How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Botrytis fruit rot, also known as gray mold decay, is a soft fruit rot that can result in significant crop losses during storage. Symptoms of decay and signs of the pathogen develop as shriveled fruit that may have gray fungal growth mostly at the stem end and occasionally around the sepals or over the entire surface of the fruit. Diseased internal fruit tissues appear water-soaked and dark green. In advanced stages of the disease black, irregular-shaped sclerotia of the fungus up to about 0.2 inch (5 mm) in diameter may form on the infected fruit.

COMMENTS ON THE DISEASE

Symptoms are uncommon on immature fruit. Infections may begin, however, at bloom and continue during fruit maturation and ripening in storage. Early infections generally do not cause immediate decay, but the pathogen can resume growth as the fruit ripen, usually in cold storage, causing postharvest rot. Infections can start on the fruit sepals and invade the fruit stem, although many can develop from the stem end where the stem is snapped off during harvest.

The pathogen Botrytis cinerea can tolerate low storage temperatures 30° to 34°F (-2° to 0°C) and grow over a wide range of temperatures commonly used to store and market kiwifruit. The pathogen can overwinter in the vineyard on fruit decaying on the ground and vines, on infected plant tissues including weeds, and leaves blown into the vineyard from neighboring orchards (e.g., citrus, almond, stone fruit). The pathogen requires wetness for spore germination and infection; thus, this disease is more severe when rainy weather occurs during bloom or especially at harvest.

In addition to wet conditions, other pests or microclimate conditions that cause fruit injury may predispose fruit to infection by the gray mold pathogen. Inoculum may be dispersed by the common brown snail (Helix aspersa) that feeds on the flowers and then on sepals of immature fruit. In addition, slime from the snail can induce B. cinerea conidia to germinate.

MANAGEMENT

Fungicide applications are required only when rainy weather or high humidity conditions from sprinkler irrigation occur during bloom and at harvest. Decisions on fungicide sprays can be based on a prediction technique (BOTMON) that assesses the potential for the development of gray mold in cold storage. The BOTMON technique involves assessing the colonization of the sepals and stem butts by B. cinerea about one month before harvest, which allows time for a preharvest spray. Regardless of bloom and preharvest microclimate conditions, the most effective method to manage gray mold of kiwifruit is with the use of postharvest fungicide treatments. This will require adoption of new postharvest handling procedures of kiwifruit that include sanitation washes and aqueous fungicide applications.

Organically Acceptable Methods
Because a large proportion of infections by B. cinerea occur at the wound where the stem is snapped off during harvest, "curing" the fruit by letting the stem end dry out before packing and cold storage can reduce the incidence of gray mold. This method can be used for organically certified fruit. Pruning the vines to open up the canopy and increase air circulation can also help in the management of this disease.

Treatment Decisions
Currently, fungicide products are only available for postharvest treatments, but registration of others are expected in the near future for preharvest use. Product labels must be read carefully for full instructions on legal use of material listed.

PUBLICATION

UC IPM Pest Management Guidelines: Kiwifruit
UC ANR Publication 3449
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
T. J. Michailides, Plant Pathology, Kearney Ag. Center, Parlier
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:
C. Arredondo, Plant Pathology student, UC Davis
K. Conn, Plant Pathology, UC Davis

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