Why were soybean yields so low in many Illinois fields in 2003? There is obviously no simple or single answer. Much has been discussed about dry weather, aphids, and charcoal rot, partly because these were common and relatively easy to see and diagnose. All of these were important and affected yields, but observations suggest that additional factors were involved. Another question has been asked: How much did disease reduce yields?
Several diseases affected soybean in Illinois. Charcoal rot was widespread and damaging late in the season due in large part to the hot and dry conditions present over much of Illinois in August. Other diseases included seed and seedling diseases, Phytophthora rot, sudden death syndrome, brown stem rot, and the soybean cyst nematode. Stem canker was a significant problem in some areas late in the season. Wet and cool weather in July prompted out-breaks of white mold in scattered areas in northern areas of Illinois. Root rots were also very common, which affected plants directly and exacerbated the effects of dry weather.
The first indication of root rot came in July, when conditions were relatively cool and wet. A disease problem was reported in soybean fields in central Illinois that damaged and killed plants about a week after heavy rain. The visible symptoms soon subsided and some plants seemed to recover, but many plants had root rot. It was suggested at that time that the root rot associated with this disease problem may continue to stress plants, especially if the soils became dry later in the summer. To some degree this appears to have happened. Root rot was common in September at many Illinois locations, and many soybean plants brought in to the UIUC Plant Clinic for diagnosis had root rot. The causes for the root rot problems are multiple and unclear. Fusarium, Phytophthora, and Pythium were associated with the root rot in July, but the roots on plants later in the season were generally too decomposed for diagnosis.
Additional scouting throughout the season would have helped to reveal and diagnose the diseases that reduced yields. In addition, proper sampling helps diagnosis. Often only the top part of a plant is brought in, but then it is difficult or impossible to diagnose the problem if it originated with unhealthy roots. Whole soybean plants (roots, stems, and leaves) always should be taken in for diagnosis.
So what caused the low yields in 2003? Unfortunately, we don't have data to suggest the relative impact of disease, drought, and aphids (or other factors). All of these were important in some areas in Illinois, and there were interactions. The root rots increased problems with drought, drought increased charcoal rot, and we are not sure yet about possible interactions with the soybean aphid. We cannot focus just on one problem because, as has often been said, causality is multiple.--Dean Malvick