Cool weather this year has resulted in different soybean diseases than we have experienced in other years. Cool and wet conditions are favorable to the development of fungal disease. This year has been cool but not wet; therefore, the disease picture will be unique. This article discusses diseases that you are likely to find during summer disease scouting.
Sudden death syndrome
This year it appears that sudden death syndrome (SDS) was found earlier than in previous years. In 2000, SDS was found earlier in the summer season. Because this year has generally been cool, conditions are favorable for SDS fungus in fields having high moisture. This growing season has been dry in most areas of Iowa, but we would expect to find the disease in fields with high moisture in June. Eastern Iowa usually sees more SDS than central and western Iowa. Jim Fawcett, Iowa State University field crops specialist for east-central Iowa, reported soybean plants with SDS-like foliar symptoms in Linn County, where the disease has been a problem for years. A sample is on its way to the Iowa State University Plant Disease Clinic for confirmation.
Preventive measures, such as scouting, are key to reducing the risk of this disease. Pay attention to early-planted soybean fields when scouting. Symptoms of this disease are characterized by interveinal necrosis. A major management measure is to use tolerant varieties, which are available from most seed companies. Other management measures can be found in Soybean Sudden Death Syndrome  (PM 1570), available by checking the Iowa State University Extension online store  or contacting your local Iowa State University Extension office.
At planting time, Phytophthora caused damping-off. There have been reports of Phytophthora in eastern and southern Iowa after planting. In midsummer, this fungus can continue to infect soybeans, causing stem and root rot. More often, infected plants have chocolate brown discoloration on the stem, especially in areas where plant stands are thin as a result of damping-off.
Management relies on the use of resistant varieties. The Rps-1k gene has been a major gene used in commercial varieties. There have been increased reports of the Phytophthora fungus overcoming this gene. If you find the fungus, be alerted for the next soybean crop.
Occurrence of white mold is unpredictable because the methodology to detect this disease is unavailable. Cool temperatures during flowering this summer provided ideal conditions for the occurrence of soybean white mold. However, soil moisture was low in many areas while soybeans were flowering. If fields where white mold occurred in the past have good soil moisture levels during flowering, we would expect this disease to occur.
During scouting, pay close attention to fields having good soil moisture levels and dense canopies. Also check production of white mold mushrooms. If mushroom production is abundant, apply a chemical treatment to control the development of this disease.
Other diseases. There have been many reports of yellowing soybean--patches with symptoms similar to potassium deficiency or iron chlorosis. They often occur in fields or areas with poor drainage and fungal root rots are often associated with the symptoms. The cause of this problem is complicated and affected soybean plants can generally survive. As the plants grow, the yellow patches disappear from soybean fields. The effects on yield, however, have not been determined.
Viral diseases were a concern early in the season. Since the weather has been cool, the population of bean leaf beetles, vectors of the bean pod mottle virus, have been lower than years when the disease was prevalent. A cool summer will not favor the disease; however, the level of this disease in the next two months is yet to be determined. This disease is likely to be in fields near woody areas where the beetles overwinter and in earlier planted soybean fields. In areas where severe disease is observed, take good notes for future management. Soybean variety makes a difference in disease severity.
This article originally appeared on page 203 of the IC-496(19) -- July 10, 2006 issue.