Botrytis Fruit Rot

Flattened black masses of sclerotia appear on the bleached appearing canes of Botrytis-infested blackberry and raspberry plants in late winter. In the presence of free water during spring, sclerotia germinate to form masses of gray mycelium and spores called conidia. Opened flowers may become infected, and the fungus will sporulate on the blighted flowers. On infected drupelets, a watery rot may precede the development of grayish brown conidia and hyphae. The conidia and hyphae eventually cover the fruit. Infected berries left on the vines become mummified. If the weather is moist after harvest, the receptacles can be colonized by the fungus and sclerotia can develop. In postharvest storage, white mycelia can cover infected berries.

Botrytis fruit rot occurs under cool, wet conditions. The pathogen requires free water for sclerotial germination, spore germination, and infection. Physical damage to the fruit increases disease incidence, especially during the rainy season. The pathogen overwinters as sclerotia on infected canes and as mycelium in infected leaves and canes on the ground. The main sources of primary inoculum are conidia from overwintering sclerotia and dead leaves, and conidia from mummified berries. Conidia are dispersed by wind, rain, and overhead irrigation. Flowers are not susceptible to infection until they are open. Infections generally remain dormant until fruit is nearly ripe or after harvest. Infections can recur throughout the season by sporulation of the fungus on unpicked, leaky, overripe fruit left on the vine.

To promote air circulation and quicken drying of plant tissue, prune and trellis the plants to open the canopy. A narrow row can be maintained by pruning, minimizing nitrogen fertilizer application, and controlling weeds. Training systems also help to maintain an open canopy. The use of macrotunnels greatly limits the amount of gray mold infestation because of the dry conditions they create for the plants.

Partial resistance to this disease is available for red raspberry cultivars. To prevent postharvest fruit rot, pick fruit when it is in the red ripe stage of development (before reaching full maturity). Avoid fruit injury when picking. Pack fruit directly into containers, and use shallow containers to avoid crushing. Pick fruit often, and pick early in the day when temperatures are cool; cool fruit to 32°F as soon as possible after harvest. Store fruit at 32°F to maintain firmness and to prevent condensation inside the closed basket or clamshell.

Pruning or using a training system to open the canopy, proper fertilization, weed control, resistant cultivars, and proper harvest techniques are all acceptable for use in an organically certified crop.

Fungicides are applied as protectant sprays at 7- to 14-day intervals from early bloom up to harvest.