Corn and Soybean Disease Update

Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu

Corn
Grey Leaf Spot (GLS) has continued to be the most common foliar disease present in fields at this time. Lesions from GLS typically begin in the lower canopy and progress up the plant under favorable environmental conditions. Due to this, irrigated corn will have higher disease pressure. In corn that is already past R3, foliar lesions that move in should not significantly influence yield potential. In late-planted, irrigated corn, continue to scout for small, tan, rectangular lesions of GLS (Figure 1). As lesions age, the rectangle can expand, following the leaf veins (Figure 2). If over 50% of plants have lesions on the third leaf below the ear leaf or higher prior to tassel, a fungicide application may be considered. Yield potential, grain prices, and cost of application are other factors to consider.

Figure 1. Corn leaf with young lesions from Grey Leaf Spot

Figure 2. Grey Leaf Spot lesions expanding on a corn leaf

Soybean
Full season soybeans across the area are R1-R4. There have been a few reports of downy mildew and low canopy Septoria Brown Spot, which do not typically affect yield. There have also been a few cases of Anthracnose caused by Colletotrichum truncatum and other Colletotrichum sp. This fungal pathogen causes irregular black spots present on the stem, pods, or petioles (Figure 3). If you zoom in on the black zones, you will see acervuli that resemble pincushions with many needle-like structures called setae (Figure 4). These structures produce conidia that spread the disease. Pod and Stem blight will also have black structures visible on the stem, but these are typically in straight lines and are most visible from R6 to R8. It is possible to have both diseases present on the same plant late in the season.

Overall, the heat of the past few weeks has kept disease pressure low across the region. Fungicide applications are typically most economical when disease is present and fungicides are applied during R1-R6 growth stages, with R3 being the most common timing. If you have disease present, and are considering a fungicide application, it is important to scout fields and monitor the weather. Most soybean diseases are favored by humid, wet conditions. If weather patterns continue to be hot and dry, disease pressure will likely remain low. However, as we saw last year, frequent rainfall can lead to serious late season disease issues, so continue to monitor rainfall and disease pressure through R6.

Figure 3. Soybean Anthracnose

Figure 4. Zoomed in soybean Anthracnose acervuli with needle-like setae

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