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.
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.