Shot hole disease, also known as Coryneum blight, is a common affliction of stone fruit trees and ornamental Prunus species, including almonds, peaches, plums, and cherries. This disease is caused by a fungal pathogen that often leaves a distinctive pattern on foliage. While the damage can sometimes resemble insect feeding, effective management strategies are available to protect susceptible trees, even though the disease can cause significant defoliation and fruit damage, especially in wet climates.
Recognizable Symptoms on Plants
The characteristic sign of this disease begins as small, reddish-purple spots on the newly emerging leaves, measuring less than a quarter-inch across. These spots may be surrounded by a lighter ring of yellow tissue. The plant attempts to isolate the damaged cells, causing the infected tissue in the center of the spot to dry out and die.
This dead tissue eventually falls away, leaving behind the circular holes that give the disease its name, making the leaves appear tattered or “shot-out.” The infection also targets fruit and woody tissue. On fruit, symptoms manifest as small, purple lesions that can develop into rough, scabby, or corky spots, sometimes leading to gummosis where the fruit oozes a clear substance.
Infected buds often appear darker than normal, and the fungus can also cause sunken, reddish spots on current-season twigs and branches. These twig infections can sometimes develop into cankers that girdle and kill the wood, providing a source of infection for the following season. Severe, recurring infections can significantly reduce the tree’s ability to photosynthesize, leading to premature leaf drop and overall plant weakening.
The Organism Responsible and How it Spreads
Shot hole disease is caused by the fungus Wilsonomyces carpophilus, which was previously known as Coryneum beijerinckii. This fungus survives the winter months primarily within infected buds and in the margins of cankers on twigs and branches. These overwintering sites are the primary sources of infectious spores when favorable conditions return.
Infection is linked to moisture, as the asexual spores (conidia) are produced and spread mainly by splashing water from rain or overhead irrigation. When temperatures are above 36°F, spores require continuous wetness to germinate and infect new plant tissue. For example, at an optimal temperature of 77°F, only about six hours of continuous wetness are needed for infection to occur.
Infections can occur anytime from spring to fall, but the disease is most severe following wet winters and periods of prolonged spring rain. Spores can remain viable for several months, and the wetness allows them to be dispersed from the infected overwintering sites to new leaves, buds, and developing fruit.
Non-Chemical Management Strategies
The first line of defense against this fungus is meticulous cleanup and sanitation, especially in the autumn, since the fungus overwinters in dead and infected material. Raking and removing all fallen leaves and mummified fruit from beneath the tree in the fall helps eliminate a significant source of fungal spores.
Proper pruning is an effective strategy, focusing on removing all visible infected twigs, dead buds, and cankered branches during dry periods, typically in the late fall or early winter. Cuts should be made a few inches below the visible canker to ensure all infected tissue is removed and destroyed away from the garden. Pruning also serves to improve air circulation within the tree canopy, which helps foliage dry quickly after rain or irrigation.
Avoiding overhead watering is important because the disease requires water splash to spread and extended leaf wetness to infect. Using drip irrigation or low-volume sprinklers angled away from the foliage minimizes the conditions needed for the spores to move and germinate. These non-chemical steps reduce the fungal population and limit the disease severity in subsequent seasons.
When to Use Chemical Controls
Fungicide application is typically reserved for trees with a history of severe infection or when cultural practices alone have proven insufficient to control the disease. The most effective chemical approach is using dormant-season copper fungicides, such as Bordeaux mixture or other fixed copper formulations. These treatments act as protectants, coating the plant surface to prevent the germination of fungal spores.
Timing the application is important to maximize effectiveness, as the goal is to protect the dormant buds and twigs where the fungus overwinters. The first application should occur in the late fall, just before leaf fall and before the onset of heavy winter rains. A second application is recommended in the late winter or early spring, just before the buds begin to swell.
Dormant sprays should never be applied once the tree has begun to leaf out, as copper can injure young foliage. If wet spring weather persists and disease pressure is high, additional protectant sprays may be necessary during the growing season. These are often different synthetic fungicides used to protect the developing fruit and foliage. Always follow label instructions precisely regarding product concentration and application safety.