Black rot is a particularly unpleasant fungal disease affecting:
- Members of the cabbage family.
- Boston ivy.
- Virginia creeper.
Several fungi species are involved.
Note that the black rot which affects squash and pumpkins is just a phase of the disease known as gummy stem blight.
The spores of the black rot fungus spread rapidly in hot, humid weather, traveling on the wind and in splashing drops of water. They can also penetrate wounded plant tissue. The fungus overwinters on the ground and in dead or dormant plants.
- Reduced plant vigor.
- Stunted plants.
- Yellow V-shaped lesions on leaf margins of cabbage crops.
- Circular tan spots with purple border on leaves of apples, often called frogeye leaf spot.
- Black leaf veins.
- Yellowed leaves.
- One-sided heads on cabbage family plants.
- Rapid head spoilage after harvest.
- Branch and shoot cankers.
- Root decay.
- Small white dots with reddish-brown ring on grapes.
- Reddish spots on apples which grow and blacken.
- Early ripening.
- Rotten apple cores.
- Mummified fruit.
Treatment of vegetables is not practicable. Affected plants should be destroyed. Fungicides (copper for organic gardens) should be applied to remaining vegetables to avoid the spread of infection. Once the growing season has ended, let the soil rest for two or three years to kill remaining spores.
Grapes and apples should be treated with regular fungicide applications. Take care to remove all mummified fruit from the plants and the ground, or black rot will return in the spring. Burying mummies through cultivation is effective.
Minimizing plant stress and maintaining good garden sanitation are the keys to preventing black rot:
- Choose varieties well adapted to your area.
- Plant in a location with plenty of sunlight and air flow.
- Rotate garden vegetables.
- Keep fruits well pruned to allow air circulation.
- Do not work with plants when wet
- Take care to avoid wetting leaves when watering the garden.
- Control weeds.
- Make sure plant nutritional needs are being met.