Crown/collar rot

Phytophthora cactorum is a plant pathogen that causes root rot on rhododendron and many other species.

Phytophthora cactorum has an extremely wide host range, and can infect over 200 species or 160 genera of trees, ornamentals, and fruit crops. In general, P. cactorum is capable of infecting both young and old plants, and causes root rots and crown rots of the many genera it infects. Although the symptoms this pathogen produces varies between the types of organisms it infects, in general disease occurs during periods that are both wet and warm. Furthermore, most infections are caused by zoospores entering the plant through wounds.