Poor Soybean Seed Quality and Preparing for the 2019 Field Season

Damon Smith, Associate Professor and Extension Specialist, Department of Plant Pathology, University of Wisconsin-Madison

Shawn Conley, Professor and Extension Specialist, Department of Agronomy, University of Wisconsin-Madison

Figure 1. Severe Phomopsis seed decay. Photo Credit: Craig Grau, University of Wisconsin-Madison.

As we finish off 2018 and look ahead to the 2019 planting season, soybean farmers need to be prepared for some potential seed quality issues. The 2018 crop was plagued by several problems, but one of the most substantial was a large amount of white, chalky, or black, and damaged seed (Figure 1). This damaged seed is impacting germination rates of soybean seed slated for the 2019 crop.

What caused this issue?

Most of this damage is a result of infection and colonization by a group of fungal species called Diaporthe. This group is implicated in diseases such as stem canker, pod and stem blight (Figure 2), and Phomopsis seed decay (Figure 3). Excessive rains at the end of August and throughout September and October resulted in a large amount of pod infection by Diaporthe. These infections combined with delayed harvest allowed for extensive seed colonization by these fungi. This resulted in Phomopsis seed decay which has led to visually damaged seed and the germination issues we are now seeing. To learn more about this group of fungi and the diseases they cause, visit the Crop Protection Network (CPN) website on pod and stem blight and Phompsis seed decay by clicking here. You can also download a PDF version of the CPN fact sheet on the same subject by clicking here.

How Do I manage this Problem at Planting in 2019?

Figure 2. Pod and stem blight of soybean

Soybean seed producers should try to clean seed to achieve less than 20% damaged seed in a seed lot. Multiple cleaning steps might be needed to achieve this level. While testing germination now is recommended, remember that testing germination again next spring and potentially just prior to delivery will also help you to understand the germination rate and determine if other management strategies need to be employed such as fungicidal seed treatments.

Seed treatments can help improve the germination rate of seed damaged by Diaporthe. However, you will need more than metalaxyl or mefonoxam active ingredients in your seed treatment. Metalaxyl and mefonoxam are good against Phytophthora and Pythium, but not effective against other organisms, like Diaporthe. Seed treatments with Phomopsis on the label have an additional fungicide (either a DMI or SDHI). Page 157 of the publication A3646 – Pest Management in Wisconsin Field Crops has a table of some of the seed treatments with Phomopsis on the label. Also available is the seed treatment efficacy table from the Crop Protection Network (CPN). You can download that publication by clicking here.

Figure 3. Damaged soybean seed as a result of Diaporthe infection.

We also recommend that as a farmer, you double check the percent germination on every seed lot prior to planting and adjust your seeding rates accordingly. Here are our recommendations for soybean seeding rate based on yield potential and white mold risk: The Soybean Seeding Rate Conundrum.

If I’m a Seed Producer, What Should I Do to Prevent this Problem Next Year?

Foliar fungicide applications during the growing season could reduce the damage from Diaporthe. Some work has demonstrated that fungicide applications between the R3 to the R5 growth stages might be useful in reducing damage. This might help improve seed quality, but not necessarily improve yield. For a list of fungicide products with efficacy ratings for soybean, take a look at this additional publication from the CPN by clicking here.

Phomopsis seed decay – An Increasing Issue for Delayed Soybean Harvest in Wisconsin

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Figure 1. Soybean seed affected by Phomopsis seed decay on the left compared to healthy seed on the right.

Figure 1. Soybean seed affected by Phomopsis seed decay on the left compared to healthy seed on the right.

As the rain continues in Wisconsin and the 2016 soybean harvest gets delayed longer, Phomopsis seed decay is going to become an increasing concern. Phomopsis seed decay (Fig. 1) of soybean is caused by the fungus Diaporthe longicolla which is the same fungus that causes pod and stem blight (Fig. 2). This fungus also causes “zone lines” that are often observed in split stems and tap roots. These “zone lines” were once thought to be cause by the charcoal rot fungus, but we now know that is incorrect. You can learn more about “zone lines” by CLICKING HERE.

What does Phomopsis seed decay look like?

The fungus that causes Phomopsis seed decay can infect soybean plants early in the season and colonize pods and infect seeds near, or at maturity. Infected seed will often be shriveled or undersized (Fig 1.) and can have a white or chalky appearance. If pods are opened in the field a white cottony “mold” (different than that of white mold) can be observed. Infected seed can pass the Phomopsis seed decay fungus on in seedlings of the next soybean crop. Therefore, it is important to identify Phomopsis seed decay especially in soybean-seed fields.

Figure 2. Symptoms and signs of soybean pod and stem blight.

Figure 2. Symptoms and signs of soybean pod and stem blight.

What conditions are favorable for Phomopsis seed decay?

Warm and wet weather during pod fill and maturity favor the development of Phompsis seed decay. The conditions were prevalent throughout much of the state in of Wisconsin in 2016. Soybean varieties that matured early are also more prone to Phompsis seed decay. Other stresses such as nutrient deficiencies or virus infections can also increase the occurrence of Phompsis seed decay. Infested seed is a likely source of Phompsis seed decay, however, the fungus can survive on soybean debris and certain weeds like velvetleaf.

How should I handle soybeans with Phomopsis seed decay?

Scout fields before harvest to get an idea of how much Phomopsis seed decay you might have in a field. Scout multiple plants in at least 5 locations in a field, opening pods to determine if Phomopsis seed decay is present. In fields where Phomopsis seed decay is observed, harvest should be prioritized as soon as combines can enter the field. Seed infected with the Phomopsis seed decay fungus will continue to rot in the pod until they are harvested.

How should I manage Phomopsis seed decay in the 2017 soybean crop?

Harvested grain intended to be seed for the 2017 crop should be cleaned thoroughly and undersized or damaged seed removed. Seed with an extremely high incidence of Phomopsis seed decay should not be used. Using a fungicide seed treatment may help improve emergence of infected seed. Resistant soybean varieties should also be used. Choose later maturing varieties appropriate for your location. Earlier maturing varieties tend to be more susceptible to Phomopsis seed decay. Finally, cultural practices such as rotation (corn or wheat are preferred) and tillage to manage infested residue should be considered in high-risk fields.

Additional Resource

A fact sheet about Pod and Stem blight and Phomopsis seed decay has been developed by a consortium of soybean extension pathologists. You can download that fact sheet by clicking here.