These methods are preferred to natural infestation in the field, which is time consuming and unreliable due to seedlings that escape infestation. Crown rot Phytophthora cactorum Phytophthora cactorum is a fungal disease that can infect both the roots and the above ground parts of the apple tree. In the first case it is referred to as crown rot, in the latter case as collar rot or aerial phytophthora. Crown rot is also caused by other Phytophthora species, especially P. megasperma , while in Japan Phytophthora cambivora is a major cause of the disease. Phytophthora is a soil fungus that enters the tree through wounds in the roots or tree parts close to the ground.
Trees are most susceptible to infection in springtime and when planted in poor draining soils.
The bark and phloem turn dark brown and die, and in advanced stages the disease spreads into the xylem. The disease often is not noticed until the stem is nearly girdled and the tree shows yellowing of the leaves in late spring or purpling in late summer, while the fruits remain small and become highly coloured. Complete girdling leads to tree death. Therefore it is important that collar rot infections are recognized at an early stage by the presence of slightly sunken bark tissue just above the bud union. On cutting of the stem, collar rot is easily recognized by the brown colour of the affected tissue.