Phytophthora Root and Stem Rot

Powerpoint presentations on Phytophthora

Phytophthora Biology Genetics and Host Resistance Phytophthora Management

Stem rot caused by Phytophthora sojae. Photo © Anne Dorrance, Ohio State University

Above: Oospores in soybean tissue. Below: Zoospores being released from the tip of a sporangium during saturated soil conditions.Photos © Anne Dorrance, Ohio State University

Phytophthora sojae is one of the most destructive soybean pathogens in the North Central region, and a major cause of stand establishment problems. Seeds and seedlings can be infected and killed at any time after the seed has absorbed moisture.

The fungus also infects plants later in the season following periods of heavy rain, causing stem rots or chronic root rot.

Phytophthora disease develops quickly in warm, saturated soils

Warm soil and periodic rains at weekly intervals are ideal conditions for Phytophthora diseases. Optimum conditions for infection are warm soils at a temperature greater than 60°F, and soils that are flooded or saturated.

Phytopthora diseases are most common in fields or parts of fields with poor drainage. But they can also occur in well-drained fields when the pathogen is present and the soils are saturated for 7 to 14 days due to heavy rain or irrigation.

A disease cycle unique to water molds

Phytophtora sojae has a disease cycle unique to fungi that are adapted to saturated soils. The pathogen survives in soil as spores called oospores which are produced in infected plants. The oospores can survive for many years in soil after plant residues decompose.

Oospores germinate when there is high soil moisture, forming sporangia. Sporangia germinate directly and infect the root, or they can produce zoospores. Zoospores are tiny, motile spores that are released when soils are flooded or saturated. The motile spores are attracted to seeds and roots by genistein and other isoflavanoid exudates released by soybean roots.

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