Some soybean fields are beginning to change color and some corn is already half-way to the milk line. While disease threat on yield is minimal at this stage, some diseases may still limit seed fill and may cause crop harvest problems. Diseases observed while scouting this week included Goss’s wilt, fungal leaf diseases, charcoal rot, and stem canker.
Goss’s Wilt
This year we are finding high Goss’s wilt incidence and for some fields, high severity (Figure 1). For several corn fields, only a handful of plants in the head rows were infected. Severe Goss’s wilt is causing plants to wilt and dry prematurely, and those plants may have light seed weight. Some fields show Goss’s wilt affecting the upper leaves of the plants (Figure 2). The hail events we have had may have helped increase its incidence and severity.
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| Fig 1. Corn plants showing Goss’s wilt symptoms. | Fig 2. Corn plants with Goss’s wilt affecting the top leaves. |
Fungal Leaf Diseases
Some fields have elevated levels of fungal leaf diseases including common rust, gray leaf spot, and northern corn leaf blight (NCLB) (Figure 3). These diseases are promoted by wet and humid weather and also hybrid susceptibility. The common rust pathogen does not survive in South Dakota, while gray leaf spot and NCLB survives in South Dakota on infected corn residue.
Figure 3. Common rust observed in high severity.
Charcoal Rot
Charcoal rot was observed at low incidence in a few soybean fields. The charcoal rot pathogen, Microphemina phaseoli, survives in soil and infects the plant through the roots. The pathogen clogs the roots which lead to a blockage of water and mineral salts being transported up the plant. Plants affected by this disease may be in patches or scattered in the field (Figure 4). Infection takes place early in the season but symptoms develop later in the season when plants are stressed by water and heat.
Figure 4. A soybean field with stem canker in Minnehaha County on Aug. 27, 2015. Notice the short plants in the center of the picture that died pre-maturely because of charcoal rot.
Stem Canker
Several scouted soybean fields had low incidence of stem canker. Stem canker can affect single plants or a cluster of plants in an area. Lesions start to develop on the stem at early flowering time but may not kill the plant. As the season progresses, the lesion on the stem elongates and may kill the plant. Lower branches below the lesion may remain green (Figure 5).
Figure 5. Soybean plants showing stem canker symptoms.
Management
There is almost nothing that can be done at this time of year to manage late season diseases in corn and soybeans. However, assessing your crop late in the season will give you an idea how the cultivar you planted is performing and what late season diseases occurred in your field.
This scouting information should help inform you when making decisions and preparations for the next growing season. Cultural practices such as crop rotation, cultivar/hybrid selection, drainage, proper plant population, weed control, and tillage to bury residue, are some of the practices that may limit development of diseases next season.
