What is Black Rot and Frogeye Leaf Spot?
Black rot and frogeye leaf spot are fungal diseases both caused by Botryoshaeria obtusa that can cause damage to the fruits, leaves and woody tissue of most pome fruits. The most affected growing regions are typically wetter and more humid, such as in the Northern and Central Eastern parts of North America. The main symptoms are leaf spots, fruit rot, and tree cankers. The degradation of harvested fruit and long-term damage to the tree structure makes this an economically important disease.
What do Black Rot and Frogeye Leaf Spot look like?
Early indications of a black rot infection in the leaves will appear as several small, purple spots on the top side of the leaf. As the spots grow in diameter, the centers will turn shades of brown and yellow, maintaining their purple edge, giving the appearance of a “frog’s eye”. Frog eye leaf spots have relatively small effects on tree health compared to fruit and branch infections. Disease-affected branches will become slightly sunken, reddish-brown, cankerous, and often have a cracked or peeling bark. Cankers can grow in size over the season and have long-term effects on tree structure. When the fungus develops in the fruit, a single dark spot will begin emerging, eventually encompassing and mummifying the fruit. These symptoms vary slightly by plant variety as well as weather conditions.
Where do Black Rot and Frogeye Leaf Spot come from?
This fungus will overwinter in any dead branches, leaves or mummified fruit left in the orchard. Dead branches from prior fire blight infections and pruning often harbor black rot. In the spring, moisture from early season rain will stimulate fungal growth and produce spores that can spread throughout the orchard. The spores travel via wind, insects, and splashing rain. These spores remain viable for long periods of time and can result in several infection events throughout a growing season. During the bloom period, spores can infect flowers and remain unobserved until fruit development and maturity. Infection of leaves and branches can happen throughout the growing season, but development is notably more frequent during petal fall. Common access points for the fungus are open wounds in the fruit, leaves or branches, as a result of mechanical or environmental damage. Black rot can be found in several alternative hosts such as pear and quince (Cydonia spp.).
How do I prevent and control Black Rot and Frogeye Leaf Spot?
- Prune out and remove infected and dead branches in the winter when the temperature is below 32oF(0oC)
- Properly dispose of any infected wood away from healthy trees (Burn, bury, or compost)
- Remove any remaining shriveled, mummified fruit from trees in the orchard
- Mitigate damage to the branches, leaves or fruit; open wounds are susceptible to infection
- Maintain well-watered and healthy trees
- Plant resistant cultivars
For more information on Black Rot and Frogeye Leaf Spot:
Fruit Tree Pathology at the Hudson Valley Research Laboratory (HVRL), Cornell University