Sometimes, as in the case of soybean stem canker, we have to be reminded that history sometimes repeats itself. Important lessons were learned during the late 1980s and early 1990s — soybean stem canker can cost growers a lot of money and using cultivars resistant to the disease can avoid the problem altogether.
Recognizing the problem, soybean breeders (both public and private) worked hard to develop and release many outstanding highly-resistant cultivars.
Growers responded by eliminating susceptible cultivars from their programs.
Problem solved — or so we thought.
Stem canker faded from our collective memory. We grew lax and began to think it must have gone away.
Southern stem canker is caused by the fungus Diaporthe phaseolorum var. meridionalis. The pathogen, found everywhere in the South, has caused total yield loss in some situations.
The fungus survives year to year on soybean stem debris. It's unclear how long the pathogen will remain in a field, but crop rotation to non-hosts for a single season is not enough to control the disease. Several common weeds also may be infected by the pathogen (with the wide use of herbicide-resistant cultivars, this may not be much of a factor in pathogen survival anymore).
In the spring, the pathogen produces spores on soybean trash and is splashed onto seedlings by rain. The more rainy days during the crop's early stages, the more likely infection will occur.
Susceptible cultivars can be infected over a long period — likely to near flowering. Cultivars with moderate levels of disease, however, must be infected during early vegetative stages for the disease to be severe.
The disease is usually most severe in con-till fields where soybean debris remained on the soil and in soybean fields planted early.
This year in Arkansas, Group 5 cultivars appear more severely affected than Group 4s, although disease symptoms in Group 4s may have gone unnoticed due to the earlier maturity.
Stem canker is one of the most discouraging soybean diseases. Most infections occur early in the season, but symptoms don't show up until R-3 to R-4. Infected plants look fine through most of the summer, and growers anticipate high yields from good-looking crops.
Then the first symptoms appear on the leaves. Interveinal areas of the leaflets turn yellow or brown while the leaf veins remain green. Leaves don't wilt, and after they die they tend to remain attached to the plant.
Leaf symptoms alone are not proof of stem canker, because other conditions may mimic leaf symptoms. However, plants with leaf symptoms and brown canker beginning at lower leaf nodes and running up one side of stems are strong indicators of the disease.
Two other diseases can be mistaken for stem canker.
Sudden death syndrome, a disease that is also widespread in some areas this year, causes similar symptoms on leaves of infected plants. However, there will be no canker on the stem with SDS alone.
Another common disease this year (because of all the rain and cool weather) is Phytophthora root and stem rot. While Phytophthora root and stem rot usually kills infected seedlings, plants of any age can be infected and die. In older plants, leaf symptoms are sometimes similar to those of stem canker.
Adding to the confusion: Phytophthora root and stem rot-infected plants may also develop stem cankers. But with Phytophthora root and stem rot, the canker starts at the soil line and completely girdles the stem. Cankers due to Phytophthora root and stem rot usually don't extend more than halfway up the stem, while those of stem canker may run most of the stem length.
The best advice for making an accurate diagnosis of these diseases is to contact a county Extension agent and ask him to confirm the disease by sending a sample to a lab. You may consider calling an Extension plant pathologist for a field visit.
Avoiding stem canker in the future will require planning. One of the most important things is to note which fields have stem canker. Write it down! From then on, plant only stem canker-resistant varieties in those fields.
With all the good resistant cultivars available, why take chances? Information on cultivars resistant to stem canker and other major diseases is available through the Extension Service in every Mid-South state. In Arkansas, the data is updated annually as new cultivars come to market and are screened.
This year in Arkansas, susceptible and certain moderately susceptible cultivars showed very high disease incidence and severity. Yield losses in the most severely affected fields were, unfortunately, high. Incidence and severity of stem canker in our moderately resistant cultivars was also higher than we would like to see, although yield losses were much less severe.
The main lessons to learn from this year are the value of stem canker resistance and recognition that the term “moderate” means just that.
Soybean stem canker is not going away. It can be managed and eliminated as a factor in production systems, but will not be eliminated as a threat. Effective management requires that we pay attention every year.
Terry Kirkpatrick is an Arkansas Extension nematologist. Mark Trent is an Arkansas Extension plant pathologist.