STUDIES OF CROWN ROT OF APPLE TREES

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Abstract
The form of apple tree crown rot that occurs in the irrigated orchards of British Columbia is confined to the below-ground bark tissues of the tree. It has been encountered in trees of all ages and of all the commercial varieties.Proof is given that this crown rot is caused by the fungus Phytophthora cactorum (L. & C.) Schroet. Typical symptoms of the disease have been reproduced in over 200 trees of various ages as a result of their inoculation with this fungus. Isolation has been possible only from the margins of active lesions, and has proved difficult even from these tissues. There is evidence that the activity of P. cactorum is inhibited in rotted tissues by the antagonistic effect of one or more secondary organisms.The influence of soil moisture and temperature on disease incidence has been studied by field observations and by the inoculation of two-year—old trees under controlled conditions in Wisconsin tank equipment in the greenhouse. The effects of these two factors seem to be interrelated, with the highest incidence of disease in an almost saturated soil at the highest temperature imposed, 32 °C. The influence of soil moisture is exerted particularly in the subsoil, rather than in the locus of crown rot attack.Certain varieties of apple have been found to vary in their resistance to crown rot. Deep wounds have proved necessary to allow entry of the fungus into bark tissues.The additional information now available is being utilized in a search for improved means of combating the disease.

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