Pod and Stem Blight

Phomopsis Seed Decay

Diaporthe phaseolorum var sojae, Phomopsis longicolla

Soybean seed pod infected by Pod and Stem Blight
Soybean seed pod infected by Pod and Stem Blight. Photo courtesy of Clemson University. Licensed under CC BY 3.0.
Pod and stem blight generally do not cause significant yield loss. It is characterized by lines of black dots arranged vertically on the stem. Phomopsis seed decay is a related symptom caused by the same fungus which results in damaged seeds.
The fungus that results in pod and stem blight inhabits crop residue and soybean seeds. Other hosts include velvetleaf, pigweed, pepper, and tomato. Infection occurs throughout the growing season. Conditions prone to infection are those with continuous soybeans, wet and humid weather, and late harvesting.
Infected seedlings can die prematurely or have cotyledons be partially to totally covered in orange lesions. The hypocotyls may also have reddish-brown streaks. More mature plants can be infected as seedlings and have the upper foliage yellow and die once they reach maturity. One way to identify the disease is through the large number of black, raised dots lined up on the stem or scattered about on the seed pod. Infected seed pods have shriveled up seeds covered in dull gray or white mold.
Infected seeds are shriveled and elongated and have cracked, whitish seed coats
Soybean pod and stem infected by Phomopsis Seed Decay
Soybean pod and stem infected by Phomopsis Seed Decay. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
The overall damage depends a lot on the weather and in severe cases can be significant. Yield losses come from incomplete seed fill as a result of premature death of the plant. The decay can also cause the overall weight and oil content of seeds to diminish. Infected seeds have reduced vigor, germination, and emergence.
Being that this pathogen overwinters in plant matter, it is very important to use proper tilling methods to make sure that the residue decays, avoiding potential spreading of the fungus. Rotating to nonhost crops, planting later when the ground is more dry combined with harvesting as early as possible, and careful selection of seed quality will also assist in gaining control of pod and stem blight.
Foliar fungicides applied between R3 and R5 will likely reduce seed infection, but yield will remain unaffected. There are other fungicide seed treatments that have been known to effectively control pod and stem blight. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
The fungus can be managed well through resistant varieties of soybeans.