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Pythium Seed and Root Rot

Causal Agent:

Pythium is in a group called watermolds. To date more than 30 different species have been associated with seed and seedling damping-off in Ohio. Each species within this group has a different optimum temperature to germinate, grow and infect seeds or seedlings. Growth and development of watermolds require saturated soils, and is favored by production practices such as no-till and reduced tillage. Fields with poor drainage systems combined with rainfall will favor the environmental conditions necessary for infection.

Symptoms and Signs:

Pythium can infect seeds, seedlings, and roots throughout the growing season. Early season infections can result in both pre- and post-emergence damping-off in soybean. Symptoms in young seedlings include water-soaked lesions on the roots, hypocotyls or cotyledons, which turn to a brown soft rot. Symptoms on roots may vary from small necrotic lesions at the tips or middle of lateral roots to a more severe lesion on the main tap root. Symptomatic areas of plants can range in color from light tan to almost black in color.

Damage caused by these species of Pythium can appear similar to many other seed, seedling, and root pathogens including those caused by Phytophthora, Fusarium and in some cases Rhizoctonia.

Most of the Pythium spp. known to infect soybean in Ohio, are also pathogens of corn seed and seedlings. Similar symptoms occur on both corn and soybean.

Symptoms of infected seedlings (A) and root rot (B) caused by Pythium spp.

Stand reduction of soybean caused by pre- and post-emergence damping off.

Symptoms of damping-off of soybean. The seedling is infected below ground and the colonized tissue will turn necrotic and become soft from the collapse of the root system.

Disease Cycle:

Water molds survive as thick-walled resting spores, called oospores, which can persist for many years in the soil. During periods of saturated soil conditions, oospores germinate to form structures called sporangia. Swimming spores called zoospores form inside the sporangia and are released into the soil. These spores are attracted to soybean roots, to which they attach and germinate. Pythium then invades the root and grows within the soybean root cells. Oospores can also germinate directly and infect the root. The oospores are released back into the soil as the roots decay.

Disease Management:

Soil drainage. An essential step to control Pythium seed rot and damping-off is to improve soil drainage. Use cultural practices that reduce soil compaction and improve drainage. Improving drainage is particularly important in no-till soils that retain moisture and require less precipitation to saturate the soil. Reducing the time that a field is saturated reduces the time available for oospores to germinate and form zoospores, which are the primary agent of infection.

Saturated soil conditions that favor infection by watermolds such as Pythium and Phytophthora.

Fungicide treatments. Currently there are several fungicide chemistries that are labeled for seed treatments to Pythium spp. However, it is important to note that the efficacy of these chemistries vary depending on the species. Currently, metalaxyl, mefenoxam, ethaboxam, oxathiopiprolin and the strobulurin chemistries are registered to control Pythium spp. For fields with a history of replant issues, a combination of at least two different chemistries targeting water molds is recommended.

Planting Date. Each of the Pythium spp. that were identified in Ohio have very different temperature requirements. At this point for Ohio, planting date per se is not going to be an effective management practice. However, planting at times that optimize seed germination and seedling emergence will greatly reduce these losses. Avoid planting right before a major storm front or periods where rain is forecast for several days in a row. This is not always possible, but conditions where soil moistures are high for several days play a major role in the development of these infections.

Host Resistance. There is partial resistance in soybean to many of the Pythium species that are expressed in the seed and/or the roots. More work is in progress to identify the key locations and develop markers to assist in the breeding of soybeans with resistance to this diverse group of Pythium spp.