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UC Pest Management Guidelines


Foliage damaged by Botrytis cinerea.

Floriculture and Ornamental Nurseries

Gray Mold

Pathogen: Botrytis cinerea

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

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 of 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; 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 readily develop 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, irrigate early in the day so that the foliage can dry as rapidly as possible. Also, maximize the period between irrigations to further enhance drying of foliage and flowers. 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. Some products can damage plants. To avoid damage and the development of fungal strains that are resistant to fungicides, growers should alternate different fungicides. The fungicides are preventives 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 Amount to Use R.E.I.+
(trade name)   (hours)

  Calculate impact of pesticide on air quality
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
 
A. FENHEXAMID
  (Decree) 16 oz/100 gal water 4
  MODE OF ACTION GROUP NAME (NUMBER1): Hydroxyanilide (17)
  COMMENTS: Apply as a spray; very effective and can be applied after infection.
 
B. AZOXYSTROBIN
  (Heritage) 1–4 oz/100 gal water 4
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply as a broadcast or banded spray targeted at the foliage or crown of the plant. A locally systemic fungicide.
 
C. IPRODIONE
  (Chipco 26019) 0.4 lb/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Apply as a drench (1–2 pt/sq ft) at seeding or transplanting. Some resistance has been reported with this material. Effective against Rhizoctonia damping‑off, Sclerotinia, and gray mold. Some iprodione is absorbed by plant parts.
 
D. THIOPHANATE-METHYL
  (FungoFlo, etc.) 20 fl oz/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: Apply as a drench or heavy spray (1–2 pt/sq ft). Some resistance has been reported with this material. Helps to control gray mold, Rhizoctonia diseases, cottony rot, Thielaviopsis rots, and some Cylindrocladium diseases. Thiophanate-methyl is absorbed by plant parts exposed to the chemical. Roots may absorb the fungicide (or its breakdown product carbendazim), which moves in the xylem to transpiring leaves.
 
E. CHLOROTHALONIL
  (Daconil WeatherStik) 54% 1.375 pt/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  COMMENTS: Do not apply to either green or variegated Pittosporum or to Schefflera. Effective for the control of Botrytis spp., Alternaria spp., Rhizoctonia spp., as well as other leaf-spotting fungi on many ornamentals.
 
F. MANCOZEB
  (Dithane) 75DF 1–1.5 lb/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Protects against leaf spots, Botrytis, rusts, and blight. Not systemic so thorough coverage is important for control.
 
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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