Botrytis Fruit Rot

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 and are exposed to free water during cool weather. 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 growth, and sporulation appears under the calyx as a gray velvety mold. 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 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.

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 fruit production.

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.

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. Fruit types with a "neck" or reflexed calyx (a calyx that grows away from the fruit) are less susceptible to the disease simply by eliminating or reducing primary infection sites.

• 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 with necked fruit or reflexed calyx.
• Remove all fruit after spring and summer rains and all plant residue after harvest, as sanitation is crucial for good control.

There are several organic fungicides available, but none have shown consistent efficacy against gray mold.

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 sufficient duration to cause epidemics, and some growers are finding it possible to grow strawberries without fungicides when strict sanitation practices are adhered to. In dense fog areas, inoculum density and environmental conditions conducive to disease development (i.e., cool, wet weather) should always determine when to apply fungicides. Because these conditions are usually seasonal, use a protective fungicide to prevent germination of spores when weather forecasts predict conditions ideal for disease development. Thereafter, set spray schedules according to disease pressure and environmental conditions.

No fungicide is recommended when conditions are not determined to be suitable for infection.

Fungicide Resistance

Botrytis cinerea has developed resistance to almost every mode of action chemistry, including pyraclostrabin/boscalid, fenhexamid, and iprodione.