How to Manage Pests

UC Pest Management Guidelines

Tomato

Gray Mold

Pathogen: Botrytis cinerea

(Reviewed 12/13, updated 12/13)

In this Guideline:


Symptoms

Gray mold appears on young plants as gray-brown velvety mold covering stems or leaves. Infections that girdle the stem cause wilting above the infected area. In the field gray spores cover dying flowers and the calyx of fruit. Under a hand lens, the spore-bearing structures resemble bunches of grapes. Infections spread from flowers or fruit back toward the stem, which turns white and develops a canker that may girdle it.

Green fruit decays and turns light brown or gray, starting at the point where it touches other infected plant parts. Small green fruit infected directly by airborne spores instead of by contact with other infections usually does not decay, but develops white, circular rings called "'ghost spots."' Infected fruit held in storage at high humidity often decays and shows the typical gray coating of spores; it may also have a white mycelium on the surface.

Comments on the disease

Gray mold is one of the main causes of postharvest rot of fresh market tomatoes, and it occasionally affects processing tomatoes when there is a rain, heavy dew, or fog before harvest. Careful management of irrigation water keeps the disease to a minimum.

Gray mold can be a problem in greenhouse-grown tomatoes. It can develop where supporting wires and strings rub against stems, causing a wound.

The pathogen also colonizes wounded stems on tomato transplants. When these plants are planted in the field, lesions can girdle the stem, resulting in stand losses.

Botrytis spores originate in the residue of tomatoes, peppers, and weeds and are spread by wind. Spores landing on tomato plants germinate and produce an infection when there is free water on the plant surface from rain, dew, fog, or irrigation. Infection is fastest when the temperature is about 65° to 75° F. Dying flowers are the most favorable sites for infection. Infection may also result from direct contact with moist, infested soil or plant debris.

Management

Cultural Control

Inspect transplants before planting them into the field. Remove and destroy plants found with severe Botrytis symptoms such as obvious active lesions or dead leaves or petioles. Also, avoid unnecessary late irrigations and keep the tops of beds dry when fruit is present to help reduce the chance of infection.

Organically Acceptable Methods

Cultural control is acceptable in an organically certified crop.

Treatment Decisions

Fungicides may be required in fresh market and greenhouse-grown tomatoes if gray mold infections are occurring. Fungicides will not suppress an established infection and are applied to protect against infection.

Fungicides cannot prevent disease development in fruit touching infested soil or plant debris, so treatments are not recommended for processing tomatoes. Infection of bush tomatoes is more likely when vines grow into furrows, allowing fruit to contact irrigation water.

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470

Diseases

R. M. Davis, Plant Pathology, UC Davis
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
K. V. Subbarao, USDA Agricultural Research Station, Salinas, CA
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier
B. J. Aegerter, UC Cooperative Extension, San Joaquin County (powdery mildew on field-grown tomatoes)
Acknowledgments for contributions to Diseases:
B. W. Falk, Plant Pathology, UC Davis

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