Shot hole, or Coryneum blight—Wilsonomyces carpophilus
Shot hole disease affects Prunus spp. Hosts include almonds, Catalina and Japanese flowering cherries, English laurel, ornamental plums, nectarines, peaches, and especially apricot. The disease will develop on cherries, plums, and prunes only when growing near more susceptible hosts during years with unusually wet weather during winter and spring.
Shot hole first appears in the spring as purplish or reddish spots about 1/10 inch in diameter on new buds, leaves, and shoots. Spots on young leaves commonly have a narrow, light green or yellow margin. The spots expand and their centers turn brown and can drop out, leaving holes.
The fungal disease also causes premature leaf drop, rough and corky or scabby lesions on fruit that are brown with dark purple margins and mostly on the upper surface, and small cankers on branches. Tiny, dark specks (sporodochium and spores) visible with a hand lens form in the centers of brown spots, especially on buds; these dark fungal spores help to distinguish shot hole from other diseases.
The fungus overwinters in infected buds and twig lesions. The spores spread in splashing water. Disease is most severe following warm, foggy or rainy winters and when it rains in the spring during young leaf growth.
Spores that land on twigs, buds, blossoms, or young leaves require 24 hours of continuous wetness to cause infection. Only the current season's growth is susceptible to infection. In California, twig and bud infections of apricot, nectarine, and peach can occur during rainy weather any time between fall and spring. Buds of peach, nectarine, and sometimes apricot are killed in the winter. The fungus can germinate and infect at temperatures as low as 36°F.
When zinc sulfate is sprayed on foliage in late October to early November to hasten leaf fall, shot hole inoculum is prevented from increasing. Otherwise, high levels of inoculum may develop and overwinter on the trees, infecting leaves the following spring.
Use low-volume sprinklers, drip irrigation, or sprinkler deflectors and prune off lower branches to prevent foliage wetting from irrigation. Prune and dispose of infected plant tissue as soon as it appears. After leaf drop, inspect plants carefully and prune varnished-appearing (infected) buds and twigs with lesions. Diligent sanitation and directing irrigation water away from foliage provide adequate control where the incidence of shot hole is low.
Where disease is severe on high-value plants, Bordeaux mixture, fixed copper, or certain synthetic fungicides such as chlorothalonil can be applied after leaf drop. Retreatment in late winter before buds swell or between full bloom and petal fall or both times may be necessary on highly susceptible apricot or if prolonged wet weather occurs in the spring.
hole on young apricots
hole damage from left to right on peach, almond, apricot,
Dark spore specks