Phytophthora cactorum (Leb. & Cohn) Schroet.
Occurrence and importance
Phytophthora rot of apple fruits has been reported from widely separated growing districts. The disease seldom causes serious damage and is rarely seen on the market. Diseased apples are most likely to be found in low-grade fruit shipments.
Phytophthora rot is a light brown, very firm decay (top photo). In the early stages, the diseased tissues are as firm as adjacent healthy tissues. Externally, the rot is usually irregularly outlined; internally, the diseased flesh is pale to light brown with no clear separation between diseased and sound tissues. The vascular bundles and strands are usually a darker brown than the surrounding tissues. In advanced rots, the affected older or central portion may become somewhat spongy, more intensely discolored, and deeply water-soaked. However, the rot rarely becomes soft and mushy.
The fruit rot is caused by a soil organism, Phytophthora cactorum, which is also responsible for collar rot, a destructive disease of fruit trees. The apples most likely to be infected by Phytophthora are windfalls and fruits hanging low on the trees. Apparently the fungus can penetrate the uninjured skin of the fruits. The practice of placing harvested apples on the ground to increase their color could result in Phytophthora infections that may develop in storage.
Phytophthora rot in marketed fruits can be reduced by eliminating windfalls from the pack.