Fungicides have traditionally been one of the last inputs that go into a corn and soybean strategy, however, between 2009 – 2013, many growers were regularly applying fungicides to their crops as a preventive measure, even without any disease pressure. Today, economics challenges the value of all inputs. Even with more scrutiny on inputs, industry surveys show overall fungicide sales remained strong during 2017. The grower community acknowledges the increased awareness of plant disease and its impact on yield. Through University and Extension education programs, the benefits of fungicide use are becoming better understood.
Soybean Disease Management
Sudden Death Syndrome (SDS)
Select soybean varieties have partial resistance to SDS. Reducing soil compaction is important. You can also reduce risk by delaying planting by a week or two and planting into warmer soils. Populations of SCN are usually associated with SDS and may increase its severity. Rotating to corn is not effective. Crop residue such as corn kernels and corn roots harbor the SDS pathogen in the soil.
Frog Eye Leaf Spot
Early season infections from infected seed, result in stunted seedlings. This fungus survives in infested crop residue and infected seed. Resistant soybean varieties are available. Foliar fungicides applied during late flowering and early pod set to pod filling stages can reduce the incidence of frog-eye leaf spot.
Septoria Brown Spot
Septoria Brown Spot is a common leaf disease. Yield losses of 5 – 8 % may occur under severe conditions when significant defoliation occurs. It can develop on the first true leaves early in the season. Because the pathogen survives on soybean residue, minimum tillage and continuous soybeans may enhance this disease. Fungicide applications made during R3 through R5 growth stages can protect yield.
Soybean White Mold (SWM)
SWM is a stem rot. This fungus survives in the soil for several years as sclerotia. No soybean variety is completely resistant. Many common weeds found in soybean fields are hosts of SWM. High weed populations of any weed in a soybean field may also increase the density of the total plant canopy and promote a moist climate that favors disease development. Legume and Brassica cover crops can act as hosts of sclerotia and should be avoided if there is any concern of SWM. A fungicide applied at the R1 growth stage provides a higher level of control than applications made at the R3 growth stage. Efficacy of fungicides for SWM declines significantly after symptoms are visible on the plants.
Corn Disease Management
Northern Corn Leaf Blight
Identified as cigar-shaped lesions unrestricted by leaf veins. This fungus overwinters in corn residue. Fungal spores are dispersed by wind and splashing water. Early infection leading to diseased upper leaves can increase yield losses. If the disease is present on 50 percent of the plants in the field at tasseling (one or more lesions per plant), a fungicide application may be necessary to protect yield. Severe NCLB development during grain fill can result in yield losses of 30 percent or more.
Eyespot is a residue-borne, fungal disease that can reach levels causing yield loss in susceptible hybrids. Higher risk of disease occurs under no-till conditions and when corn is planted sequentially for two or more years. The first visible symptoms of eyespot are small, circular spots, water-soaked with yellow halos on leaves. These can occur as early as the V3/4 stage. Eye spot favors cool temperatures, humid and wet conditions, continuous corn, conservation tillage, and susceptible corn hybrids. Fungicides are economically beneficial if applied early.
Gray Leaf Spot
Gray leaf spot can occur every growing season. The fungus survives in corn residue and is often more severe in corn following corn. Lesions always start in the lower canopy. GLS can result in high yield loss when the disease spreads to leaves above the ear after tasseling. High GLS severity may also increase the risk of stalk rot. Fungicides are effective at reducing disease and protecting yield.
Yield losses from Goss’s Wilt can exceed 30% for susceptible hybrids. Goss’s Wilt primarily infects leaves that have been wounded by circumstances such as hail, sand-blasting, rain, wind, and strong storms. The primary symptoms on leaves are elongated tan lesions with irregular margins extending parallel to the veins. Dark, water-soaked spots (‘freckles’) develop in the lesions. Goss’s Wilt is caused by bacteria, so we do not recommend applying fungicides. This disease can be managed using resistant hybrids, rotating away from corn production, using tillage that buries corn residue after harvest, and by controlling grassy weeds.
In summary, during the season it’s not uncommon to see corn and soybean diseases. Since conditions change from year to year, it’s important to be prepared by learning how to identify key diseases and take action to protect against hidden yield losses. With today’s economics, it’s about creating tomorrow’s success today.