NPIPM:Anthracnose on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Anthracnose occurs in all soybean growing states in the U.S. but damage is most extensive where warm and humid conditions prevail. Yield loss is mainly attributed to pod infection. Infection limited to the stem does not usually cause extensive yield loss.
Causal Organism
The most common pathogen causing anthracnose on soybean in the Northern Plains is the fungus Colletotrichum truncatum. Other Colletotrichum fungi have also been found to be associated with anthracnose on soybean.
Symptoms and Signs
Infection may occur at all stages of soybean development, but symptoms are most evident when the plants reach maturity. Common symptoms include irregular dark blotches on the stem, pods and leaf stalks. Necrotic leaf veins and leaf rolling are also common. Leaf stalk cankering (gradual decay) may cause premature leaf drops. The irregularly shaped blotches may resemble the symptoms of pod and stem blight. In fact, on mature soybean, both anthracnose and pod and stem blight may occur together. During the advance stage of anthracnose, acervuli, small fruiting bodies that look like black pincushions, can be seen using 10x hand lens on infected tissues. Fungal mycelium may completely fill infected pods, reducing the size of seeds within the pods or causing no seeds to form at all (pod blanking) (Image). In cases where seeds are formed, the pathogen may colonize the seeds and becoming seed borne. Colonized seeds may appear moldy and shriveled or it may look normal.
Disease Impacts
Anthracnose is a common secondary disease on soybean in the Northern Plains and does not usually cause serious yield loss.
Life Cycle and Epidemiology
Pathogen inoculum survives between seasons in infected crop residue or within seed. Seed embryos infected with C. truncatum may be killed and the seedlings fail to emerge altogether. Soybean seedlings infected by C. truncatum may be killed, causing post-emergence damping off. It is also possible for soybean seedlings to be infected without showing symptoms until they reach maturity. Conidia act as the primary inocula and are disseminated by rain splash. Warm temperatures and wet weather (i.e. rain, dew or fog for more than 12 hours) are the optimum conditions for infection.
Management Approaches
Crop rotation prevents the inoculum buildup within a field and crop residue incorporation by tillage reduces the inoculum carry over from the previous season. Pathogen-free seed is recommended to avoid damping off. Seed treatment with fungicide is warranted when planting seeds from heavily infected fields. Since the pathogen is seed borne, scouting for anthracnose is especially important in seed production fields.
