Issue 8, June 11, 2012
Bacterial Spot of Stone Fruit
Bacterial Spot of Stone Fruit (Xanthomonas campestris pv. Pruni) is a disease that can infect a number of stone fruits, from peaches and plums to cherries and almonds. It was originally discovered in 1903 on a Japanese Plum tree in Michigan. Since then it has spread to nearly every stone fruit producing country in the world. This disease can affect leaves, branches, and fruits, and is more severe in areas where fruits are grown in light, sandy soils with a warm and humid environment. Prunus species and their cultivars vary widely in susceptibility.
Bacterial spot appears on leaves as angular, gray or brown to purple, water-soaked lesions that are about 1-3mm in diameter, and are most concentrated around the midrib, leaf tip, leaf margin, or a combination of these (Picture 1). The lesions will increase with size as they age, eventually becoming purple and necrotic. After this, the lesions will often abscise, leaving what is called a shot-hole in their place (Picture 2). Many lesions can result in leaf chlorosis or yellowing and eventually premature abscission. This can often be confused with a nitrogen deficiency, so make sure you have a correct diagnosis before treating.
Bacterial leaf spot on peach.
Bacterial leaf spot on peach with "shot-hole" symptoms.
Lesions can also occur on twigs or branches and are designated as one of three types: "spring" or "summer" cankers or as "black tip." Spring cankers will appear at about the same time as leaf emergence on last autumn's growth. These will look like slightly raised, blister like areas that will extend along the twig for several centimeters. "Summer" cankers will form on new green growth and appear in late spring or early summer. These are similar in appearance to "spring" cankers. "Black tip" becomes visible late into the winter and is limited to the terminal bud of the previous year. This terminal bud will usually fail to open, and turn into a dark cankered area that can extend down a few centimeters on the twig. Fruit can also become infected, and these symptoms will become obvious at about 3-5 weeks from petal fall. Symptoms will appear as small, water-soaked, brownish lesions. As the season goes on these lesions will become cracked and sunken. Note that the frequency and the severity of these symptoms do not necessarily correlate between leaves and fruit. Severe symptoms on one do not mean severe symptoms on the other.
Both "spring" cankers and "black tip" are results of infections of the previous year, and the disease can overwinter in lateral buds. Once symptoms of the disease are noticeable, it is already at a point where it is very difficult to control. Successive years of infection will weakened trees and result in reduced fruit production and quality.
A cultural strategy aimed at disease prevention is always our primary recommendation. Purchase trees that are healthy and disease free. When available, select disease resistant species and cultivars. Avoid introducing and spreading material that might contain the bacteria to environments conducive to the pathogen's growth. One way to avoid spreading the bacteria is by sanitizing pruning tools. Soil should be kept fertile to promote steady but not excessive foliar growth. Well-draining soil in an area with good air circulation as well as keeping foliage and fruits dry can help considerably.
Chemical options are available for commercial orchards, but are not practical for homeowner use. Commercial orchards should refer to the 2012 Midwest Tree Fruit Spray Guide for options. Spray programs help to control bacterial spot by suppressing development of disease, but they do not eliminate it. Because of the cost and uncertainty of chemical control, the best way to control bacterial spot is the use of the cultural practices outlined above.
(Travis Cleveland and Sean Mullahy)