Soybean Seedling Disease Identification: Pythium Damping-off and Root Rot

Soybean seedlings suffering from post-emergence damping-off.

Environment is the key component in the expression of every plant disease.  Especially when considering the occurrence of seedling diseases we typically have a situation in place where inoculum is present in the soil, more than likely in high quantities, and a susceptible host is planted, in this case soybean.  Presently, resistance/tolerance to the organisms responsible for the seedling disease complex is not available.

Diagnosing the particular seedling disease can be a challenging task.  Above, I alluded to a situation where seedling diseases are caused by a complex of organisms.  Put more simply, more than one fungus is generally responsible (e.g. Fusarium, Pythium, Rhizoctonia).  In addition, in situations where members of the genus Pythium are responsible for the seedling disease typically more than one species involved in causing the particular disease and there are essentially two different disease symptoms that can be observed.  In the past we have focused some of the attention of the Mississippi Crop Situation towards discussing the presence and symptoms involved when non-lethal Pythium occurs.  However, over the past two weeks we’ve observed soybean seedling death as a result of the lethal form of Pythium.  The two different forms of the disease will manifest themselves in two different distinct symptoms.  Lethal Pythium will kill seedlings prior to emergence (more commonly termed “pre-emergence damping-off”) from the soil profile and can also be responsible for the death of seedlings that have emerged through the soil surface (more commonly termed “post-emergence damping-off”).  Over the past two weeks several consultants have called and stated they are observing brown, shriveled, dead soybean plants and when probing around in the soil can find plants that were not able to emerge through the soil surface with similar symptoms (refer to photos).  Upon closer inspection in the field, some seedlings had developed sunken lesions on either one side of the root or in rare cases the lesion can girdle the entire root.  In some cases, the lesions that develop on the emerging seedlings, especially when the lesions develop at the soil line, can be easily confused with herbicide injury.  But, this situation will depend on the herbicide used and in most cases herbicide injury can be excluded depending on the environment encountered following planting and/or seedling emergence.

Typically, Pythium can be more of an issue in situations with:

-sandier soil characteristics

-when cooler temperatures occur after planting or seedling emergence

-when moisture is prevalent (or excessive in the form of standing water in the field) since the fungus produces a motile spore stage that occurs when moisture is present

Note lesion on stem of seedling as well as pre-emergence damping-off symptoms.

Over the past few weeks when I’ve made observations in the laboratory by slicing through some of the sunken lesions on the developing root the reproductive structures of Pythium have been present.  In all of the specific cases a seed treatment was applied.  However, keep in mind that seed treatments are only effective at preventing some losses due to seedling disease.  Seed treatments are most effective for a short period of time (generally 21 days) and are typically considered to be effective at allowing the plants to emerge from the seed and produce a good stand.  But, if the environment changes to a more conducive environment for disease to occur (e.g. cool/wet conditions) all bets are off.  Pythium is a fast-moving organism and can infect and kill seedlings in as little as 24 hours if a conducive environment occurs.

When scouting emerged fields of soybean seedlings keep in mind that there are a range of symptoms that can occur depending on the specific seedling disease encountered.

Stands with skips can be indicators of Pythium seedling disease.