By: Teo Spengler
Bacterial spot is a disease that attacks stone fruit, including plums. It is found throughout fruit-growing states in the eastern half of the country, affecting a fruit tree’s leaves, twigs, and fruit. If you have or plan to have plum trees in your home orchard, you’ll want to learn about bacterial spot on plums. Read on for information about plums with bacterial spot and tips for controlling plum bacterial leaf spot.
Plums are not the only fruit susceptible to bacterial spot. The disease also affects nectarines, apricots, prunes, and cherries. Severe infection can result in poor quality fruit and even devastating fruit loss. Ornamental trees can also get this disease.
Bacterial leaf spot on plums is caused by Xanthomonas, a bacterium that thrives in rainy summer weather– typical summer weather in many regions. Currently, there is no effective plum bacterial spot treatment.
The first symptoms you may see on plums with bacterial spot are numerous tiny leaf spots. They begin as water-soaked circles, but quickly develop into deep purple or brown lesions. The dry centers often tear away leaving a shot-hole or wind-torn effect. That’s why bacterial leaf spot is also known as bacterial shot-hole.
Bacterial spot on plums also attacks small twigs as well as fruits. This makes the fruit unappealing to eat and severely reduces the quality too.
You can control bacterial spot in some types of fruit trees by applying the antibiotic oxytetracycline. However, products containing this substance are not labeled for use on plums with bacterial spot. This means that there is no effective plum bacterial spot treatment.
While chemical control has not been effective, you can try controlling plum bacterial leaf spot with cultural practices. Providing your plum trees with good care is important, including all the nutrients they require to thrive. Vigorous trees are not as susceptible to the disease as stressed or neglected trees.
Any cultural practice that makes the fruit and foliage of the plum tree dry faster reduces the risk of infection. For example, trimming the inside branches to allow sun and wind in the canopy can help prevent this issue.
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Bacterial Canker (bacterium – Pseudomonas syringae): Cankers develop at the base of infected buds on trunk and scaffold limbs. Cankers spread more rapidly above the point of infection than below and only slightly to the sides. This results in a long, narrow canker. Cankers develop during the fall and winter but are not visible until late winter and early spring. Damaged areas are slightly sunken and somewhat darker in color than surrounding bark. As the trees break dormancy in the spring, gum is formed and flows down the outside of the tree. Cankers have a soured smell. The bacterium is a weak pathogen and causes serious damage only when a tree is in a near dormant condition or weakened due to unfavorable growing conditions. Avoid using high fertilizer rates in late summer. Succulent, late fall growth is more easily infected. Prune when trees are fully dormant (January and February). Trees showing signs of bacterial canker should be left and pruned after all other trees have been completed.
Brown Rot (fungus – Monilinia fructicola): The brown rot fungus can cause blossom blight or fruit rot. Surface moisture and moderately warm temperatures encourage its development. Fruit damaged by wind, hail, insects, or mechanical means is more susceptible to this organism. Infected blossoms are brown and water-soaked. The fungus grows down the pedicel into the stem which may cause twig dieback. Diseased blossoms and fruit generally become covered with “tufts” of brown fungal material (See Photo). Fruit infection usually occurs near maturity. The fungal organism overwinters in mummies, stem cankers and old fruit peduncles. Control is by repeated fungicide applications (See Table Below) and sanitation.
Bacterial Spot of Plum (bacterium – Xanthomonas campestris pv. pruni): Symptoms are observed first as small, irregularly shaped lesions. The spots are pale green in contrast to the dark green surrounding tissue. In advanced stages, angular lesions are formed, surrounded by a halo of lighter colored tissue. The inner portion of the lesion turns black and drops out. This gives the leaf a “ragged” or “shot hole” appearance. Leaves heavily infected with bacterial spot turn yellow and fall. Leaf spots are concentrated toward the distal end of the leaf. Fruit infection is not as common as foliage infection. When it occurs, small spots develop and gum may flow from these spots. Highly susceptible varieties like Methley and Santa Rosa are more likely to have fruit infections than Morris, Bruce or Ozark Premier. The bacterium overwinters on infected twigs. Chemical control (See Table Below) has not been highly effective. Early and late dormant copper sprays will aid in control. Optimum nutrition is also important.
Rust (fungus – Tranzschelia discolor): Initial symptoms are minute yellow spots on the upper leaf surfaces. As the fungus develops, reddish to dark brown colored rust lesions break open on the lower leaf surfaces (See Photo). The specific rust fungus attacking plums does not attack peaches. Rust is usually a late season problem that causes rapid defoliation. If defoliation is repeatedly premature, production can be affected. Chemical control (See Table Below) is a viable option but generally not necessary. Bruce and Morris are susceptible. Methley, Santa Rosa, Ozark Premiere and Allred are tolerant or resistant.
Cherry Leaf Spot (fungus – Coccomyces hiemalis): Infected leaves have small circular purple lesions which turn brown and eventually drop out. This gives the foliage a ragged or shot hole appearance. Rainfall is necessary for disease development. Protective fungicides applied for other diseases gives economical control. This is considered a minor disease on plums.
Black Knot on Plum (fungus – Apiosporina morbosum): Affected limbs and twigs are malformed as a result of black woody growths (galls). The galls are similar to those caused by crown gall on the root system. The disease is more prevalent on the small twigs, but under severe disease conditions it may be found on large limbs. Infection of new shoots occurs from bloom until shuck split stage. Primary inoculum comes from one to two year old galls. Control of black knot is by sanitation, chemicals and resistant varieties. Prune and destroy the galled tissue. Make cuts two or three inches below the gall to insure complete removal of the fungus. Fungicides applied (See Table Below) in the early bloom periods will help reduce the occurrence of this disease.
Plum Pockets (fungus – Taphrina communis): Plum pockets are a fruit disorder of little economic importance. Infection occurs soon after bud break under cool, wet conditions. The skin of the fruit is first a reddish color and then a velvety gray. Infected fruit becomes distorted and puffy. The disease is similar to peach leaf curl except fruit rather than leaves are attacked. A fungicide program (See Table Below) used to control peach leaf curl will also control plum pockets.
Prunus Stem Pitting (virus-Tomato ringspot): (See section on Peach)
Armillaria Root Rot (fungus – Armillaria tabescens): (See section on Armillaria Root Rot/Mushroom Root Rot)
Cotton Root Rot: (See section on Cotton Root Rot)
Purple-leaf ornamental plum trees (Prunus cerasifera) offer beautiful foliage in addition to their colorful flowers. When winter’s chill is still in the air, “Krauter Vesuvius” bursts into bloom as early as January in temperate climates. Friends of the Urban Forest cites this cultivar is one of the first flowering trees in spring, with fragrant blossoms opening on bare limbs before the purple leaves break bud. “Thundercloud” and “Mt. St. Helens” are other ornamental plum trees cultivars that maintain purple leaves throughout the growing season. The trees grow best in U.S. Department of Agriculture plant hardiness zones 5 through 8.