As we head into late summer, some mid-season diseases become visible in soybean fields. Based on our observations and samples submitted to the ISU Plant Disease Clinic, we think that overall disease pressure is low this year. However, we have noticed three relatively new diseases that you may find while scouting crops.
Fusarium wilt. Also called Fusarium blight, this disease was found in late July. It is caused by Fusarium oxysporum, a very common soilborne fungus. Plants killed by this disease appear from a distance to have Phytophthora root rot, and may be scattered or in small patches in the fiel. One symptom is wilting of stem tips, especially on young plants, during or before flowering. Upper leaves are wilted and seem to be scorched. The middle or lower leaves turn yellow or have pale yellow spots, then wither or drop prematurely. Unlike Phytophthora root rot, there should be no evidence of any lesions on the stem and root rot is minor. The stem pith may be brown, so the disease can be mistaken for early-season brown stem rot. Soybean cyst nematodes and herbicide injury can predispose soybeans to infection by this fungus. To prevent future problems, plant high quality seed, use SCN-resistant varieties, and reduce other stress factors.
Downy mildew. Usually this disease is not severe in Iowa, but we have noticed an increased prevalence this year compared to previous seasons. Downy mildew symptoms first appear on the upper surface of topmost leaves as light-green to yellow spots, which enlarge over time to form large yellow lesions. The lesions may spread and cover the entire leaf surface and cause the leaf to turn brown and fall off the plant. Seeds from infected pods will be small, have cracked seed coats, and will be covered with a pale coating of spores. If contaminated seed is planted the next season, the fungus will systemically infect the seedling and cause stunting and mottling of the leaves. Growers planning to save seed for planting next season should be aware of the risk factors involved with this disease.
Cercospora leaf spot. This disease was observed last year in many areas of Iowa but this year has mostly been seen only in the north central part of the state. Frequent rains in this area relative to the rest of Iowa have increased the incidence of this disease.
Cercospora leaf spot can be identified at the beginning of the pod-filling stage as a mottled purple to orange discoloration of the uppermost leaves. The discoloration is most visible on the undersides of leaves but can be observed faintly on the upper leaf surfaces as well. Later in the season the leaves will become dark purple and have a leathery appearance. When the plant is approaching maturity, the leaves will become orange or bronze in color. Severe infection may cause early death of the leaves and defoliation. This may be mistaken for natural leaf yellowing when mature. The fungus also can infect seeds and cause a purple discoloration of the seed coat that can cause poor seed vigor and reduced germination if planted next year. Yield loss from Cercospora leaf spot is usually slight, however, the incidence of Cercospora has been increasing in recent years. We will continue to monitor the situation and alert growers if the need arises.
White mold. As far as the first week of August, we have observed very little white mold activity, which is much less than what we saw last year. Lack of rain in July and early August apparently slows the activity of this fungus. Early-planted soybean fields that have already flowered should be at a low risk because the fungus enters plants from flowers. After this time, the fungus is unlikely to cause measurable damage.
However, among our five white mold research sites in central and northern Iowa, the one near Mason City has a considerable amount of apothecia, mushroom-like fungal structures that produce spores. George Cummins, extension field crop specialist for Charles City, also reported observing a field where plants were progressively killed by white mold. Since northeastern Iowa has received more rain than other parts of the state, we can anticipate more disease in that area.
This article originally appeared on pages 155-156 of the IC-478(20) -- August 18, 1997 issue.