It's Getting Harder to Get Top Soybean Yields

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

            In 1999, Fond du Lac County farmers harvested 43,600 acres of soybeans.  Just ten years ago, soybeans accounted for only 10,600 acres of county terra firma.  No crop species in our area comes even close to experiencing this magnitude of growth.  There are several reasons for this increase in acreage.

            First, dairy farmers have found soybeans to be an excellent high protein feedstuff that can be grown instead of purchased.  Second, soybeans fit nicely into cash crop rotations and provide both a rotational benefit for corn as well as a significant nitrogen credit.  Finally, recent crop economics supports a more favorable loan rate price for soybeans than for corn.  Hence, soybean acres have displaced some corn acres in the past several years.

             When I first started working in the county some twelve years ago, there were few soybean pests of major significance.  Weed control was the biggest obstacle to high yields.  We consistently grew higher yielding soybeans on less productive soil with a shorter growing season than our corn belt neighbors to the south.  Although this is still often the case, our increase in soybean acreage has also brought an increase in soybean disease and insect pests.

Diseases like sclerotinia (white mold), brown stem rot, and Phytophthora root rot have increased in direct proportion with the frequency that soybeans are grown in rotation.  As if that weren't enough, insect pests like the soybean aphid are causing direct damage to plants and transmitting a multitude of virus diseases that we are just beginning to learn about.  Let's take a look at each of these pests in a little more detail and discuss some management factors that can help minimize their impact.

Sclerotinia stem rot (as seen in the picture to left), or white mold as it is often called, is a disease that frequently occurs when soybeans are grown in rotation with susceptible crops and environmental conditions are favorable for its development.  Yield reductions can be significant with severe disease infestations.  Plants are infected during flowering and eventually develop stem lesions that become covered with a white mycelium mold.  Soybean varieties differ in their susceptibility to sclerotinia and resistance ratings can be found in the UW Soybean Variety Performance bulletin.  The disease is enhanced with narrow-row planting and a humid environment during the flowering stages of the crop.  White mold can significantly reduce yields with severe infestations and it appears to be widespread in year 2000.

As with sclerotinia, Phytophthora root rot in soybean is prominent in many area fields this year.   There are several races of the Phytophthora root rot fungus in Wisconsin soils.  Variety selection and good soil drainage are the keys to controlling this disease.  Genes can be incorporated into varieties to provide resistance to the specific races.  The Rps1-k gene is the most effective in Wisconsin.  "Tolerance" or "field resistance" to Phytophthora root rot is sometimes cited for specific varieties.  This is non race-specific and can help overcome a lack of gene resistance in some situations.  The performance of soybean varieties with the field tolerance reaction to Phytophthora root rot is generally good unless soil conditions are highly favorable for the disease to develop.  It should be noted that reliable tests for tolerance have not yet been developed.  Field tolerant varieties benefit favorably from fungicide seed treatments in most years.  Phytophthora root rot kills plants from the seedling stage through maturity.

Brown stem rot is another fungal disease that has increased proportionally with the increase in soybean acres.  It becomes a problem when soybeans are grown frequently on the same field, especially under no-till conditions where crop residue can harbor the fungal inoculum.  Research has shown that three or more years of a non-host crop like corn grown between soybean crops can effectively suppress this disease.  Where this isn't possible, variety selection becomes important.  This disease seems to be most prevalent when high yield conditions exist.  Symptoms of the disease develop after pod development and include browning between leaf veins and within the stem pith.

Much has been written and discussed this summer regarding the explosion of soybean aphid populations throughout Wisconsin and the upper Midwest.  After getting over the initial shock of seeing soybean plants coated with little green aphids, it was at least somewhat comforting to know that natural predators like fungal pathogens and ladybugs would eventually be our best and least cost control option in the vast majority of situations.  However, what we still don't know for sure is the severity of viral diseases that these little critters spread from plant to plant.  Viral diseases like bud blight, soybean mosaic, and bean pod mottle are becoming more commonplace.  Most become evident at harvest time and include symptoms like green stems, mottled or distorted leaves, lack of leaf drop, and abnormal pod development.  This is an area of soybean research that is currently receiving much attention.