This article was originally published on the Soybean Research & Information Network, a checkoff funded website.

Phytophthora sojae is one of the most destructive soybean pathogens in the northern growing regions 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 root & stem rot develops quickly in warm, saturated soil.
  • Phytophthora sojae belongs to a group of organisms called Oomycetes, also commonly known as “water molds”. This group includes some of the most damaging plant diseases including Pythium seedling blight, Aphanomyces root rot of peas, downy mildews and late blight.
  • 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.
  • Phytophthora diseases are most common in fields or parts of fields with poor drainage. 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.

The disease cycle is adapted to saturated soil.
Phytophthora root and stem rot has a disease cycle unique to fungi that are adapted to saturated soils. Towards the end of the disease cycle, the organism produces reproductive structures called oospores that can survive for many years in the soil after plant residues decompose. Oospores germinate when soil moisture is high. Germinated oospores then produce sporangia, another type of reproductive structure. Sporangia release tiny, swimming spores, known as zoospores, that are released when soils are flooded or saturated. Soybean seeds and young roots produce chemicals that are released into the soil and serve as an attractant to the newly released zoospores.

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

  • 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

Damping-off phase
Where soils are saturated by heavy rains following planting, scouting can begin as early as plant emergence. Where gaps in the row appear, carefully dig down to the seed zone. Infected seeds may rot before germination. Others may germinate but fail to make it to the surface. In other instances, the plants may emerge from the soil and then die. These dying or dead plants often show a “pinching” of the stem just at or slightly below the soil line. This symptom is commonly referred to as “damping-off”.  Keep in mind, it is difficult to distinguish Phytophthora root rot from Pythium root rot at this stage. Both diseases cause damping-off and a rotting of the young root system.  Generally, Pythium is more active in cold soils and Phytophthora more active in warm soils. Recent research, however, has shown that some Pythium species from the more southern area of the growing region are adapted to causing disease in warmer soils.

Stem and root rot phase
If damping-off is a problem in the spring, be on the lookout for Phytophthora infection following rainy periods later in the season. The stem rot phase is easily recognizable by the presence of a distinct chocolate-brown lesion moving up the stem from the soil line. Initially, the upper stem of the plant may remain green, but eventually the plant will wilt, turn yellow and then die with leaves remaining attached.

Soybean stem canker can sometimes be confused with the stem rot phase of Phytophthora. It produces slightly sunken dark cankers at the nodes. Typically, the stem remains green between the nodes, but in severe instances, the cankers may completely girdle the stem and enlarge to the point where there is little green tissue visible on the stem making it hard to distinguish between the two diseases. An identifying diagnostic feature between the two diseases is that roots of stem canker infected plants will still look healthy.

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. A close inspection of the root system will reveal a significant reduction in the number of secondary roots and fine root hairs that help the plant take up moisture and nutrients. Plants with the root rot phase may mature a week or two earlier than healthy plants in the field.

Phytophthora root and stem rot can affect soybeans at any stage of development but is often most damaging when it occurs early in the season.

Risk factors for Phytophthora losses are:

  • Field history of Phytophthora or a history of stand establishment problems
  • Years in soybean production – risk increases with more years in soybean production
  • Heavy rains following planting; disease development is most rapid at soil temperatures above 60°F when soils are saturated
  • Poorly drained fields due to flooding, low spots, compacted soils, or a high clay content
  • Susceptible soybean variety planted

Continue reading and find more research funded by the soybean checkoff at the Soybean Research & Information Network.

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