Stone Fruit Disease - Cornyeum Blight

Cornyeum blight can affect peaches, apricots, and sweet cherries. Economic loss results when fruits are disfigured and blemished (spots and lesions) by the fungus. The fungus is difficult to eradicate because it produces spores from infected buds and twigs for two to three years. Currently, this is not a severe problem in Pennsylvania.

Symptoms

The fungus overwinters in infected leaf buds, flower buds, and small twig cankers. Spore production begins in early spring. First symptoms are observed on young leaves as small red spots that enlarge and become purple with a white center. These spots drop out leaving a "shot hole." On fruit, early season infections are characterized by the presence of a reddish-purple halo surrounding a light tan, scab-like center spot, which is the dead fruit skin killed by the fungus. These symptoms are similar to San Jose Scale damage. Depending on environmental conditions, the spots may remain tiny or enlarge to ¼ inch in diameter. In severe cases, lesions coalesce and cause skin cracking. Similar to bacterial spot on fruit early in the season, fruit infections can also have gummy ooze on the surface. Late season fruit infections (up to four weeks before harvest) are different and lack the scab-like center and pronounced reddish-purple halo. Infections on the maturing fruit produce sunken brownish spots up to ½ inch in diameter in a fairly short time.

Disease cycle

Cornyeum blight is serious in years when frequent light showers occur during the summer. Wind currents disperse the spores from infected twigs in and leaves to uninfected branches. These spores require four hours of contact with free water droplets on the fruit, leaf, or twig surface in order to germinate and cause infection. The blight may spread rapidly within an individual tree, with movement from tree to tree somewhat slower. Leaf infections are a constant threat to fruit infection, since leaf lesions produce spores that can infect fruit whenever weather conditions are favorable. Temperatures of 70°F to 80°F are optimum for Cornyeum infections. Lesions can develop at 45°F, but at a much slower rate. It takes from two to five days for a spore to initiate infection and cause a visible lesion.

Disease management

Once established in an orchard, Coryneum blight is difficult to eradicate. Bud and twig lesions may continue to produce spores for two to three years, but the fungus does not overwinter in old infected leaves. The best preventive approach is application of chlorothalonil products (like Bravo) or copper-containing products in the fall when the leaves are easily knocked off the shoots. This protects twigs and buds from infection during wet fall weather and reduces disease carry-over to the next season.

Where disease incidence is or has been high, fungicides may be needed throughout the growing season. Applications should begin between the petal fall and shuck fall stages. Chlorothalonil products give best protection but cannot be applied to fruit after shuck fall. Copper products such applied at the low rate described for bacterial spot control will also help to provide protection. Chemical control for brown rot throughout the season will also aid in controlling Cornyeum blight.