Late Season Crop Diseases


Plant Nurtrients

Late season diseases are occurring in corn and soybeans. Tar spot in corn and sclerotia or white mold plus soybean cyst nematodes (SCN) in soybeans. In many cases, it’s too late to prevent the diseases. However, there are some management practices to reduce future incidences of the diseases.

For corn tar spot, the symptoms include irregularly shaped black structures on leaves. The black structures are firm, appear mostly smooth on the surface and the spots do not rub off or break open. Tar spot can also produce black spots circled by tan lesions with dark borders. Tar spot overwinters on surface crop residue. The spores are dispersed by the wind and rain droplets splashing the inoculum onto plants.

The inoculum likes cool summer conditions with adequate moisture and high humidity. The recent August rains with cooler days and nights is causing good growing conditions. Field with a history of tar spot are most susceptible. The best time to apply a fungicide is at tasseling (VT) up to the blistering stage (R2). Most of our corn is past this stage, so it is too late. In many cases, the tar spot got a late start, so the disease is not rampant this year. But it is a source of inoculum for future years.

Crop management practices that help control tar spot are fungicides and crop rotation. Once the tar spot inoculum is in the field, it hard to get rid of it. Even tillage has not proven to be effective. Getting residue to break down quickly and even some other fungus associated with the tar spot are known to reduce tar spot inoculum. The following information comes from Purdue University. White mold is caused by the soilborne fungus Sclerotinia which is capable of causing disease on hundreds of different plant species including soybean, pea, canola, potato, sunflower, and tomato. The pathogen can also infect several weeds such as red root pigweed, ragweed, common lambs quarter, velvetleaf, and dandelion.

White mold symptoms started to appear in August when the soybean plants began to flower. Disease signs include fluffy white mycelium (mold) growing from bleached lesions. Small black structures called sclerotia may also form on or in the stem which are capable of surviving harsh weather conditions. The sclerotia can be 1/8-inch to 3/4-inch long and are initially soft but harden with age. When cut, the inside of the sclerotia are white. When soybeans are harvested, the sclerotinia are about the same size as the soybeans and look like black rat turds!

The disease thrives on cool summer temperatures and wet conditions. Often the best-looking tall growing soybeans get infected. The sclerotinia are a source of infection for following years. One strategy is to plow them under. However, if tillage is continued, the preserved sclerotinia can become a source of infection in future years if they get within 2 inches of the soil surface. The other strategy is to leave them on the soil surface where they can degrade quicker. Sclerotinia may last in the soil for several years.

Fields infected with Sclerotinia or white mold infected fields should be harvested last. Once in the combine, it may be spread easily from field to field. Thoroughly clean the combine before moving to another field. Prevention is the first and best strategy.

Row spacing and planting population are strategies to fight white mold. In infested fields, increasing air circulation by planting in 15-inch row spacing or wider and planting populations of 125,000 to 150,000 plants per acre may lower the chance for future white mold infections. Rotating to non-host crops reduces the chance for white mold development. Corn and wheat are non-host crops. Also select soybean varieties for white mold resistance. Several fungicides and bio-fungicides are available and labeled to limit or suppress white mold, but proper timing and good canopy coverage are important. The fungicides and bio-fungicides should be applied no later than beginning pod (R3) growth stage at 15 to 20 gallons per acre (GPA).

For Soybean Cyst Nematode (SCN) look for yellow and stunted plants. The soybean cysts can survive for several years in soil without a host. Cereal rye and annual ryegrass are two known cover crops that may reduce SCN populations. Plant the cover crop early in the fall when soil temperatures are above 500F Cysts will come out and since these grass plants are non-host plants, the SCN population may decrease 50% to 80%. The SCN can not distinguish the grasses (non-host plants) until they come out of their cysts and then do not have time to complete their life cycle before winter. Have a safe harvest!