Phytophthora Sojae - Hosts and Symptoms

Hosts and Symptoms

Phytophthora sojae infects soybean plants (Glycine max) and many members of the genus "Lupinus". They have the ability to infect soybeans at any point during its development process, including during seed development.

They cause seed decay and pre- and post-emergence damping off when the soil is flooded after planting. Seedling roots may appear to have light brown soft rot as soon as the seedlings begin to sprout from the soil.

It also causes root and stem rot and the severity of the infection depends on how susceptible or tolerant the plant is to pathogens. In a highly-tolerant soybean plant, the root rot will simply cause the plant to be stunted and slightly chlorotic instead of killing the plant. In contrast, infection of a low-tolerant soybean plant will most likely lead to the death of the plant. Infection initiates in the roots and then progresses several nodes up the stem, turning the root and the stem brown and the leaves yellow. As the pathogen progresses, the entire plant transforms into an orange-brown color. The wilted leaves bend towards the plant and remain attached as it succumbs to death.

Foliar blight is also a symptom of Phytophthora sojae, especially when the plant has recently experienced heavy rain. The soybean plant has an age-related resistance in which the older leaves are not susceptible to foliar blight.

Soybean fields infected with Phytophthora sojae can be easily spotted by looking for stunted soybean plants or looking for empty patches where the soybean seed had been planted.

Microscopic identification of an oospore that measure around 40 micrometers in diameter from a soybean plant sample is a definite sign of Phytophthora sojae. Oospores, in general, measure around 20-45 micrometers in diameter and have very thick cellulose cell walls for overwintering.