White rot cankers were also found in some Hudson Valley orchards as well. It is not always easy to tell the difference between black rot and white rot cankers, but there are some characteristic differences. Black rot cankers are usually initiated at pruning cuts, whereas white rot cankers may be initiated in unwounded bark on trunks and scaffold limbs. Black rot cankers have distinct margins, and the bark and cambium within the canker margins are completely killed. White rot cankers frequently begin as depressed areas in the bark with less clearly defined margins.
Green cambium is often evident beneath superficial white rot lesions.
Bark areas affected for several years may develop large wart-like structures surrounding lenticels. The superficial white-rot lesions do not seem to cause economic damage until the trees become drought stressed. Under drought stress conditions, the white rot fungus penetrates through the cambium and the resulting cankers suddenly become obvious. In some cases, white rot cankers that develop during summer may ooze, almost like active fire blight cankers. Field observations in New York suggest that black rot cankers develop primarily on trees where xylem tissue has been sequentially damaged by cold injury and by basidiomycete wood-invading fungi such as Schizophyllum commune and Trametes versicolor.