Bacterial Spot, caused by the bacterium Xanthomonas campestris pv. pruni, is a disease of stone fruits that has rarely been seen in western Colorado orchards. Apricots, peaches, nectarines, and plums are more severely affected than cherries, and cultivars vary greatly in susceptibility to damage for each of these crops. However, no cultivar is completely immune when environmental conditions are highly favorable to disease development.

Fruit, foliage, and shoot growth can be affected, with the greatest impact coming from fruit infections since they often result in fruit not being salable. Fruit and foliage infections begin as small, typically angular, water-soaked spots on the surface of leaves and immature fruit (Fig. 1). Under the right conditions, these spots can expand on fruit and produce a sunken rotted lesion (Fig. 2). It is this type of lesion on the fruit that makes affected fruit non-salable. On the foliage, the spots can also increase in diameter and result in death of the affected leaf tissue, which then can drop out of the leaf to give a "shot-hole" appearance to the leaf. On both fruit and leaves, the spots and lesions tend to be irregular in shape rather than round. On shoot growth, the lesions can cause elongated cankers that can kill the shoot portions above them.

Control is difficult, virtually impossible once the infections have occurred. It is far better to prevent infection from occurring. Control options thus focus on preventative treatments. Copper sprays applied in the fall at leaf-fall and in the spring before bloom will reduce bacterial populations before the season gets underway and have some carry-over as they remain on the bark surface on into the season. Materials containing zinc, including zinc sulfate foliar sprays and ziram, can also help control bacterial population build-up. Use of the antibiotic tetracycline (terramycin) is available only in states that has this material registered for such use on stone fruit crops, and that doesn't include Colorado at this time. It is possible that some of the new plant defense elicitor products (e.g., Serenade^TM, Elexa^TM, Messenger^TM) will be effective in controlling this disease, but they are still in the research stage on this. Captan by itself is not effective. Good sanitation, i.e. removal of any over wintering cankers from affected trees, will help considerably.

The other control option is avoiding cultivars that are highly susceptible to this disease. These include the following: for peach - Elberta, Elegant Lady, Monroe, O'Henry, & Suncrest; for nectarines - Firebrite, Flavortop, & Redgold; for plum - Santa Rosa; for apricots - Blenril, Early Golden, & Veecot. Severe symptoms have been observed in Colorado on Goldrich with lesser incidence on Goldstrike.

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