Stress often increases the likelihood of disease and fungicide often decreases stress to ward off certain disease.
Gray leaf spot (GLS) emerges anywhere from silking to maturity and prefers moderate to warm temperatures, above 75° F. Scout for rectangle-shaped lesions on leaves that appear yellow to tan in early infection and gray as the disease progresses. Fields with a history of the disease are especially susceptible to GLS.
Unlike GLS, northern corn leaf blight (NCLB) needs moderate to cool temperatures, less than 75° F. If you see cigar shared gray to green lesions from silking to the end of the season it’s likely NCLB. The disease overwinters in corn residue so corn on corn and fields with disease history are most likely to have the disease.
Anthracnose stalk rot in corn appears at pollination and is most visible just before maturity. The disease occurs when the plant has undergone stress from insect or weather when the pathogen is present. Check the stalk for shiny black blotches and note the inner stalk could also be black—but isn’t always. The stalk will crush easily and could cause the plant to fall. Anthracnose can also appear in soybeans in their leaves and pods. Soybean pods and stems will show irregular brow spots, pods have fewer or reduced seeds per pod and the plant might defoliate.
If it stays wet and gets warm, keep an eye out for frogeye leaf spot in soybeans—especially in more southern states. If infected, you’ll see round brown or gray lesions on leaves with reddish margins. New leaves are most likely to show infection and more likely to be found in fields with a history of frogeye.
Finally, look for white mold in soybeans. It can appear after flowering in cool, wet weather especially in narrow rows or where the infection has occurred before. If present you’ll see plant tops turn gray, with and die and hard black fungal masses on or inside pods and stems.Replant Considerations Double check the calendar, acres left to plant and cost before pulling the replant trigger. The later in the season it gets, the higher the risk for replant. “First figure out what your stand is—how many plants per acre, is it even across the field or patchy?” Grossnickle says. “Look at the time of year, can you replant to corn again or do you need to switch to soybeans? What herbicide did you use—are soybeans even a viable alternative?” Calculate growing days find out how much yield is still available. If there is 70% of a stand left in a field and the calendar indicates there’s only 70% yield potential, it’s better to stick with what’s out there. In addition, keep an eye on weather forecasts to see if there could be more rain coming that could drown out more plants, which could make replanting virtually useless. If replant is the best option, experts recommend keeping the same maturity for corn or soybeans as long as possible. It’s also important to wait for the field to be ready, just because the top ½” is dry doesn’t mean below ground is. Travelling over fields too early could lead to sidewall compaction and poor root growth throughout the season.