Phytophthora Root and Stem Rot - Scouting

Damping off of soybean seedlings. Photo credit: X.B. Yang. Identification of soybean seedling diseases (link to ISU ICM newsletter)

During scouting, Phytophthora stem rot can be easily misidentified as stem canker. Photo credit: Anne Dorrance, Ohio State University

Scout for Phytopthora 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.

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. It is difficult to distinguish Phytophthora root rot from Pythium root rot at this stage. Both diseases cause taproot and lateral root rot and root pruning. Generally, Pythium is active in cold soils, and Phytophthora active in warm soils.

Stem and root rot phase

If damping-off is a problem in the spring, be on the look-out for Phytophthora infection following rainy periods later in the season, too. You'll recognize it by a distinct chocolate-brown lesion moving up the stem from the soil line. P. sojae slowly girdles its host, causing wilting, yellowing and death.

The root rot phase of Phytophthora 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.

For disease scouting, you are most likely to find the disease in the following places:

1. low and wet spots in a field,
2. fields with high clay content,
3. fields that have been in no-till for a few years, and
4. weedy areas that may be the result of stand reduction earlier in the season.

Look-alikes

Stem canker, caused by the fungus Diaporthe can look similar to stem rot symptoms of Phytophthora. One way to separate stem canker and Phytophthora stem and root rot is to check whether diseased plants have root rot because stem canker causes only stem lesions and not root rot.

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