Shot hole disease, also known as Coryneum blight, is a common plant condition that creates distinct perforations in leaves, making them appear as though they have been hit by small pellets. This disease primarily affects the foliage, resulting in a tattered appearance. It is a widespread issue for various plants, particularly certain fruit and ornamental trees.
Identifying Shot Hole Disease
Shot hole disease is recognized by specific visual cues on plant leaves. Symptoms begin as small, circular spots, usually reddish-purple or brown, measuring 1/10 to 1/4 inch in diameter. These spots sometimes have a lighter green or yellowish ring surrounding them.
As the disease progresses, the tissue in the spot centers dries and falls away, creating the characteristic round holes. While most noticeable on leaves, the disease can also affect other parts of the plant, including twigs, buds, and fruit. Common host plants include stone fruit trees such as cherry, plum, peach, and apricot, along with ornamental varieties like cherry laurels.
Causes and Spread
Shot hole disease is primarily caused by the fungus Wilsonomyces carpophilus. However, certain bacteria, such as Xanthomonas arboricola pv. pruni or Pseudomonas syringae pv. syringae, can cause similar symptoms, especially on cherry laurels. The pathogen overwinters in infected buds or twig lesions, harboring the disease until favorable conditions arrive.
In spring, rising temperatures and increased moisture release spores or bacteria, spreading them to new leaves, shoots, and fruit. Splashing rain or irrigation water is a primary dispersal method, carrying pathogens from infected to healthy plant parts. Cool, wet spring weather, with continuous leaf wetness for 24 hours or more, provides ideal conditions for infection.
Management and Prevention
Managing an active shot hole infection involves several steps. During the dry season, prune and destroy infected twigs, branches, and buds to remove overwintering pathogen sources. Also, rake and dispose of all fallen leaves and fruit from around affected plants, as these can harbor fungal spores or bacteria.
Long-term prevention focuses on cultural practices and timely treatments. Apply dormant sprays containing copper-based fungicides in fall after leaf drop and again in late winter before buds swell. These sprays protect dormant buds and twigs from infection. Avoid overhead watering, which keeps leaves wet for extended periods and reduces disease spread. Ensure good air circulation through proper pruning to promote faster foliage drying, creating a less hospitable environment for pathogens. Maintaining plant health through appropriate nutrition and irrigation helps plants better resist diseases.