Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
Extreme heat followed by lots of moisture over the past few weeks has led to the appearance of several abiotic symptoms, particularly leaf scald in corn. Sunburn on the leaf can resemble foliar lesions (Figure 1) but is often present on the youngest leaves with most exposure in the upper canopy (Figure 2). However, as we continue with wet and humid weather patterns, this creates a favorable environment for fungus among us. In corn, yield reductions are typically observed when lesions are present on the two leaves below the ear leaf or higher, so these are the leaves to pay close attention to when scouting. If corn has not yet tasseled and over 50% of plants have lesions on 5% or more of this leaf surface (ear leaf or 2 below), you may want to consider a fungicide application. If applying a fungicide, VT/R1 timing has shown the greatest chance of economic return for grey leaf spot. The 2025 Fungicide Efficacy for Control of Corn Diseases, provides ratings of product performance across multiple diseases based on trials conducted by Extension specialists across the country.
Field areas saturated for extended periods may begin to look yellow. If soybean plants do not recover after the soil has dried, you may want to check for Phytophthora root rot (PRR). Phytophthora in soybean is one that we don’t consistently see, but the weather conditions this year have been conducive, so it is one to keep an eye on. In mature plants yellow leaves and wilting are the first symptoms. When dug up roots will be brown and rotting, these plants will easily pull from the ground. You may also see a dark or purple discoloration on the stem (Figure 3). With this disease leaves remain attached, and you may see dead plants in a row or as patches, particularly in those low field areas. The PRR that impacts soybean is caused by a different species (P. sojae) than the ones we see in vegetable production, so crop rotation is a good management option. Other management approaches include soybean variety selection for genetic resistance and seed treatments.
In addition to diseases like PRR, nematodes are also in many of our soils. Soybean cyst nematode (SCN) and root-knot nematode (RKN) may act alone or could be in combination with PRR or other diseases like soybean sudden death syndrome, Diaporthe diseases, brown stem rot, or others. Root-knot often has more visual symptoms than SCN and any time the plant has been stressed, these tend to be more pronounced. With root knot nematode, you will be able to see galling when roots are dug (Figure 4). We will discuss more about RKN in next weeks article.
As far as foliar disease, I have been seeing a bit of low canopy Septoria Brown Spot. Leaf symptoms begin as small brown spots that can have a yellow halo around them, as spots coalesce the leaf may turn yellow (Figure 5). This disease usually stays to the bottom of the plant, limiting yield impact. Fungicide applications in soybean are most economical when disease is present and fungicides are applied during R1-R6 growth stages, with R3 being the timing with best return on investment. 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. The 2025 Fungicide Efficacy Recommendations for Foliar Diseases of Soybean provide an overview of product ratings based on trials conducted by Extension specialists across the country.
Figure 1. Leaf scald on corn following high temperatures.
Figure 2. Leaf scald symptoms on corn present on younger leaves near the upper canopy.
Figure 3. Stem lesion from Phytophthora root rot.
Figure 4. Root galling on soybean caused by root knot nematode.
Figure 5. Low canopy Septoria brown spot.