Soybean Root Rot, Stem, Foliar, and Seed Diseases
Soybean production in the United States (USA) has increased from about 27 million acres in 1960 to around 75million acres 50 years later. In 2010, the USA produced about 50% of the world’s soybean crop, with about 15% of this coming from Illinois alone. Along with the increased production, there has been an increase in the number of reports of soybean diseases. In 1979, when the first Soybean Disease Compendium was published, about 50 diseases were listed; the new edition (5th) of this book now lists more than 100 diseases. Much of this increase is based on more intense production and more focused soybean disease research. A general estimate of the losses caused by diseases ranges from 10 to 15% depending on the location and year. There is documentation of much greater losses when levels of disease epidemics are maximized. Pathogens of soybean infect all parts of the plants from roots to seeds. Some pathogens attack multiple plant parts. One of the most devastating root and stem diseases is Phytophthora root and stem rot. To manage this disease, growers make a contribution by selecting resistant varieties. Another stem disease that can be devastating is Sclerotinia stem rot (also known to some as white mold). Unlike Phytophthora, this disease is not effectively controlled by host resistance, but requires other management options, including fungicide applications, and has prompted us to search for novel sources of resistance. One of the most globally important foliar diseases is soybean rust. Since its introduction into the continental USA in 2004, this disease has had little economic impact in Illinois and most of the USA though it is still very important in Brazil and other countries. Control options include host resistance and fungicides. Research has shown promise in both areas, although there are no commercially available resistant varieties in the USA. In addition to the pathogens that attack the root, stem, and leaves, there are some that attack the pods and seeds. One of these is called Phomopsis seed decay. This disease reduces seed numbers and seed quality, and is often overlooked by growers and researchers. The occurrence and devastation of any one disease is the result of interactions among the pathogen, the soybean variety, and the environment. It is our responsibility as plant pathologists to understand these interactions and use this information to develop management practices that minimize the impact of disease epidemics.
Figure 1. Inoculation of soybean seedlings with a fungal pathogen in a controlled environment.