Disease Description and Management
Crown rot continues to be a major cause of tree death in Pennsylvania orchards. It often is observed on 3- to 8-year-old trees. Certain rootstocks are more susceptible to the pathogen than others. The disease often occurs in low-lying areas of orchards with heavy, poorly drained soils. The incidence of this disease has increased with the introduction of more dwarfing rootstocks.
Apple scab is Pennsylvania's most important apple disease, attacking wild and cultivated apple and crabapple. Early season disease management is primarily directed at controlling apple scab.
Bitter rot, Glomerella cingulata, is an important disease in the southern states and infrequently occurs in Pennsylvania. Its hosts are apple and pear trees.
The black rot fungus, Botryosphaeria obtusa, covers a wide geographical range and attacks the fruit, leaves, and bark of apple trees and other pomaceous plants. The fungus is a vigorous saprophyte and can colonize the dead tissue of many other hosts; however, its parasitic activities are confined mainly to pome fruits.
Blossom end rot of apple is not a major problem in Pennsylvania orchards. Because it occurs only infrequently, very little is known about its disease cycle and control.
Blotch is caused by the fungus Phyllosticta solitaria, which can infect the fruit, leaves, and twigs of apple and crabapple trees. Only occasionally seen in Pennsylvania fruit orchards, this disease does not pose a very large problem for apple producers here.
Blue mold, a common rot of stored apples and pears, is caused by the fungus Penicillium expansum. Other names for the disease are "soft rot" and "Penicillium rot."
Caused by the fungus Mycosphaerella pomi, Brooks fruit spot is also known as Phoma fruit spot. The disease attacks apple and crabapple trees and rarely is found in well-sprayed orchards.
Nectria twig blight, caused by the fungus Nectria cinnabarina, is a minor disease that breaks out occasionally.
This disease should not be confused with the fire blight or leaf spot diseases of pears. Leaf blight and fruit spot are caused by the fungus Fabraea maculata, which infects the leaves, fruit, and shoots of pear and quince and the leaves of apple trees.
The pear leaf spot fungus, Mycosphaerella pyri, infects the leaves of pear, quince, and occasionally apple trees. Numerous leaf spots can produce defoliation. Fortunately, this does not occur often before fall, except in nurseries.
Pear scab resembles apple scab in nearly all respects and is caused by the closely related fungus Venturia pirina. Although it is not particularly common, pear scab is very destructive when it does occur.
Powdery mildew, caused by the fungus Podosphaera leucotricha, attacks buds, blossoms, leaves, new shoots, and fruit of wild and cultivated apples and crabapples. It interferes with the proper functioning of leaves, reduces shoot growth and fruit set, and produces a netlike russet on the fruit of some varieties. It often is a serious problem in apple nurseries.
There are three rust diseases: cedar-apple rust, hawthorn rust, and quince rust. All three fungi spend part of their life cycle on the eastern red cedar and are problems only when red cedar is found close to the orchard. The most common is cedar-apple rust.
Sooty blotch and flyspeck of apple are separate diseases affecting apple, crabapple, and pear trees. Oftentimes both diseases are normally present on the same fruit.
Sooty mold fungi of the genus Capnodium cause an unsightly blackening over the surface of fruit and leaves. Sooty mold attacks many plants and is most common on pear, although it can affect all tree fruits and tree nuts.
The white rot fungus, Botryosphaeria dothidea, often referred to as "Bot rot" or Botryosphaeria rot, occurs most commonly on apple trees, but also attacks crabapple, pear, grape, and chestnut trees. On apple trees, it can be observed as a distinct canker on twigs, limbs, and trunks; leaf infections do not occur.
Stony pit of pear is presumed to be caused by a destructive virus, but the virus has not been isolated. Affected fruit are unsightly and unmarketable. This disease is sometimes referred to as "dimpling" because of the symptoms observed on fruit.
Crown gall occurs on a wide range of herbaceous and woody plant species including pome and stone fruit trees. The disease occurs worldwide and is especially troublesome in nurseries. Losses in orchards are sporadic. The disease is caused by the bacterium Agrobacterium tumefaciens.
Mucor rot is a fungal disease of apples and pears. The disease is a postharvest storage problem. It does not occur as frequently as blue mold, however, losses due to Mucor infection can be serious.
Apple union necrosis and decline is primarily a problem on trees propagated on MM106 rootstock. It is especially serious on red delicious trees, which are on MM106. The disease is caused by the same virus, which is vectored by the dagger nematode causing Prunus stem pitting.