Phytophthora sojae is one of the “Big 4” seedling pathogens among Pythium, Phytophthora, Fusarium and Rhizoctonia that cause stand establishment problems in soybeans.

Scout for Phytophthora infection during the early vegetative growth stages, especially if heavy rains occur shortly after planting and soils are warm. If you spot stand establishment problems or no emergence, dig up the seed and check for seed rot.

The disease is known to be found in:

• Low and wet spots in a field
• Fields with high clay content
• Fields that have been in no-till for a few years
• Weedy areas that may be the result of stand reduction earlier in the season

To help maintain yield potential, know what to look for at different times of year.

Damping-off Phase

• Phytophthora-infected areas of the stem will look water-soaked or bruised and will disintegrate easily. Infected plants are easily pulled from the ground since the root system is damaged.
• At this stage, it is difficult to distinguish Phytophthora root rot from Pythium root rot. Both diseases cause taproot, lateral root rot and root pruning. Generally, Pythium is active in cold soils, whereas Phytophthora is active in warmer soils.
• If damping-off is a problem in the spring, be on the lookout for Phytophthora infection following rainy periods later in the season.

Stem Rot Phase

• Phytophthora-infected areas of the stem can be recognized by a distinct chocolate-brown lesion moving up the stem from the soil line.
• The pathogen girdles the host, causing wilting, yellowing and death.

Root Rot Phase

• The root rot phase is not as readily recognized as the stem rot phase.
• Infected plants can be stunted and less vigorous, although this is hard to spot unless the infected plants are near a healthy comparison.
• A “Shepherd’s Crook” can be identified by a general severe wilting of the plant. This can give it a gray-green look from a distance since you are seeing the undersides of the leaves as they droop.
• Different from diseases found in somewhat circular areas, Phytophthora-infected plants “Shepherd’s Crook” can be found in the same row.

Look-alikes

• The stem canker pathogen can look similar to a Phytophthora-infected stem rot. One way to separate stem canker and Phytophthora stem and root rot is to check whether diseased plants have root rot. This is because the stem canker pathogen only causes stem lesions and not root rot.
• Flooding injury can occur in the same conditions. This injury allows the outer area of the roots to be easily pulled off, leaving a “rat tail” look.

If you suspect Phytophthora is impacting your fields, speak with your NK® retailer for management recommendations.

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