisture again becomes
available.
Gray mold is most severe during times of the year when the humidity is high. In
California, this is usually in the late fall and winter months when rainfall is
common. The worst time for disease development is from September to December
because there is an abundant amount of herbaceous vegetative material (crop
refuse and dying summer plants) available for fungal colonization and, as a
consequence, many spores are present in the air and on plant parts.
Cultural Control
Botrytis cinerea produces innumerable asexual spores (conidia) that are
moved about by air currents. Because spores may originate in decaying
vegetation and old flowers, elimination or reduction of sources of the spores
is an important part of any control program. Also, removing old flowers before
they become infected and function as spore sources can be important and
sometimes essential to control. The fungus can develop and sporulate at low
temperatures, so do not overlook old flowers and foliage in refrigerators.
Because free moisture is necessary for germination and infection, great
emphasis is placed on avoiding condensation of water on susceptible plant
parts. Avoid overhead watering during blooming. If this is the only method of
irrigation available, then do it early in the day so that the foliage can dry
as rapidly as possible. Also, extend the period between irrigations to the
maximum consistent with good growth. Wider
plant spacings to increase
ventilation and minimize leaf
wetness can also help reduce disease incidence.
Chemical Control
Numerous fungicides are effective against Botrytis cinerea but not all
of them can be used on all crops. With some, this is because of damage to the
plants or some plant parts. To avoid damage and the development of fungal
strains that are resistant to fungicides, some growers alternate different
fungicides. The fungicides are preventatives and must be applied before
infection. In some crops, such as chrysanthemum, the lower foliage of crowded
plants becomes infected and acts as a source of spores that then infect the
flowers. In these crops, it is important to apply fungicides at an early stage
when the lower foliage can be adequately covered by the chemical.
| Common name |
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Amount to Use |
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| A. |
FENHEXAMID |
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(Decree) |
16 oz/100 gal water |
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COMMENTS: Apply as a spray; very effective and can be
applied after infection. |
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| B. |
AZOXYSTROBIN |
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(Heritage) |
1-4 oz/100 gal water |
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COMMENTS: Apply as a broadcast or banded spray targeted
at the foliage or crown of the plant. |
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| C. |
IPRODIONE |
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(Chipco 26019) |
0.4 lb/100 gal water |
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COMMENTS: Apply as a drench (1-2 pt/sq ft) at seeding
or transplanting. Some resistance has been reported with this material. |
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| D. |
THIOPHANATE METHYL |
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(FungoFlo, Cleary's 3336F, etc.) |
20 fl oz/100 gal water |
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COMMENTS: Apply as a drench or heavy spray (1-2 pt/sq
ft). Some resistance has been reported with this material. |
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| E. |
CHLOROTHALONIL |
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(Daconil Weather Stick) 54% |
1.375 pt/100 gal water |
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COMMENTS: Offers protection against Botrytis. Do
not apply to either green or variegated Pittosporum or to Schefflera. |
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| F. |
MANCOZEB |
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(Dithane) 75W |
1-1.5 lb/100 gal water |
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UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
R. D. Raabe (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego Co.
A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM),
UC Berkeley
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