Growing Fruit Trees in Maine - Diseases
Apple scab is recognized by the brown or olive green spots on leaves and the black spots on fruit. In severe cases, scab causes defoliation which weakens the tree and inhibits flower formation.
Apple scab is spread from spores released from previously infected apple leaves that remain on the ground through winter and from unsprayed trees nearby. One way to lessen the number of infecting scab spores is to rake up leaves and remove them from the orchard before May. Another way to lessen this disease is to remove wild or abandoned apples trees within 100 yards. Unsprayed apples trees with scab infections provide spores that spread the disease to nearby trees. McIntosh, Cortland, Gingergold and Macoun are very susceptible to scab.
The most effective strategy for managing apple scab is to plant resistant varieties. Akane, Ashmead’s Kernal, Belle de Boskoop, Bramley’s Seedling, Chehalis, Chestnut Crab, Duchess, Discovery, Ellison’s Orange, Esopus Spitzenberg, Fortune, Haralson, Honeycrisp, Hudson’s Golden Gem, Gravenstein, Greensleeves, Jonathan, Keepsake, Northern Spy, Rariton, Red Baron, Strawberry, Sweet Sixteen, Wealthy, and Wolf River are varieties less likely to get scab. Crimson Crisp, Dayton, Enterprise, Fiesta, Florina, Freedom, Holstein, Jonafree, Liberty, MacFree, Novamac, NovaSpy, Nova Easygro, Priscilla, Pristine, Red Free, Richeleau, Scarlet O’Hara, and William’s Pride are resistant to scab.
Scab prevention is possible with spray applications of captan, sulfur or other fungicides. Captan and sulfur are “protectant” fungicides, meaning they should be applied before scab infection occurs. Timing is important for good results. Infections occur as early as the green tip stage of flower bud growth, which occurs in early May when green tissue emerges from the bud, and continue to occur all season with each rainfall. The six-week period from green tip until a week after blossom petals fall is the most important time to maintain protection against apple scab on susceptible varieties. Scab protection requires approximately weekly fungicide applications during this period unless there is an extended period without rain. Scab infections only occur during and after rainfall. Periods of frequent rains and prolonged leaf wetness during bloom cause the most severe scab infection conditions. If infections are not controlled, leaf spots will appear about two weeks later. These initial infections produce additional generations of spores that can continue to spread the disease. If early season infections are effectively suppressed, then monthly application of an effective fungicide in June, July and August is usually adequate to prevent other summer fungal diseases.
Flyspeck and sooty blotch
Flyspeck and sooty blotch are two summer diseases of apple that grow on the surface of the fruit, causing blemishes but not affecting the apple flesh. Flyspeck appears as a group of tiny and sharply defined black dots. These groups are often circular and about half the diameter of a dime, but the specks can also be randomly distributed. Sooty blotch is another surface fungus that causes a green, gray, or black stain that covers a large portion of the fruit surface. Flyspeck and sooty blotch are slow growing fungi. Visible symptoms take a month or more to appear after initial infection. Sooty blotch and flyspeck are both favored by high humidity, such as occurs in unpruned, large trees with thick canopies, or trees located in a pocket where morning dew persists into the day. Warm, humid weather in August or September leads to higher risk of noticeable flyspeck and sooty blotch on unprotected apples.
Sooty blotch and flyspeck are easier to prevent than scab. A fungicide application in mid to late July, with a follow-up application in August usually provides adequate control in hobby orchards. Captan and sulfur are effective fungicides.
Black knot of plum and cherry
Black knot of plum and cherry is a fungal disease that produces black, elongated swellings on the limbs. Infections occur most often on small twigs beginning in spring at the green tip stage of bud growth and continuing until shoots stop growing. The green tip bud stage of plum occurs in mid May when trees are in bloom. Most infections occur in the period just before bloom until two weeks after bloom. The fungus requires rain to infect shoots. Several months after initial infection, soft, green knots will appear, usually in late summer or the following spring. As they age, the knots turn black and harden.
Remove all infected branches as soon as possible. Prune them out by cutting two to three inches below the swelling. In some cases, entire limbs may need to be removed. Infected limbs bear few fruit and continue to spread the disease to healthy shoots. Removing wild plum or cherry trees from the surrounding area will help prevent this disease since they are also susceptible to the disease. Fungicides alone will not effectively control this disease, so removal of infected tissues and wild fruit trees is essential for good control.
Plum varieties vary in their susceptibility to black knot. Asian plums are less susceptible than European plums. The Asian varieties, Shiro, Santa Rosa, Formosa, and Methley are moderately susceptible. The European varieties, Stanley, Damson and Shropshire are highly susceptible. Bradshaw, Fellenburg and Early Italian are moderately susceptible, and President is resistant.
Brown rot is the major disease of stone fruits. No variety has complete resistance to brown rot, although plums are generally less susceptible than peaches or cherries. Rainy weather during bloom leads to infection of flowers and young shoots. Infected flowers and shoots wilt, turn brown and die, but remain in the tree to spread the disease to fruit. As fruit ripen, they become susceptible to infection. Infected fruit have soft, rotten areas covered with masses of fuzzy spores.
Sanitation prevents this disease from spreading. Remove infected flowers, shoots and fruit. Infected fruit shrivel into “mummies,” but remain in the tree to spread the disease the following spring. Mummies can be easily removed when pruning in winter or early spring.
To protect flowers, the fungicides captan, propaconazole or sulfur can be applied just before or after bloom. Do not apply a ready-to-use tree fruit spray product to fruit trees in bloom if it contains insecticide, which most multipurpose sprays do. A second high risk period for brown rot infection is during the final three weeks before harvest. Apply fungicide at 21 and ten days before expected harvest if wet weather occurs. Sulfur should not be used on apricots, and captan should not be used on plums since they can damage their foliage.
Cedar apple rust
Cedar apple rust is an uncommon disease of apple trees in Maine. It occurs when apple trees are grown in close proximity to Eastern red cedar, Juniperus virginiana. Both apple and Eastern red cedar can harbor the disease and spread it to the other species. Golden Delicious is highly susceptible and should not be grown near Eastern red cedar trees.
Peach leaf curl
Peach leaf curl causes leaves to become thickened and puckered. In some cases, the leaves develop an orange or red color. Infection occurs just as buds begin to swell in spring, but symptoms do not appear until a month after bloom. Wet weather favors infection. A single fungicide spray before bud burst in spring will give nearly compete control. Effective fungicides include copper sulfate, Bordeaux mixture and chlorothalanil. When a high degree of control is desired, apply one of these before buds swell. Once infection occurs, this disease cannot be controlled.