As planting season nears, we need to talk more about tillage. Growers benefit from using no-till because it offsets production costs and reduces soil erosion. In 1994, Iowa no-till acreage increased significantly. No-till acreage is expected to increase again this year because of the USDA/ASCS soil conservation program.
Last year, Iowa State University staff conducted a survey using 121 soybean samples submitted to the Plant Disease Clinic. We classified the disease samples according to the tillage systems from which they were taken. We then compared disease data with data of the Iowa tillage survey, conducted by USDA/ASCS. We found that about 44 percent of our diseased soybeans were from no-till fields, although only 24.5 percent of 1994s total soybean acreage was no-till. Only 3.5 percent of our samples were from clean tillage, which accounted for 23 percent of 1994 soybean acreage. This means that more soybean diseases were reported from no-till fields than from plowed fields. It is possible that the occurrence of disease in no-till can be overestimated because no-tillers scout for disease more often. The trend, however, indicates an association of higher disease risk with conservation tillage.
Why do we see more disease in no-till fields? Since many pathogens concentrate on aboveground parts of plants and reproduce on crop debris, leaving residues untilled increases pathogen inoculum in fields. Fungi of many soybean diseases, such as brown stem rot and Phytophthora root rot, survive in crop residues. Large amounts of residue also reduce soil temperature and increase soil moisture. Low soil temperature favors the development of seedling blight and brown stem rot. Increased soil moisture also favors Phytophthora root rot and Pythium seedling blight. Furthermore, an increase in soybean diseases is associated with the increasing weed problems in conservation tillage. The weeds provide some pathogens a reproductive base in the absence of soybean.
Keep an eye on soybean disease in no-till fields. Normally, disease does not occur immediately after no-till is begun. Disease causes damage after building up for a few years in no-till. Early detection of diseases is an effective means of prevention. You should scout for disease in no-till fields more often than in conventionally-tilled fields. Early spring, especially May, is the time to find seed rot or death of seedlings caused by Pythium and Rhizoctonia. In late spring when temperatures warm up, the common disease is seedling blight by Phytophthora. If you find seedling disease this year, consider seed treatment when you next plant soybeans. If you find Phytophthora, consider using Phytophthora-resistant varieties next time to control build-up.