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By Alison Robertson, Department of Plant Pathology and Microbiology
If you are not out scouting corn fields for leaf diseases, and soybeans for disease, you should be. I have seen and had several reports and emails of various diseases occurring throughout the state. Most diseases on corn are hybrid specific, so scouting can be targeted to those hybrids that are rated on the susceptible end of each company's rating scale.
Northern leaf blight (NLB) and gray leaf spot (GLS) can each be found in the lower canopy of corn in southern and central Iowa depending on the hybrid. Infection by the NLB fungus is favored by frequent rains and overcast conditions, like those we had in late June. Warm and very humid conditions favor GLS development.
Our threshold for application of a fungicide is: corn foliar disease (not including common rust) observed on the third leaf below the ear or higher on 50 percent of the plants. We have a fungicide trial evaluating this threshold in Ames. This same trial is being done in by colleagues in Wisconsin, Illinois and Ohio and is part of a research project funded by the USDA. We have inoculated the trial with the NLB and GLS fungi. Severe NLB disease has developed in the inoculated plots.
Holcus leaf spot has also been reported but at very low incidence and severity — a few plants with as many spots. Holcus is a bacterial disease. Warm (75-85 F), wet, windy conditions early in the season favor infection and the development of Holcus leaf spot. Symptoms are light tan (sometimes almost white), round, oval spots, which may appear water soaked at the margins or have a light brown border occur on the lower leaves. The spots are initially about 1/4 inch in diameter, but sometimes grow larger and coalesce into irregular spots and streaks of dead tissue. Later the lesions dry out, turn light brown and have a papery texture.
Holcus spot symptoms can resemble those of eyespot. Eyespot symptoms are smaller, round spots with a distinct brown border and a yellow halo. Eyespot was reported earlier in the season on specific hybrids, but I suspect the warm weather we have had since the beginning of July has stopped further disease development. Eyespot is favored by cool (70s), wet conditions.
I continue to receive reports of Goss's wilt; once again it is very hybrid-specific. Physoderma brown spot is also prevalent.
Soybean stem diseases
Soybean stem diseases, Phytophthora root rot and northern stem canker have also been reported. Both diseases cause brown lesions to develop on the outside stem of plants. Infected plants wilt and die. Phytophthora is usually more prevalent in low wet spots in a field that are prone to flooding. The pathogen, an oomycete, infects the roots of plants via swimming zoospores, thus the brown lesion on the stem extends from the tap root for several nodes above the soil level (Figure 1). Infection by the stem canker fungus usually occurs at the second or third node of the plant, and the canker (brown lesion) extends up the plant. So, to distinguish between these two stem diseases of soybean, check for green stem tissue at the soil level. If the lesion extends from below the soil surface, the disease is likely Phytophthora root rot, if the lesion begins above soil level, stem canker is likely.
Phytophthora- resistant varieties are available although the pathogen continues to diversify within fields so that one resistant gene will not usually be effective. Try to select varieties that also have some partial resistance or field tolerance. Northern stem canker is reported from time-to-time in Iowa. There is no known resistance to the disease. Good reviews of both diseases are available at the Plant Health Initiative website.
Figure 1. Wilted plants with a brown lesion extending from the tap root up the plant stem are characteristic symptoms of Phytophthora root rot.
Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at firstname.lastname@example.org or phone 515-294-6708.