Phytophthora sojae is a soilborne pathogen that causes Phytophthora root and stem rot, a disease in soybean production globally. This organism is not a true fungus but an oomycete, commonly known as a water mold. This classification is meaningful because its biology, particularly its reliance on water for infection, influences how the disease spreads and is managed. The economic consequences of this pathogen lead to considerable yield reductions and stand losses in soybean crops.
Identifying Phytophthora Root and Stem Rot
Early in the season, the pathogen can cause seed rot, preventing germination entirely, or it can lead to pre-emergence damping-off, where seedlings die before they can break through the soil surface. If seedlings do emerge, a post-emergence infection can cause them to wilt and collapse, a condition also referred to as damping-off.
A key diagnostic feature in more developed plants is a dark, chocolate-brown discoloration that starts on the stem at the soil line and progresses upward. This lesion has a clear boundary between the infected dark tissue and the healthy green part of the stem. Infected plants will often wilt, and their leaves turn yellow between the veins, a symptom called interveinal chlorosis.
Below the ground, the disease aggressively attacks the root system. The taproot turns dark brown, and the smaller lateral and branch roots can be almost completely destroyed by the rot. Often, the withered leaves will remain attached to the dead plant for a week or more.
How Phytophthora Sojae Spreads and Thrives
Phytophthora sojae survives in the soil for several years as oospores, which are resilient, thick-walled survival structures. These oospores can remain dormant in field soil and crop residue, waiting for favorable conditions to return.
The disease cycle is initiated when soils become saturated with water. In these waterlogged conditions, the dormant oospores germinate and produce structures called sporangia. The sporangia, in turn, release numerous zoospores, which are motile spores equipped with flagella that allow them to swim through the water present in soil pores.
These swimming zoospores are the infectious agents of the pathogen. They actively move through the saturated soil to locate and attach to soybean roots, beginning the infection process. Two environmental factors are required for an outbreak: saturated soils, often resulting from heavy rainfall or poor drainage, and warm soil temperatures. Fields with high clay content or areas prone to compaction are especially vulnerable because they hold water longer.
Managing Phytophthora in Soybean Fields
Management of Phytophthora root and stem rot relies on an integrated approach that combines genetic resistance, chemical seed treatments, and cultural practices. The most robust line of defense is planting soybean varieties that possess genetic resistance. These varieties contain specific Resistance to Phytophthora sojae (Rps) genes, which can provide strong protection against specific pathotypes, or races, of the pathogen. Another form of genetic protection, known as partial resistance or field tolerance, does not provide complete immunity but reduces the severity of root rot under disease pressure.
Seed treatments offer another layer of protection, particularly for seedlings during their most vulnerable early growth stages. These treatments involve coating the soybean seed with fungicides specifically designed to target oomycetes, such as those containing metalaxyl, mefenoxam, ethaboxam, or oxathiapiprolin. This chemical barrier protects the young plant from early-season infections that cause damping-off, allowing for better stand establishment, especially in fields with a known history of the disease or when planting into cool, wet soils.
Cultural practices that modify the field environment are also a component of management. Improving soil drainage is a primary goal, as the pathogen depends on saturated conditions to initiate infection. Installing subsurface tile drainage can significantly shorten the duration of soil saturation after heavy rains. Reducing soil compaction through practices like tillage can also enhance water infiltration and aeration. Avoiding planting just before major rainfall events can help susceptible or moderately resistant varieties escape infection during the critical early season.