Botrytis Fruit Rot

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.

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 ripens, usually in cold storage, causing postharvest rot. Infections commonly develop from the stem end where the stem is snapped off during harvest or from surface wounds that occur during harvest and handling. Infections may also start on the fruit sepals and invade the fruit stem.

The pathogen Botrytis cinerea can tolerate low storage temperatures 30° to 34°F (-1° to 1°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). Fruits that have been partially consumed by rodents and birds and were left behind in the vineyard are even more likely to harbor the pathogen.

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, other pests or extreme microclimate conditions (e.g., hail) may cause fruit injury and allow infection by the gray mold pathogen and other decay fungi. 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.

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 assesses the colonization of the sepals and stem ends, as well as the blossom ends of fruits by B. cinerea about one month before harvest, which determines the need of a preharvest spray.

BOTMON protocol for sepal and receptacle evaluation

Randomly collect 60 pieces of fruit for every five acres four months after fruit set or one month before harvest. Sample only pair rows of female vines, following a zigzag pattern. Select vines and fruit with a normal growing pattern,avoid weak and stressed vines and fruit (sunburned fruit, snail damage). Collect fruit with their stems attached and place them in flats with individual cells. If sepals break off while in the flats, collect them for later processing. Collect fruit in sets of 20 fruits per flat, if possible.

Fruit analysis:

1. Slice part of the fruit bearing the sepals and stem end.
2. Remove sepals (by hand) and stem end (with a cork borer) from each fruit.
3. Surface disinfect sepals and stem ends in 0.5% chlorine household bleach plus 2 drops of Triton-X-100 surfactant (per L water) for 1 minute.
4. Rinse the sepals and stem ends in sterile water and dry them in a positive-flow hood for 10 to 15 minutes.
5. Place the sepals and stem ends onto acidified potato-dextrose agar (pH=3.2-3.5), and incubate at 7ºC for 6 days.
6. Record B. cinereacolonies growing from the sepals and stem ends in each plate (first Botrytis recording).
7. Incubate at 23°C for 3 more days and record additional B. cinerea colonies in each plate (second Botrytis recording).
8. Combine data from the two recordings (=total B. cinerea colonies) and calculate incidence (%) of colonization of sepals or stem ends.
9. Use Table 1 for sepal or stem end colonization to predict Botrytis gray mold expected to develop after 3 or 5 months storage of fruit in controlled atmosphere and to make management decisions.

The BOTMON protocol requires experience and skill in identifying fungi on decaying plant material. The technique also allows for the development of saprophytic fungi, which may lead to misdiagnosis of the plant health problem.

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.

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.

Most fungicide products are only available in the form of a postharvest dip or spray. Cyprodinil (Vangard) however, is intended as a preharvest application for the prevention of Botrytis fruit rot. Apply 7 to 14 days before harvest in a 7 to 10 day interval. Read product labels carefully for full instructions on legal use of materials listed.