By Kevin Ong and Corinne Rhodes
Bacterial spot is a serious disease in stone fruit (Prunus spp.) that occurs worldwide. In the United States, it was first reported on plums in Michigan in 1903 and is still a threat east of the Rocky Mountains.
Susceptibility varies among different varieties, but bacterial spot (caused by Xanthomonas arboricola pv. pruni) affects all stone fruit species and can significantly affect tree vigor as well as fruit quality and yield. It is of greater concern in areas with high humidity and rainfall during the early part of the growing season.
Leaf symptoms begin as small (1 to 2 mm in diameter), angular, water-soaked spots that become dark purple to brown and necrotic (dead), sometimes with a chlorotic (yellow) halo. Leaf spots may increase up to 5 mm and, after a couple of weeks, the necrotic tissue falls out, leaving a shot hole appearance (Fig. 1). Often, a narrow margin of necrotic tissue remains around the hole.
Leaf spots are typically more abundant toward the tip of the leaf and along the major veins where water collects, sometimes making the leaves look tattered (Fig. 2). Severe leaf infection can result in yellowing and premature loss of leaves.
Fruit symptoms can include:
- Olive to brown, round spots that become deep pits, particularly when the infection starts during early fruit development
- Fruit cracking (Fig. 3)
- Water-soaked regions around lesions
- Yellow gum exudation (gummosis)
- Reduced fruit size and yield, especially when heavy leaf infections and early leaf drop occur
Cankers may develop on twigs in spring and summer. Spring canker symptoms occur near the previous year’s twig tip and appear as darkened, water-soaked blisters. They sometimes contain gummosis (oozing sap) (Fig. 4), which can rupture, release bacterial spores, and leave cracks in the bark. Summer cankers develop on the current season’s twig growth and are dark, water-soaked, elliptical lesions that can become sunken. Heavy twig infection can lead to dieback of the twigs and branch tips.
The pathogen can overwinter in twigs, buds, and leaf scars, providing sources of infection for leaf and fruit lesions to develop in the spring. All infected tissues will then contribute to further disease spread throughout the growing season.
Since bacterial cells spread by water droplets, warm, humid conditions, dew, and rain (particularly wind-driven rain) promote disease spread.
The peak period for the development of leaf and fruit lesions is between petal fall and shuck split, so weather conditions at this time are critical in determining the severity of the infection for the current year. Wounds serve as infection reservoirs; any activity that promotes wounds, such as small abrasions on leaves, fruit, or twigs will help spread the disease.
It is much more difficult to control bacterial spot once it is established. To prevent it:
- Choose resistant Prunus varieties with a history of performing well locally. Varieties developed in dry areas tend to have severe problems when grown in humid areas.
- Maintain tree vigor with a proper pruning, fertilization, and watering regime. Pruning increases air circulation, helping fruit and leaves dry faster from wet conditions.
- Plant trees away from roads and plant wind breaks to protect fruit and leaves from wounds caused by blowing dirt or sand.
Chemical treatments are an option, although they provide only limited disease control. To effectively treat bacterial spot:
- Treat preventively because treating after symptoms appear requires more intensive applications.
- Follow the recommendations on the chemical label to determine the number of sprays, application timing, and chemical concentration.
- Apply copper fungicides such as copper ammonium or copper hydroxide in the fall or early spring because peach leaves are susceptible to copper phytotoxicity. Oxytetracycline is an effective chemical that can be an alternative to copper fungicides as the season progresses and leaves and fruit develop. But, repeated applications can lead to bacterial resistance; alternate with a copper fungicide treatment.
- Schedule applications between petal fall and shuck split, particularly when weather conditions favoring the disease are present.
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