What to look for in soybean seedling diseases

CHAMPAIGN, Ill. — Managing soybean seedling diseases is challenging, but there are ways to minimize the impacts.

Nathan Kleczewski, University of Illinois research assistant professor, conducts research and Extension activities on plant pathology issues in the state and shared his expertise in an Illinois Soybean Association’s ILSoyAdvisor webinar.

“Soybean seedling diseases are caused by many different types of pathogens. Because we are working in fields with environments that are very diverse, oftentimes these are going to work as complexes,” Kleczewski said.

“You’ll have multiple species, multiple types of pathogens in a given field. So, managing them can be pretty challenging, but there are some things we can do to minimize some of the impacts that we have with these diseases.”

Kleczewski detailed each of the prevalent soybean seedling diseases in Illinois and new one.

Pythium

Pythium, a soilborne mold, is the most common soil pathogen in Illinois and thrives in saturated soils because it produces a spore that swims in the water and detects roots for colonization.

It can cause “mushy” roots that results in the complete rotting in the outer cortex. Another symptom is rat-tailing of the roots.

Pythium is a complex disease with diverse species that affect soybeans differently. This can lead to differences in how aggressive and severe pythium can be in a field or region and affects management differently.

The species diversity also impacts how sensitive they are to fungicides.

“Even though we’re getting the majority of our pythium controlled by these active ingredients, some of these pythiums are not going to be controlled. If those are at high levels in the your field you might not see the efficacy you’re expecting,” Kleczewski said.

Temperature can impact the amount of disease cause by pythium and the benefits of fungicides.

For example, pythium aphan at 59 degrees is not having much of an effect on emergence or germination of the seedlings. It’s much more effective at reducing the seedling emergence at warmer temperatures. However, pythium ultium is not impacted by temperature and is able to cause disease at 59 and 68 degrees.

Minimizing compaction and addressing drainage issues are important steps in managing pythium. Plant into warm soils to help the seed germinate quickly produce leaves and a larger root system. The bigger the plant, the more it’s going to tolerate pythium root rot later in the season.

Use a seed treatment with pythium-specific active ingredients that provide two to three weeks of initial protection.

Phytophthora

Similar to pythium, phytophthora has spores that can move in water. It favors saturated soils in warm conditions. With soils greater than 60 degrees is can infect throughout the season. Early infections typically lead to the most significant symptoms.

Early in the season, phytophthora will present itself similar to pythium and is easier to differentiate later in the season when a canker develops on the plants and moves up from the roots.

“The roots are oftentimes pretty much jet-black and they’re not going to have any fine feeder roots on them. The late season canker can cause girding of the stems and plant death later in the season,” Kleczewski detailed.

Phytophthora sojae is the most aggressive and is specific to soybeans. Phytophthora sansomeana is not as aggressive, but can cause issues.

As with pythium, managing phytophthora begins with avoiding compaction and improve drainage.

For fields with a history of phytophthora, select a variety with a good field tolerance rating and use a variety that has a Resistance to Phytophthora sojae gene associated or stack Rps genes. Also, consider a phytophthora specific fungicide seed treatment.

Rhizoctonia

The rhizoctonia fungus includes several different and diverse species. It is problematic in warm weather and wet, not saturated, fields of high organic matter.

The disease causes a pre-emergence dampening off. Plants wilt and die soon after emergence. Red-brown lesions can be seen at the soil line on the tap root. Lesions are often sunken, a canker.

“A late infection is typical, especially after a drought when you’re going to see yellow foliage and wilting plants. That’s where you’re going to see yield loss. The symptoms are also very similar in our other root rots, as well. Rhizoctonia does tend to do better under moderate to dry warm conditions. So, wet followed by a drought is usually a good condition for rhizoctonia,” Kleczewski said.

To manage rhizoctonia management, reduce compaction and improve drainage in problematic areas or fields. Rotate to corn or wheat to reduce inoculum. Reduce plant stress from herbicide burn, soybean cyst nematode and so forth. Select a soybean variety with good tolerance to rhizoctonia. There are also rhizoctonia seed treatments that are available.

Sudden Death Syndrome

“This is one that we don’t even realize is a seedling disease because we’re so familiar with the symptoms later on in the season when at R4 we start to see leaves with inner venal necrosis and maybe they’ll blight and the leaves fall off,” Kleczewski said.

“That’s when we’re thinking SDS. That’s very noticeable and the early season symptoms that oftentimes go unnoticed. We may blame them on one of the other organisms or in combination of some of those organisms.”

SDS favors cool, wet weather soon after planting. Early season seedling issues are often overlooked or misdiagnoses. There are significant infections on the tap root, with the finer feeder roots eaten and destroyed, leaving only a black top root.

To manage SDS, utilize a combination of a cultivar with moderate resistance and seed treatments, improve drainage and avoid compaction, plant into warm soils and manage soybean cyst nematode. SDS is worse with SCN present.

Red Crown Rot

This disease was first detected in Illinois in 2018 near Pittsfield, and Kleczewski called it a warm season cousin to SDS that likes it wet and soil temperatures in the 70s and 80s. The disease was typically seen in peanuts in the south.

It acts much like SDS where it infects the roots, causing pre- and post-emergent damping off with a black tap root. It also produces a toxin later in the season that goes up into the leaves and those leaves will then develop an inner venal necrosis, looking similar to SDS.

“If you see inner venal necrosis in fields, don’t assume it’s SDS. It could be a number of things. It could be red crown rot, it could be stem canker, or it could be brown stem rot. There are a lot of different things it could be,” Kleczewski said.

The base of infected plants are covered in a white mat of fungal growth. The lower stems have a red appearance. When a stem is split in half, the center pith of the lower stem is gray and discolored.

To help manage red crown rot, avoid a soybean-soybean rotation and rotate to corn and manage residue. Avoiding legumes for two seasons is optimal.

There are not varieties with any tolerance or resistance, and there are no labeled seed treatments.

Further research is underway, supported by the Illinois soybean checkoff, to determine the disease’s spread and its risk to producers.