Plant Pathology: Extension: Plant Disease Library

Phytophthora

Important diseases:  Late blight of tomato and potato, Little leaf disease of pine, Black shank of tobacco.

Phytophthora causes damping off, root, crown, stem, tuber and corm rots, and leaf blights of numerous plants including vegetables, fruit and forest trees, field crops, and ornamentals. Phytophthora is a water-mold and its infection is favored by wet, saturated conditions.

Phytophthora initially causes water-soaked or dark lesions on infected leaves that enlarge to blight the entire leaf. Black lesion are formed on infected stems and fruits. Plants infected with Phytophthora in the roots often exhibit chlorosis, marginal necrosis and eventually death. Cutting the lower stem at the soil line often reveals brownish necrotic tissue progressing up the stem.

Phytophthora mycelium is elongated, wide, and without cross walls (aseptate). The fungus overwinters in infected roots. Lemon-shaped sporangia are produced at the tips of the sporangiophores.

 

Oospores and sporangia can be sometimes seen within infected plant tissue. If using the stain acid fuchsin, the structures will be stained pink. Can be easily confused with Pythium, but it can be distinguished by the production of lemon-shaped sporangia, hyphal swellings and fewer and often larger oospores found within infected plant tissue.