An Overview of Pod and Stem Blight and Diaporthe Seed Decay

Pod and stem blight and Diaporthe (formerly Phomopsis) seed decay are caused by fungi in the genus Diaporthe. Pod and stem blight and Diaporthe seed decay are found in the United States and Canada and can cause disease on soybean when conditions are conducive.

Pod and stem blight often damages maturing plants and can result in premature death. Diaporthe seed decay can greatly affect seed quality. Both diseases can occur when susceptible varieties are planted and there is greater than normal rainfall during the pod-filling growth stages.

Symptoms and Signs

Pod and stem blight is characterized by black, raised specks that appear in linear rows on mature soybean stems (Figure 1). These specks are fungal reproductive structures known as pycnidia. These black structures may also cover pods, but they may not follow the linear pattern seen on stems (Figure 2). These signs are most prevalent when soybeans are nearing maturity, from R6 (full seed) through R8 (full maturity).

Diaporthe seed decay is characterized by cracked, shriveled seed with white chalk-colored mold on the seed surface. Seeds may also have pycnidia (Figure 3).Seedlings that develop from infected seed may have small reddish-brown lesions or streaks on the cotyledons or lower stems.

Disease Cycle

The fungi that cause pod and stem blight and Diaporthe seed decay overwinter on soybean residue. The fungus may also infect seeds and serve as another source of inoculum. Weeds such as velvetleaf and pigweed are also hosts of the fungi that cause the disease.

The fungus infects soybean plants early in the season when rain splashes spores from residue or from planting infected seeds. However, the fungus infects pods between R5 (beginning seed) and R6 (full seed) growth stages, which is important since only infections initiated in the pods can infect seeds and cause seed decay.

As soybean plants reach the R7 growth stage (beginning maturity), pod colonization declines drastically as seed moisture drops. Seeds will not become infected once moisture is below 19 percent. However, during periods of wet and warm weather, seed infection and colonization can continue or resume if seed moisture increases to more than 19 percent. Therefore, delayed harvest can increase the risk of Diaporthe seed decay.

Disease Diagnosis

Look for the linear alignment (rows) of black pycnidia. These rows will distinguish pod and stem blight from anthracnose stem blight and charcoal rot, both of which have randomly scattered black fruiting bodies on soybean stems. The Diaporthe species of pod and stem blight produce black zone lines that are visible when the stem is sectioned transversely or longitudinally (Figure 4).

Diseases With Similar Symptoms

The pod and stem blight fungus produces black pycnidia in linear rows on mature soybean stems; however, these can be confused with symptoms of other soybean diseases.

Yield Loss and Impact

Yield losses from pod and stem blight can be significant, but depend on the cultivar’s susceptibility and the weather during maturity. In years where rainy, wet conditions persist from early to midseason, damage and yield loss will be higher.

Yield losses from pod and stem blight result from in- complete seed fill caused by early plant death. Diminished seed quality and reduced seed vigor, germination, and emergence are all consequences of Diaporthe seed decay. Diaporthe seed decay can also reduce seed test weight and oil content.

Management

This pathogen is very well managed through choosing varieties with the best resistance package. If pod and stem blight or Diaporthe seed decay do develop in a cultivar you planted, avoid that cultivar in the future and practice techniques to reduce the amount of inoculum in a field, including tillage and rotation to nonhost crops such as corn.

Delaying harvest increases the risk of pod and stem blight, because plants are likely to be exposed to cool and wet conditions that favor disease development. Thus, timely harvest will help reduce the risk of seed decay and preserve seed quality (Figure 8).

Do not plant seed with a high incidence of Diaporthe seed decay. Infected seed can be treated with fungicide seed treatments, which usually increases germination and emergence. Do not use seed lots with more than 20 percent Diaporthe, because severely infected or moldy seed will fail to germinate even after being treated. Currently available fungicide seed treatments generally appear to be effective against Diaporthe seed infection.

Foliar fungicide applications for pod and stem blight between R3 (beginning pod) and R5 (beginning seed) may reduce stem and seed infection especially in seed production fields. Although fungicide applications may reduce disease and improve seed quality, yield may not be affected.

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Acknowledgments

Authors

Daren Mueller, Iowa State University; Carl Bradley, University of Illinois; Martin Chilvers, Michigan State University; Loren Giesler, University of Nebraska; Febina Mathew, South Dakota State University; Damon Smith, University of Wisconsin; Albert Tenuta, Ontario Ministry of Agriculture and Food; and Kiersten Wise, Purdue University.

Reviewers

Emmanuel Byamukama, South Dakota State University; Anne Dorrance, Ohio State University; Doug Jardine, Kansas State University; Dean Malvick, University of Minnesota; Sam Markell, North Dakota State University; Adam Sisson, Iowa State University; and Laura Sweets, University of Missouri.

All photos were provided by and are the property of the authors and contributors.

Sponsors

The Soybean Disease Management series is a multi-state collaboration sponsored by the North Central Soybean Research Program (NCSRP) through the Soybean Checkoff. This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture, the Grain Farmers of Ontario, and United Soybean Board for their support. Contributors to this series come from land-grant universities in the North Central states and Canada. 

This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.