Root rot (Phytophthora megasperma var. sojae)

Root rot: biology

Phythoptora cactorum is the causal agent of root rot in over 200 species including strawberry and woody ornamentals and fruit crops. Pathogenicity may vary across species but symptoms can appear as root and collar rot, fruit rot, cankers, leaf blight, wilts, and seedling blight.  Reproduction is asexual where spores produce more spores over about a 10 day period. Sexual reproduction may occur later in the season to produce spores which then overwinter. Infection is caused by tiny conidia which germinate on a thin layer of moisture on the leaf’s surface. Germ tubes then form that penetrate between leaf cells which is where they find nutrients to feed on. Cool humid conditions are conducive to fungus emerging which envelop infected leaves and cause them to die off. As the upper portions of plants are infected and die off, the disease travels down to the next level of leaves. Conidia are produced at night and spread in the morning when wind drafts pick up and temperatures rise. They can travel to nearby fields or hundreds of kilometers away depending on wind and weather conditions.

The most damaging incidents can occur in apple, strawberry and Rhododendron. In apple orchards crown rot begins at the soil line and continues down into the root mass. Foliar symptoms of dark brown to orange lesions below leaf surface tissue are an indication of what is going on with the vascular system of the tree. Trees have poor terminal growth and can become stunted and eventually die over several seasons. In strawberries Phythoptora cactorum can cause Leather Rot as well. Symptoms include rot and dysfunction of the vascular system and eventual death of the plant. Brown discoloration in the crown starting from the top down is key to diagnosing this problem.