Coryneum Blight (Shothole) caused by Stigmina carpophilum (= Coryneum beijerinckii)

Dr. Harold Larsen, Colorado State University, Agricultural Experiment Station, Western Colorado Research Center, Orchard Mesa

Coryneum blight, caused by the fungus Stigmina carpophilum (Synonym: Coryneum beijerinckii), is a disease of stone fruits. In Colorado, it affects mainly peaches, apricots, and, to a lesser degree, sweet cherry. Both leaves and fruit may be attacked; severe leaf infections with extensive shot holing may weaken a tree, while infections on the fruit produce the most apparent damage and economic loss.

The fungus typically overwinters on infected dormant leaf and/or blossom buds, and in small twig cankers. Spore production begins in early spring and continues until fall. First symptoms of infection are observed on young leaves as small red spots (fig. 1) which enlarge and become purple with a tan-white center (fig. 2). The spots then drop out of the leafblade leaving a "shothole". Severe leaf infections produce numerous holes and give the affected leaves a very tattered appearance. Shoot infections also appear as reddened spots (fig. 3); these are helpful in providing confirmation that the shotholing is in fact the result of the fungal coryneum infection rather than the virus disease prunus necrotic ringspot.

Economic loss from coryneum blight results from infection of fruit. The purple-red spots on the epidermis blemish or disfigure the fruit. Spot development depends on the occurrence of wet weather periods, and spots may appear on the fruit from 10-12 weeks before harvest to throughout the postharvest period. Early infections when the fruit is small generally produce the largest (to 6 mm in diameter) and the roughest, scab like spots. In severe cases, spots coalesce and cause the skin to crack and gum (fig. 4). Late infections on apricots when the fruit is large generally produce smaller red spots, but these also reduce fruit quality and increase cullage. However, late infections on peaches as the fruit approaches harvest are the most obvious. The fungal infection spreads very rapidly to produce finger- to thumb nail size, sunken, grayish lesions that render the fruit non-marketable (fig. 5). Particular attention to preharvest sprays will likely be necessary in orchards with a history of coryneum blight problems, especially if rainy weather is anticipated at that time.

Under Colorado conditions, most infections appear to occur during wet periods in spring and early summer, but cool, wet weather prior to harvest can trigger blight outbreaks even at that time. The wet, rainy weather spreads the spores from infected twigs and leaves to uninfected branches, young leaves, and developing fruit by splashed and wind-blown rain. Four hours of contact with free water is required for spore germination, and wet periods can make rapid spread of the disease within a tree possible, although movement from tree to tree is usually somewhat slower. Leaf infections are a major threat to developing fruit as they produce spores that can infect the fruit. Lesions can develop very slowly even at 45o Fahrenheit., but their development is much faster at optimal temperatures of 70-80o Fahrenheit.

Once established in an orchard, coryneum blight is difficult to eradicate. Infected buds and twigs may produce spores for 2 to 3 years. A conscientious annual program of chemical control with particular emphasis on fall sprays and removal of dead wood is necessary over a 3-year period to alleviate the problem. Fall copper sprays (applied at leaf fall) provide a good preventative program to minimize overwintering infections. In locations where severe infections occur, spring copper or chlorothalonil sprays are advised, and summer sprays (usually captan or ziram) also may be needed until control has been obtained.

Coryneum beijerinckii
Coryneum beijerinckii
Fig. 1. Coryneum blight infections on peach foliage resulting from a short infection period. Note small size of reddish lesions (arrow). [HJL photo] Fig. 2. Coryneum blight infections on peach foliage resulting from a long infection period. Note the large size of reddish lesions (arrow). [HJL photo]
Coryneum beijerinckii
Fig. 3. Coryneum blight infections on current season peach shoot growth. Note the red spots on twig growth (arrow) and on leaf. [HJL photo]
Coryneum beijerinckii on fruit
Fig. 4. Coryneum blight infections on maturing peach fruit. Note the scab-like lesions from infections incurred shortly after shuck fall and exuding gum (arrows) from some of these. [NSL photo]
Coryneum beijerinckii on fruit
Fig. 5. Pre-harvest Coryneum blight infection on peach fruit after 2 -3 weeks in cold storage. Note the sunken rot areas and the one lesion with a ring of spore production (arrow). Fruit showed no obvious infection when placed into cold storage. [HJL photo]

Placed on the Internet 5/3/01 9:01:52 PM; Uodated August 8, 2009