Farm Progress

Conditions favor another year of early-season soybean diseases

With cooler soils in recent weeks, in addition to heavy rains, keep a watch on fields for common soybean seedling diseases.

June 14, 2017

3 Min Read
SYMPTOMS: Phytophthora root and stem rot causes dark stem discoloration extending from the soil up the stem. As soil conditions warm up, Phytophthora will become more common with heavy rain.

By Loren Giesler

Overall cooler soils in the past few weeks and heavy rains in many areas of the state are affecting soybean stand and emergence. In addition, the current heat will lead to some of these patches being further damaged as a result of poor rooting.

That's why it's important to watch fields for our most common seedling diseases and make sure you are getting your seedling disease problem identified correctly. Injury from PPO-inhibiting herbicides, which has also been common this year, can be confused with soybean seedling disease injury.

Watch for these diseases
Following are brief descriptions of some of the soybean diseases which may be injuring seedlings:

• Pythium. Wet conditions are favorable for Pythium, which is the most common seedling disease of soybeans in Nebraska. Cooler soil temperatures will make this worse as the seedling will be stressed and grow more slowly. Typical symptoms of pythium will include seed decay, preemergent seedling rot and seedling damping off after emergence. If the plant has emerged, it often can have a root system where the outer layer can be easily pulled off and the center of the root will stay intact.

• Phytophthora. As soil conditions warm up, phytophthora often will become more common with heavy rains. Fields will typically have a history of this disease, which will flare up when rain events saturate soil profiles. Phytophthora is often indicated when a field was planted with a standard rate of seed treatment but still has significant stand reduction when wet conditions occur. This will be a field-specific issue and usually does not occur as often as pythium.

Typical symptoms of Phytophthora are seed decay and preemergence seedling rot and seeding damping off after emergence. Typical symptoms on seedlings are darkened stems at the base of the plant coming up from the soil line. When young plants are cut at the lower stem, often there will be a dark center to the stem. Phytophthora can kill plants at any stage of development, but pythium typically does not kill plants much past the V5 growth stage.

• Rhizoctonia root rot and fusarium root rot. A couple other diseases that have been observed this year are rhizoctonia root rot and fusarium root rot. Both of these are typically favored by dryer conditions and will occur more commonly in sandy or well-drained loamy soil types. Rhizoctonia will be evident as reddish brown lesions on the lower stem (typically at the soil level). Fusarium will have stunted plants with brown to black discoloration on the roots (often in the lower portion of the root system).

More information on product rates and management for seedling diseases can be found in Seed Treatment Fungicides for Soybean. An updated version of this Nebraska Extension NebGuide is in progress.

Another great resource to help identify and differentiate soybean seedling diseases and other injury is the Crop Protection Network website.

Further information, photos and videos are also available in the Soybean Disease Management section of the University of Nebraska-Lincoln's CropWatch website. Also check the Soybean Management Calendar to create a location-specific, customized calendar of when to expect which diseases.

Many soybean seedling diseases have similar appearing symptoms. To identify what you're seeing in the field, submit a sample to the UNL Plant and Pest Diagnostic Lab

Giesler is a Nebraska Extension plant pathologist. This report comes from UNL CropWatch.

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