Roundup-Ready soybean had great success last year. It's reported that more acres will be planted with herbicide-tolerant varieties this year, although final figures are not yet available. Use of herbicide-resistant crops is a major change in agriculture. It provides growers greater flexibility in weed management and also may help them manage soybean diseases. In this article, I will discuss disease management in herbicide-tolerant soybeans using a proactive approach.

Use of herbicide-resistant soybean may reduce some root disease problems commonly observed in conventional soybean, which are not tolerant to herbicides. In conventional soybean crops some root rot diseases, such as Rhizoctonia and Furarium root rot, often are associated with improper application of herbicides or herbicide residues. Herbicide injury can predispose plants to rapid growth (colonization) of some fungi. Root rot diseases that may be negligible in the absence of stress can be enhanced when plants are under severe stress. This means that when herbicides put too much stress on soybean, fungi will colonize in the weakened plants. Because herbicide-tolerant soybean would tolerate these conditions, these varieties may be used to reduce root rot disease. ISU plant pathologists, agronomists, and weed scientists are working on ways that herbicide-tolerant soybean could be used to manage root rot diseases.

On the other hand, you should always follow label instructions when applying herbicides. Herbicide-tolerant soybean plants are not free from stress. Applications that are above the labeled rate may still produce stress and induce root disease problems, as recently reported by researchers from two universities. In these studies, Roundup-Ready soybean that received twice the recommended application rate had more root diseases compared to a control group. These studies show that it's important to correctly apply herbicides even though you have herbicide-tolerant soybean.

Furthermore, many current glyphosate-tolerant varieties do not have a very good disease package against some diseases prevalent in Iowa, unlike conventional cultivars. Herbicide-tolerant soybean fields that appear unhealthy or unusual during the season may have problems caused by pathogenic fungi or other pests. I have received growers' questions about this issue. Keep an eye on your herbicide-tolerant crops.