While every season has its challenges, some fields can face major threats to yields before the crop is even planted. These threats come in the form of diseases that can overwinter in soil and are particularly challenging to continuous corn and soybean programs.
Most pathogens only survive if a host is present. However, several common corn and soybean diseases can survive in debris, such as stalk, leaves and roots. Once the debris decays, the pathogens left in the soil are greatly reduced.i
Most of the time, plant debris does not fully decompose over the winter between harvest and planting. Plant material left behind from last year’s crop is still present in the spring. With host plant debris left behind, problematic pathogens from the last growing season survive and are ready to infect the host crop when it is planted again. If the host crop is not planted, the pathogen will no longer be able to survive once the debris is fully decayed. However, it may take longer than one season for debris to fully decay in no-till and reduced tillage fields. Even so, crop rotation is an important strategy that helps reduce several common corn and soybean diseases that overwinter in soil.
Corn diseases that overwinter in soil
Gray leaf spot: This corn disease is caused by the fungus Cercospora zeae-maydis. It most commonly occurs after extended periods of high humidity and warm temperatures, or wet weather. At first, the disease appears as small spots with halos. These spots elongate to rectangular lesions about 1/8-inch-wide and up to three inches long. The lesions are usually gray to brown in color.ii Gray leaf spot is most often found in the corn belt states and may lead to reduced yields by up to 40 bushels per acre.
Northern corn leaf blight: This disease is caused by the fungus Exserohilum turcium. It is most common in wet and humid weather conditions. Northern corn leaf blight is characterized by grayish-green or tan oblong lesions that range from one to seven inches. These lesions begin on the lower leaves and can progress throughout the plant up to the husks. If the disease occurs during corn tasseling or silking development, it can greatly reduce yields by up to 50 percent.iii
Tar spot: This corn disease is caused by Phyllachora maydis. Tar spot develops when there are cooler temperatures with high humidity and frequent rainfall. It appears as a series of black spots containing spores on corn leaves and can cause reduced photosynthesis, resulting in poor stalk quality and standability. Tar spot is newer to the United States, arriving in the Midwest in 2015. The tar spot fungus seems to overwinter in corn debris. However, it is unknown how effective crop rotation is in reducing the disease.iv
Anthracnose leaf blight: This fungal disease is caused by Colletotrichum graminicola. Anthracnose leaf blight develops when there are extended periods of cloudy, wet and warm weather. This disease first appears as a leaf blight on lower leaves with uneven brown spots. These spots can spread to the upper leaves and can grow to be three-quarter inches long. Eventually, the spots will appear tan in the center and bordered by yellow, red, or brown. It is possible for anthracnose leaf blight to advance to stalk or stem rot. This could lead to death of the plant, stopping grain fill and causing yield loss. The premature plant death may also cause lodging, which complicates harvest.
Soybean diseases that overwinter in soil
Frogeye Leafspot: This soybean disease is caused by Cercospora sojina. This fungal disease prefers consistent, high humidity for development. Frogeye leafspot appears as irregular lesions with purple borders and are observed in the upper canopy. When young, silvery spores can be seen on the underside of lesions. Leaves, as well as stems and pods can be affected.v Frogeye leafspot has the potential to cause yield losses of up to 30 percent.vi
Brown Spot: This common fungal disease is caused by Septoria glycines. It develops shortly after planting and can progress through upper canopy, causing yield losses. Brown spot develops when there is warm, wet weather. It first appears as purple lesions on young plants and develops into irregular-shaped, dark brown lesions. Lesions may cause leaves to turn yellow and drop. Brown spot can damage plant leaves, stunt pod fill, and may reduce soybean yields.vii
White mold: This soybean disease caused by several species of Sclerotinia. This disease occurs more often when there are cooler temperature or above average rainfall at the time of flowering. In soybeans, white mold appears as a cotton-like fungal growth on the stems. It can cause pockets of dead and drying plants during flowering. Affected plants also wilt and stems may appear bleached. White mold can reduce yields by two to five bushels per acre for every 10 percent increase of the fungus. This disease is common throughout the north central United States. White mold can survive in the soil for many years, making it difficult to prevent and control with crop rotation alone.viii
Crop rotation breaks the disease life cycles
“Although there are pathogens that can survive for many years in a given field, some of the most important ones overwinter only on the residue from previous plantings,” said Randy Myers, fungicides product development manager at Bayer. “Crop rotation, along with tillage, if used, promotes degradation of the crop residue before the next time the vulnerable crop is planted.”
Integrated disease management
While crop rotation is an important strategy for mitigating disease, not all diseases will be managed by crop rotation alone. Integrated disease management programs involve combining techniques, such as crop rotation, tillage, hybrid selection and fungicide selection.ix
“Pairing crop rotation with a fungicide is an effective combination of techniques that mitigates disease and improves plant health,” said Myers. “Crop rotation reduces the size of the pathogen population in the soil from the beginning of the season. Fungicides provide an added defense to help manage and control disease all season long.”
Bayer Solutions for disease management
Myers recommends using Delaro® for disease control in corn and soybeans. Delaro provides preventative and curative effects for unmatched, broad-spectrum disease control.
With two modes of action, Delaro helps manage and prevent the development of disease resistance and helps maximize control of tough diseases that overwinter in soil and plant residue. For corn such as gray leaf spot, northern corn leaf blight, tar spot, and anthracnose leaf blight. For soybeans, this includes frogeye leafspot, brown spot, and white mold. In addition, Delaro can be used to control diseases, such as common rust and southern rust in corn, that are blown in by the wind and are not well managed by crop rotation.
Delaro improves plant health by promoting healthy, dark green leaves for improved photosynthesis. Use of Delaro also increases plant stress resistance to utilize the full genetic potential and yield of the seed to help growers achieve their personal best yields. To learn more visit, delaro.us.
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