Cankers are irregular excrescences due to the perennial struggle between tissues attempting to heal up a wound, and some organism or other agent which keeps the lesion open. A canker always originates in a wound affecting the cambium, and usually in a small wound such as an insect puncture or frost nip; if undisturbed the dead parts would heal over by cork and callus, but if recurring frost-cracks break open the coverings, or if insects or fungi penetrate the callus and invade the cambium, irregularities of growth due to the occluding tissue on the one hand, and continued growth of the still unimpaired cambium on the opposite side of the injured shoot on the other, result in the canker. Frost cankers occur on fruit-trees, Vines, Beeches, etc.
Cankers due to insects are found on Apples, the cortex of which is punctured by the woolly Aphis (Schizoneura) while the twigs are young, and the wound is kept open by the insects nestling in crevices in the occlusion tissues. Species of Coccus, Lachnus, and Chermes also produce cankers on forest trees.
Cankers due to fungi usually originate in a wound primarily due to an insect puncture or bite, or to frost, the invading fungus hyphae making their way into the wounded tissues and gradually extending more and more into the cambium and the occluding callus. Among the best known of these wound fungi which cause cankers are Dasyscypha Willkommii the peziza of Larch disease, Nectria ditissima and N. cucurbitula on Beech and Conifers; less common are Scleroderris on Willows, Aglaospora on Oaks and some others.
Peridermium Pini and Aecidium elatinum also cause cankers under certain conditions, as also does Gymnosporangium, but in these cases the fungi are more truly parasitic.
In some cases - e.g. Ash, Pine, Olives - bacteria are concerned as associated organisms in the cankering of trees.
For further information on Cankers the student should read Marshall Ward, Timber and some of its Diseases, Chapter X. Further, the discussion as to the causes of canker in Frank, Krankheiten der Pflanzen, B. I., p. 207, and B. III., pp. 167 and 172, and various papers in Zeitschrift fiir Pflanzenkrankheiten.