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Disease Cycle and Epidemiology

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Phytophthora sojae is a soilborne plant pathogen that survives as oospores in soil and plant debris. Oospores can survive for a number of years in a dormant state, and can withstand freezing and long periods of cold temperatures. Oospores of P. sojae have endogenous dormancy and not all will germinate at the same time even when conditions are highly favorable. Oospores germinate to form mycelia under high soil moisture conditions, and this mycelium then produces sporangia and zoospores under continued or subsequent saturated soil conditions. Zoospores are attracted to soybean roots, specifically to the soybean specific root exudates daedzein and genestein (Morris and Ward, 1992). Once they reach a root, the zoospores lose their flagella, produce a cell wall (encyst), germinate, and subsequently infect root tissues. P sojae colonizes the root and stem tissues where it produces oospores.

Phytophthora sojae is readily baited from soils collected at any time of year. To bait, soils are planted with susceptible soybean lines and flooded for 24 hours (Figure 8). Seedlings with characteristic symptoms can be collected and the pathogen isolated from infected tissues. Alternatively, soils can be submerged and leaf discs floated on the surface to capture zoospores, then leaf discs can be plated on selective medium. However, soils which are dry or have been frozen require saturation followed by an incubation period of 1 to 2 weeks of continuous moist conditions and temperatures of 60-65°F. This incubation period helps “break” dormancy of the oospores.

Figure 8

Soybean root and stem infections can occur throughout the season and are thought to arise directly from oospores. In most fields, inoculum of the pathogen survives as oospores, often in high numbers. The diseases caused by P. sojae are considered monocyclic, i.e., having only one effective infection cycle per growing season, primarily due to the nature of oospores which do not all germinate at one time. The role of secondary (root-to-root) inoculum in enhancing disease severity is largely unknown. Infection often appears random; in the field it is common to see one symptomatic plant, then 10 healthy plants followed by another symptomatic plant. Conversely, large areas of damping-off can occur in low areas of a field or areas with reduced drainage (Figure 9).

Figure 9

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