As discussed in the white mold article in this issue, moderate to excessive rainfall in some parts of Illinois may encourage a number of diseases, including stem canker. Stem canker symptoms usually appear in late July or early August, when the pods are starting to fill out, and persist until the crop matures. The fungus is favored by warm temperatures, normal or excessive rainfall, high canopy humidity, and crop damage (for example, due to hail).
Stem canker is characterized by reddish brown to black and slightly sunken lesions usually located near lower leaf nodes (Figure 3). In addition, the fungus produces a toxin that causes leaflets above the canker to develop a bright yellow color between the veins. Enlarging lesions will eventually girdle the stem, causing the plant to wilt, wither, and die.
Phytophthora root rot (Figure 4).has plagued a number of growers this season, and in terms of future variety selection it is important that Phytophthora and stem canker not be confused with each another (see Tablle 2 for comparison). For example, let's say you have a field planted to a Phytoph-thora-resistant variety and you incorrectly identify a fair amount of midseason Phytophthora root rot in it this year. Based on this incorrect information, you will likely assume that the type of Phytophthora resistance you used is not appropriate for your field or area and may wrongfully select a different source of resistance for the 2002 crop year. If you are unsure about the diagnosis, seek assistance from an Extension educator, agronomist, or the University of Illinois Plant Clinic.
Fifty years ago, stem canker was a major problem. However, most varieties available today have good resistance to this disease. If you are experiencing problems with stem canker in certain areas or with certain varieties, consider tillage and rotation to reduce the amount of infested residue, and consider using other varieties that have a good track record against stem canker.
If conditions are right for stem canker in your area, what can be done at this point? Due to the depressed crop price, it is very unlikely that fungicide applications are economical. There is a checklist on page 110 of the Illinois Agriculture Pest Management Handbook 2000, which will help you determine if a foliar fungicide application should be made to soybean fields. Provided you can achieve good canopy penetration, Benlate, Bravo, or Topsin M can be used to manage a number of foliar diseases (for example, anthracnose, stem canker, brown spot, and pod and stem blight). For maximum yield and seed quality, two applications are suggested, beginning at full bloom to early pod set (depending on the product) and once again 10 to 14 days later.--Bruce Paulsrud