Shot hole disease

Shot hole disease (also called Coryneum blight) is a serious fungal disease that creates BB-sized holes in leaves, rough areas on fruit, and concentric lesions on branches. The pathogen that causes shot hole disease is Wilsonomyces carpophilus.^[1]

Hosts and symptoms

The fungal pathogen Wilsonomyces carpophilus affects members of the Prunus genera. Almond, apricot, nectarine, peach, prune and cherry trees can be affected. Both edible and ornamental varieties are vulnerable to infection.^[2] Shot hole disease produces small (1/10-1/4”) reddish or purplish-brown spots. There may be a light green or yellow ring around these spots. Damaged areas become slightly larger and then dry up and fall away, leaving BB-sized holes in leaves. As the fungus spreads, more leaf tissue is damaged until the leaf falls. Significant infections can reduce the amount of photosynthesis that can occur, weakening the plant, and decreasing fruit production. The fungi can also affect fruit, beginning as small purple spots that develop into gray to white lesions. Gummosis may occur. These lesions leave toughened spots on the skin, and in some cases the fruit may be lost. Infected buds may appear darker than normal. Branches may develop concentric lesions when infected.^[2] These lesions may girdle a twig and kill it.

Disease cycle

W. carpophilus overwinters in infected buds and in twig cankers. Asexual spores (conidia) are dispersed in spring when moisture levels increase or as a result of overhead watering. Spores are pigmented and remain viable, in a dormant state, for months. Infection can occur any time moisture is present for at least 24 hours, as long as temperatures are above 36 °F. At higher temperatures, infection occurs more quickly. W. carpophilus infection takes only 6 hours at 77 °F.

Management

W. carpophilus can remain viable for several months and spores are often airborne. Since the fungi thrive in wet conditions, overhead watering should be avoided. Remove and dispose of any infected buds, leaves, fruit and twigs. In fall, fixed copper or Bordeaux mixture can be applied.

Importance

Shot hole disease is a major concern of the stone fruit industry. It is estimated that 80% of the California almond crop may be infested with shot hole disease, resulting in a potential yield loss of 50-75%.^[3] In the 1930s, it was found that applications of Bordeaux mixture reduces shot hole disease on peaches from 80% to 9%.

References