Plant Pathology



Soybean Diseases Control Series: Are We Missing Opportunities?


Donald E. Hershman
Extension Plant Pathologist


Series Introduction

Kentucky soybean producers are continuously searching for ways to enhance the profitability of producing soybean. One common concern voiced by many producers is that they might not be aware of all available, cost-effective, soybean disease management tactics. The purpose of this PPFS Series is to considers ways that soybean producers may "beef up" their existing soybean disease management programs and, perhaps, increase profitability. The Series has been developed so that farmers can focus in on specific diseases and/or disease groups. Most of the diseases covered in this Series are also addressed in other publications offered by the University of Kentucky Cooperative Extension Service. Please consult disease-specific publications for more detailed disease management information.

Part 3: Root and Lower Stem Diseases

With the exception of soybean cyst nematode and soybean sudden death syndrome discussed in the first two parts of this series, root and lower stem diseases of soybean are not usually a serious concern in Kentucky. There are exceptions, but these disease situations tend to occur sporadically.

All fields are subject to seed and seedling disease caused by Pythium ultimum and/or Rhizoctonia solani. These pathogens are present in every soybean field to one degree or another. However, both pathogens require rather specific conditions before serious crop damage occurs. P. ultimum, for example, is a "water mold" that kills and /or damages seed and young seedlings when soils are cool and wet, and seed/seedlings are stressed. Very early planting dates, especially when no-till practices are used; planting marginal quality seed, and the combination of below normal temperatures and above normal moisture following planting, encourage Pythium diseases. Having said this, I have not seen a field during my 19-year tenure in Kentucky that had to be replanted due to Pythium.

Rhizoctonia solani usually attacks stressed seedlings. I have seen widespread Rhizoctonia lesions on plants in a field a few times in 19 years. However, in each instance there was an overriding herbicide injury or stress problem. Even then the plants usually "outgrew" the Rhizoctonia problem and it was debatable whether or not crop yields were negatively impacted by the disease. Soybean plants have a tremendous ability to compensate for early damage. This favorable characteristic probably plays a major role in negating the negative impact of Rhizoctonia and other chronic disease situations.

Perhaps the most serious seed and seedling problems arise when marginal quality seed, infected by the pod and stem blight fungi (Phomopsis spp.), is planted. Infected seeds have two strikes against them before they are even planted. Then, if seed encounter stressful conditions once planted, serious stand problems can result. Treating seed with a variety of different fungicides can help to an extent, but few producers consider treating soybean seed with fungicides due to fear of getting "stuck" with treated seed. The best option is to plant high germ, high vigor seed. This is critical when planting saved seed. If you are considering planting seed you harvested last year, I would greatly encourage you have it properly cleaned and tested for germination at a qualified seed testing laboratory.

Charcoal rot is a severe problem during years when moisture stress is an issue, particularly late in the season. When late-season drought conditions exist, I have seen fields where 100% of plants are affected by charcoal rot. Of course in those instances, the drought was the main problem and farmers had already written off the crop. In 2001, there were many fields on the fringe of having a serious problem with charcoal rot due to inadequate soil moisture. In those fields, diseased plants could be found under tree lines (i.e., tree roots competing for soil moisture), in compacted areas, and in areas where soil was "thin" due to underlain bedrock and/or hardpan. Conserving soil moisture through no-tillage or timely irrigation is the only proactive means of managing charcoal rot. All soybean varieties are susceptible to the disease and crop rotation is of little value. Fortunately, most soybean fields in most years get and retain adequate soil moisture and simply escape charcoal rot.

Every once in a great while I see a field that is heavily damaged by Phytophthora root rot (PRR). However, the Phytophthora fungus is finicky and infection is favored by cool, wet conditions and soils with a high organic matter content. These conditions are frequently met in the upper mid-west, but not in Kentucky. Interestingly enough, research I conducted in the mid-1990's determined that Phytophthora sojae, the cause of PRR, is very widespread in Kentucky soils. Similarly, many soybean varieties we plant are susceptible. The take-home message here is that the soil environment....a component of the disease what usually keeps Kentucky soybean fields free of PRR. However, for fields with a rare historical problem with PRR, successful management can be achieved by planting certain resistant varieties or by planting "field tolerant" varieties protected early with a fungicide effective against P. sojae (e.g., metalaxyl).

Southern stem blight is a rather exotic disease in Kentucky soybean even though the disease is very common in home gardens. The disease organism, Sclerotium rolfsii, is heavily influenced by soil organic matter content. The rather low organic content of most agricultural soils in Kentucky may explain the rare and sporadic occurrence of southern stem blight here. All soybean varieties are susceptible to the disease.


CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.



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