Phytophthora root rot (Phytophthora sojae)

Phytophthora root rot: biology

Phytophthora root rot of soybeans is caused by the soil borne pathogen Phytophthora sojae. The fungus oospores can survive for years in the soil or on plant debris, and develop into disease when conditions are favourable. These conditions include high soil moisture such as that found in poorly drained, compacted soils or in low areas of a field, and temperatures above 15˚C.

The Phytophthora sojae fungus has high genetic diversity, so is identified by different races or biotypes. The fungus is prevalent in almost all Canadian soils, so it is important to know which races are prevalent in your area.

The most prevalent races of P. sojae in Manitoba fields are Races 3, 4, 25, and 28. Race 4 is the most predominant.

An Ontario study found more than 22 races of P. sojae. Race 25 is the predominant race, and races 3, 4, 5, 6, 7, 9, 28 and 45 are often detected.

Phytophthora root rot can infect all plant stages when conditions favour the pathogen. Symptoms will vary by soybean variety and usually become apparent two weeks after heavy rains. One unique symptom is a dark brown lesion that extends from the taproot up to several nodes on the stem. This lesion girdles the stem and causes the plant to be stunted and wilt. Leaves will usually stay attached to the plant after it dies. Once infected, leaves of susceptible varieties will eventually turn yellow, wilt and plants can die.