I have received some requests for more information on soybean seed treatments since my article appeared in the April 21 ICM newsletter. One common concern is how to determine when treated seeds are no longer needed. As we know, growers can benefit by using treated seed when they're planting soybeans early in cool, wet soil. If not, the decision about treated seed is a judgment call based on an assessment of the disease risk.
In early spring, disease problems in cool soils (below 60° F) usually are seed rot or death of the seedlings caused by Pythium and Fusarium. In late spring when the soil temperature gets warmer (70° F), the most common seedling blight is caused by Phytophthora and Rhizoctonia. An Iowa State University study shows that most problematic Pythium species in Iowa prefer cool soil temperatures (below 60° F) for infection to occur.
According to historical data recorded at Ames, the long-term average morning soil temperature (7 a.m.) at a 1-inch depth is around 50° F before the second week of May, and above 60° F after the end of third week of May. Keep in mind that temperatures increase later in the day and vary from field to field. Also, this year's soil temperatures are cooler than normal.
Generally speaking, if you want to use treated seed to control Pythium, the risk of Pythium damping-off should not be a concern when average soil temperatures are above 60° F. Phytophthora and Rhizoctonia should be a concern for later planted soybeans, mainly in fields where the diseases have occurred in the past. Apron is effective to control Pythium and Phytophthora; Rival is very good to control Rhizoctonia. Phytophthora damping-off also can be controlled by planting resistant varieties.
The disease risk may vary, however, with tillage systems. In no-till fields, the risk is higher because many pathogens are concentrated on above-ground parts of plants and survive on crop debris. This leaves residues undisturbed, which allows pathogen numbers to increase in the field. Further, a large amount of residue also reduces soil temperature and increases soil moisture. Low soil temperatures favor the development of Pythium seedling blight. Increased soil moisture also favors Phytophthora root rot and Pythium seedling blight, especially in compacted soils.
Normally, a pathogen causes damage only several years after it builds up population density. Pay attention to fields that have been no-till for years. Be sure to scout for diseases more often in no-till fields than fields prepared with conventional tillage.
This article originally appeared on page 66 of the IC-478 (9) -- May 19, 1997 issue.