Apple tissue necrosis

Infectious bark and wood necrosis is caused by the fungi Cytospora leucostoma and Cytospora cincta (cytospora canker), characterized by sunken bark with the formation of gray-brown pustules, and by Tubercularia vulgaris, which manifests as brick-red sporulating cushions. Bacterial necrosis, caused by Pseudomonas syringae, is accompanied by the formation of blisters and depressed spots with purple margins, followed by wood decay with a sour odor. Root infection by the fungi Fusarium solani and the oomycetes Pythium ultimum leads to necrosis of the root system and general wilting of the tree.

Viral pathologies, such as apple rough bark virus, manifest as deep cracking and pitting of the trunk, while the apple blister canker virus causes the formation of rupturing brown blisters. Physiological necrosis is most pronounced in the form of bitter pit of fruit, where dark sunken spots form in the flesh. Breeding for resistance to necrotic bark diseases is carried out primarily on the basis of complex field resistance, since specific gene markers for these types of damage are less defined than for scab.