How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

The fungus that causes Botrytis fruit rot, also known as gray mold, is widespread in the environment. It can infect strawberry flowers when spores landing on them are exposed to free water and cool temperatures. Infections can either cause flowers to rot or Botrytis can become dormant in floral tissues. Dormant infections resume activity on the berry later in the season anytime before or after harvest when sugars increase and conditions become favorable to disease development.

Infections first appear as small brown lesions, often under the calyx. Lesions begin to sporulate within a day after resumption of activity, and spore structures appear under the calyx as tiny stalks with clusters of spores at their tips. Lesion size increases rapidly. Both green and red berries are susceptible. Infected berries maintain their original shape and take on a velvety, gray-brown coat of mycelium and spores. Initially, rotted areas are soft and mushy, becoming leathery and dry in the absence of high humidity. Millions of spores are produced on each berry and become airborne at the slightest touch or breeze.

Direct infection of the berries also occurs if berries are exposed to free water. These infections develop in the same manner as flower-infected berries, but differ in that multiple initial lesions may appear anywhere on the berry's surface.

COMMENTS ON THE DISEASE

During the growing season, the fungus is constantly present and is often found in new plantings. Nothing can be done to escape the presence of this fungus, but the level of inoculum in a particular field can be reduced by removing dead leaves and infected fruit. After harvest, the fungus survives in the soil as small, black, inactive sclerotia on tilled-in leaves and fruit. In addition, the fungus lives on decomposing, dead organic matter of many plant species in and around the growing area. Because wet, cool weather is necessary for development of this disease, it is mostly limited to the coastal growing regions and northern nurseries and causes very little damage in inland growing regions except during periods of unusually wet weather during active growth.

MANAGEMENT

Presently, control of Botrytis fruit rot ranges from repetitive fungicide treatments with no cultural control to intensive cultural methods with no fungicide applications. Environmental conditions in various microclimates play an important role in determining control strategies. Planting in areas where wind can rapidly dry out the plants and interrupt disease progress helps to reduce disease incidence.

Cultural Control
Remove and destroy dead or infected plant material to help reduce the amount of inoculum capable of producing new infections. Also, remove all ripe fruit during harvest as well as any fruit with signs of decay or rain damage. Growing strawberries in plastic tunnels has proven to effectively reduce the incidence of Botrytis infections. Using plastic mulches to prevent berry-soil contact also reduces disease except where water puddles under the fruit on the plastic.

Some cultivars have flowers and fruit that develop with an upright stature, which allows fruit to be exposed to better air movement and sunlight, and this reduces the risk of infection, but fruit tend to be more exposed to rain and hail.

Organically Acceptable Methods
Select fields that are isolated from conventional growing areas and have environmental conditions that are not conducive to disease development (i.e., warm, dry spring weather or areas where wind is prevalent at some point during the day). Use varieties that are suited to the growing area. Sanitation is crucial for good control; therefore remove all fruit after spring and summer rains and all plant residue after harvest.

Monitoring and Treatment Decisions
In areas without heavy coastal summer fog, inoculum levels may be low enough in clean fields that early sprays in spring can be omitted. In dry areas, leaf wetness seldom is of long enough duration to cause epidemics, and some growers are finding it possible to grow berries without fungicides when strict sanitation practices are adhered to. In dense fog areas, inoculum density and environmental conditions conducive to disease development (i.e., wet weather and cool temperatures) should always determine when to apply fungicides. Because these conditions are usually seasonal, use a protective fungicide to prevent germination of spores when conditions ideal for disease development are anticipated. Thereafter, set spray schedules according to disease pressure and environmental conditions.

A Botrytis infection risk model has been developed that allows growers to define an infection period as light to severe based on field level temperature and wetness data. No fungicide is recommended when conditions are determined not suitable for infection.

PUBLICATION

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468
Diseases
S. T. Koike, UC Cooperative Extension, Monterey Co.
W. D. Gubler, Plant Pathology, UC Davis
G. T. Browne, USDA Crops Pathology and Genetics, UC Davis

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