Apples

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 directed primarily at controlling apple scab.

Bitter rot, Glomerella cingulata, is an important disease in the southern states and is becoming more important in Pennsylvania. Its hosts are apple and pear trees. On peach and nectarine the same fungus causes a disease known as anthracnose; on grape it causes ripe rot; and on chestnut it causes blossom-end rot of green burrs. The discussion below is limited to the disease as it affects apple and pear trees.

The black rot fungus, Botryosphaeria obtusa, covers a wide geographical range, attacking the fruit, leaves, and bark of apple trees and other pomaceous plants. The fungus is a vigorous saprophyte and may colonize the dead tissue of many other hosts. However, its parasitic activities are confined mainly to pome fruits.

Blue mold of apple is a common rot of stored apples and pears and is caused by the fungus Penicillium expansum. Other names for the disease are soft rot and Penicillium rot. Blue mold is a disease of ripe fruit and develops mostly on apples that are picked before they are mature. Firm fruit in the same container as decaying fruit may absorb a moldy odor and flavor.

Brooks fruit spot is a minor disease that occurs throughout the northeastern and mid-Atlantic apple growing regions in the United States. The disease is caused by the fungus Mycosphaerella pomi, Brooks fruit spot is also known as Phoma fruit spot. The disease attacks apple and crabapple trees.

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.

Fire blight is destructive to apple and quince trees and is the most serious pear disease in the eastern United States. In recent years significant losses to the apple industry have resulted due to this disease. Caused by the bacterium Erwinia amylovora, the disease can attack some 75 species of plants in the rose family. Fire blight also occurs frequently on pyracantha, spirea, hawthorn, and mountain ash. In fruit trees, the disease can kill blossoms, fruit, shoots, limbs, and tree trunks. Certain varieties of apple are more susceptible than others. Susceptible varieties include Gala, Ginger Gold, Braeburn, Jonathan, Rome, Yellow Transparent, and Idared.

Nectria twig blight, caused by the fungus Nectria galligena, is a minor disease that breaks out occasionally in Pennsylvania. This disease is a problem, however, in areas of coastal California where temperatures are more moderate. Because its symptoms are similar to those of fire blight, growers need to be able to recognize it. The chemical controls used for fire blight would not be used on nectria twig blight.

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, reduces fruit set, and produces a netlike russet on the fruit of some cultivars. It is often 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. These diseases can cause economic losses in several ways. Severe leaf infection and defoliation may make trees susceptible to winter injury. Severe defoliation reduces fruit size and quality, and infected fruit is deformed, sometimes very seriously. Cedar-apple rust occurs on leaves and fruit of apple and crabapple trees. Hawthorn rust occurs on leaves of pear, hawthorn, apple, and crabapple. Quince rust occurs on the leaves and fruit of quince and the fruit of pear, apple, and crabapple.

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. They cause only surface blemishes that detract from fruit appearance and lower fruit quality and market value. Sooty blotch also shortens fruit storage life because of increased water loss. Sooty blotch is a disease complex caused by several unrelated fungi. Flyspeck is caused by the fungus Zygophiala jamaicensis.

The white rot fungus, Botryosphaeria dothidea, often referred to as "Bot rot" or Botryosphaeria rot, is most important on apple trees, but also attacks crabapple, pear, grape, and chestnut. On apple trees it can be observed as a distinct canker on twigs, limbs, and trunks. However, leaf infections do not occur. Losses from fruit rot can be considerable, especially in southeastern regions of the United States. Drought stress and winter injury have been associated with an increase in infection and canker expansion in northern states.