Bacterial Leaf and Fruit Spot

The foliage and fruit of many plum, peach and apricot varieties is susceptible to infection by bacterial leaf spot, Xanthomonas arboricola, that causes small yellow spots and a shot-hole appearance in foliage.  Fruit infection leads to round, sunken spots that detract from the appearance and marketability of fruit.  These symptoms can be confused with spray injury.  Warm weather and moisture favor its spread, as do nearby susceptible peach and apricot trees.  Other names for this disease are bacterial shot-hole and bacteriosis.

Plum and peach varieties vary in susceptibility. Shiro and Early Golden have a high degree of susceptibility. Obilinya and Vanier have some resistance. In general, European plums are more tolerant than Japanese varieties. The most effective management strategy is to plant resistant varieties of plum and peach since chemical sprays are of limited use in preventing this disease.  Removal of branches and limbs with cankers during dormant pruning can reduce disease pressure. 

In early summer, signs of bacterial leaf spot first appear as watersoaked spots on the leaves. The shape of the spots is more angular than round. They eventually turn yellow and fall out which creates the shot-hole appearance. The shot-hole effect is more pronounced on plum than peach, whereas in peach, the lesions turn brown with extended areas of yellowing.  Fruit infections lead to sunken spots that easily crack with fruit expansion.

Copper sprays are more effective for prevention than control, and are of limited use due to phytotoxicity. Consequently, planting resistant varieties is the most effective management strategy. 

• Black knot infections occur in springtime during the active phase of shoot growth when rainfall favors the spread of this fungus. Wild cherry and plum serve as alternate hosts, spreading the disease to cultivated plums and making chemical control more difficult in susceptible varieties.
• This disease is common in plum orchards. Symptoms appear in new shoots as green swellings of the stem and oddly shaped growths covering the shoot that eventually turn black with warty surface.  In severe cases, the fungus girdles the branch.  Cultivated plums vary in susceptibility with Stanley and Methely being highly susceptible, and Obilinaja fairly resistant.
• The most important period for black knot sprays is from white bud to shuck split.  Infections require rain and are favored by temperatures above 55 F.

Disease Cycle and Management

Black knot is caused by the fungus Dibotyron morbosum, also known as Apiosporina morbosa.  It overwinters on infected trees, and is common on wild cherry and wild plum, as well as, ornamental stone fruit species.  Dispersal occurs from ascospores released during rain beginning soon after budbreak at the green cluster stage and continue throughout shoot elongation which continues into summer, but most occur just before and during petal fall.  Symptoms become visible in fall.  Initially, infections appear as green swellings on new shoots, but they eventually turn black in their second year.  Elongated swellings that can vary in length from 1 to 12 inches. Because of their green color, it is easy to miss infections in their first year.  The knot continues to develop through the second summer with ascospores maturing the second winter after infection. 

Infections will affect the woody part of the tree, so with severe infestation, there is a yield reduction. Once established in an orchard, it is difficult to control solely by chemical means. Well-time fungicide sprays can prevent infection. However, where wild infected hosts are near the orchard, complete control is not possible in susceptible varieties.  In this case, infections should be removed during dormant pruning by cutting back several inches into healthy tissue.

Activity Rating of Fungicides

• Copper (Cuprafix, etc.) - slight
• Captan - good
• Chlorothalanil (Bravo) - excellent
• Sulfur (Microthiol Disperss, etc.) – none (ineffective)
• Thiophanate-methyl (Topsin M) - fair

Nonchemical control is primarily by removing wild plum and cherry, and by removing infections during pruning.  Where this is not possible, select varieties with partial to complete resistance:

Spring Satin plumcot, Obilinaja, Early Italian, Green Gage, Fellenberg, and President.

• 
  Strong resistance to brown rot does not exist in commonly grown varieties, but plums are generally not as susceptible as peaches.  However, European plums are more susceptible than Japanese plums. 
• The brown rot fungus infects flowers when wet weather occurs during bloom and infects fruit when warm, wet weather occurs during the ripening stage. Infected fruit that is not harvested will dry and persist into the next season to continue the spread of infections, and should therefore be removed during dormant pruning.  
• Fungicides applied to protect blossoms and ripening fruit should be timed according to susceptibility and weather that favors disease.

Disease Cycle and Management

Brown rot, a fungal disease caused by Monilia fruiticola, infects blossoms, shoots and fruit, and causes significant losses for growers when not controlled.  Plums are less susceptible to brown rot than peaches and cherries, but can be severely infected when conditions are ideal for the fungus.  The brown rot fungus overwinters on mummified fruit that remain attached to twigs, infected blossoms that cling to twigs and on infected shoots.  Fungal conidia are dispersed by wind and rain.  The disease is more severe with wet weather. 

Conidia produced on infected blossoms and shoots spread to fruit.  Infected blossoms wilt, turn brown and cling to the shoot.  The infection can move into the twig and cause a canker.  Blossom infections are not common on plum.

Fruit are susceptible to infection.  Immature fruit can be infected when insect control is poor.  Thinned fruit can serve as a source of innoculum, but thinning fruit so that they do not contact each other will reduce disease at harvest.  Signs of infection begin as small, tan colored circular lesions on the fruit surface.  In warm, humid weather, lesions develop conidia, a mass of grayish brown spores over the fruit surface.  During warm, humid conditions, grayish brown spores are produced which can be diagnostic of brown rot.  Infected fruit that remain in the orchard will shriven and cling to the branch, and serve as a source of disease the following spring.  These shriveled “mummies” should be removed during pruning.

The brown rot fungus can be spread by insects and can also spread from alternate hosts such as wild plum and cherry. Improve air circulation with annual pruning and sucker removal.  Thin fruit so that they are not in contact with other fruit.  Harvest fruit before they become over ripe.

Fungicide resistance has occurred.

Activity Rating of Fungicides

• Copper – not applicable
• Captan - good, but phytotoxic to some varieties
• Chlorothalanil (Bravo, Echo) – excellent for blossom blight, not labeled for fruit rot
• DMI (Indar, Tebuzol, etc.) – excellent, except for Rally
• Strobilurin (Pristine) - excellent
• Strobilurin (Flint, Gem) - suppression
• Sulfur (Microthiol Disperss, etc.)– fair for blossom blight, slight for fruit rot
• Biofungicides (Double Nickel, Serifel) - suppression
• Thiophanate-methyl (Topsin M) – not recommended because of resistance

Possible causes are phytoxic fungicides, particularly Captan.  Russetted spots appear in early summer and can be more prevalent on the sun-exposed side of the fruit.  Captan should not be applied from shuck split through early July, and chlorothalonil from shuck split through harvest.