Coryneum blight or shot-hole blight is a fungal disease that causes damage on peach, nectarine, apricot, and sometimes cherries. It is caused by the fungal pathogen Wilsonmyces carpophilus. It can infect buds, twigs and branches but most often we notice it on leaves and fruit.
On leaves it causes multiple small holes to appear, giving it a ‘shot’ appearance. On apricot it produces a speckled surface on the fruit. You have probably seen this before. On peaches and nectarines, the fruit can develop grey sunken lesions, causing it to rot and fall from the tree.
On current-season and one-year-old fruiting wood, infections consist of round, reddish spots, which are slightly sunken. Fruit buds often are killed, and fruiting wood may be badly damaged or girdled. Rough, grey cankers up to two or three inches long develop on two- to four-year-old wood. These are a source of the overwintering spores. I’ve seen an increase of infections on tender young peach branches in our area.
The disease is most damaging in the moist conditions of spring, although it can occur and cause damage any time there are prolonged wet conditions. During the growing season, only short periods of moisture are required for germination and infection. Temperatures between 70- and 80-degrees Fahrenheit and wet foliage is a bad combination. Any type of sprinkle irrigation that wets foliage and fruit is notorious for spreading the disease.
Remove infected twigs and branches by pruning. Pruning during the dormant season is very effective and recommended for the homeowner as a major component of management. In the spring at shuck split (just as fruits become visible), common fungicides such as Captan, Spectracide Immunox, Bravo Weather Stik, Bonide Fung-onil, Daconil and other products containing chlorothalonil are effective. I see this disease frequently in our area. It decreases fruit quality and reduces the overall health of trees.
If you have experienced coryneum blight in your orchard, sprinkle irrigation is going to exacerbate this disease. In fact, the same is true for fire blight, a bacterial disease which affects apples and pears. You must be the judge, but sprinkle irrigation wets fruit and foliage when temperatures are prime for fungal and bacterial infections. If these diseases have plagued your trees in the past, it may be time to look for an alternate method of irrigation. Additional resources and symptom images at: https://intermountainfruit.org/dbm/shothole