Disease often goes unnoticed in soybean fields, although it may be causing significant yield losses. Most major soybean diseases are associated with cool, wet conditions and heavier soils. The most cost-effective means of controlling disease is through genetic resistance. Learn more about soybean diseases and other yield robbing pests to help protect yield in your fields.
Soybean cyst nematode (SCN) is responsible for millions of dollars in yield losses each year. Yield losses of up to 30 percent can occur without obvious symptoms. SCN removes nutrients, disrupts water uptake and retards root growth. The result is a stunted, yellow plant with a poorly developed root system. Nematode damage also makes soybean roots more susceptible to invasion by root-rotting fungal diseases.
Soil sampling is the best way to detect the presence of SCN. Rotation to nonhost crops (mostly nonleguminous) and genetic resistance are the best control methods. There are many races of SCN, and soybean varieties offer different sources of race-specific resistance. The source of resistance for most CROPLAN® soybean varieties is traced to PI 88788, resistant to common races 3 and 14.
Brown stem rot (BSR) is a fungus that usually manifests itself after pod development has begun. Foliar symptoms generally appear from August to early September and start as gradual yellowing, followed by wilting, curling and death of leaves. Groups of infected plants have a brownish cast and often lodge.
Losses from BSR are greatest when cool weather occurs during podfill and is followed by hot, dry conditions. The fungus survives in soybean straw and other crop debris, making crop rotation and clean tillage effective controls. Several CROPLAN® soybean varieties are resistant to BSR.
Frogeye is characterized by “eyespots” on the leaves with gray or tan centers and purplish-brown margins. Eventually the entire lesion turns black. Frogeye can also grow on stems and pods. Affected plants may have premature leaf drop.
Frogeye is common in soybean growing areas in the southern United States. Its development and spread are facilitated by warm, moist weather. The disease may be seed-borne or air-borne. Control measures include planting resistant varieties, planting disease-free seed and rotating crops after an infection. Infected fields should not be planted back with soybeans for at least two years.
Asian soybean rust is a highly aggressive and destructive fungal disease with approximately 50 to 60 specific races, which makes control by resistance alone very difficult. Under optimum environmental conditions, wind-borne spores are produced in large quantities and spread quickly on prevailing wind currents. Favorable conditions for severe soybean rust outbreaks include moderate temperatures of 60° F to 82° F with relative humidity of 75 to 80 percent and six to 12 hours of leaf wetness.
Asian rust overwinters on living host plant species such as kudzu, which grows throughout the Southeast. Finding soybean varietal resistance to Asian rust may take 10 or more years because of the large number of races involved. In the meantime, the only effective way to control Asian rust is with fungicides.
Sudden death syndrome (SDS) is a disease common to cool, moist growing seasons. A fungal disease caused by Fusarium solani, SDS symptoms begin with yellowish lesions forming on upper leaves in interveinal areas. The veins remain green while the lesions spread, and, eventually, the entire interveinal surface of the leaf turns brown and the leaf drops off. Unlike BSR, the pith stays white, but surrounding vascular tissue turns gray-brown. The disease is most severe after earlier infections.
Varieties differ in tolerance, but since the disease is most often associated with soybean cyst nematode (SCN)-infested soils, SCN-resistant varieties have improved tolerance. Generally, the disease is more prevalent in the eastern and southern Corn Belt areas, especially in Iowa, Illinois, Indiana and Ohio. Several CROPLAN® soybean varieties have very good tolerance to SDS.
Stem canker symptoms are usually first observed at midseason during stages R5 to R6, when scattered dead plants are seen with leaves still attached. A brown, sunken lesion develops on the leaf scar after the petiole has fallen. This usually appears on one side of the stem near the soil line. Eventually, a reddish-brown sunken stem lesion girdles the stem, and the upper part of the plant dies. Stem tissue above the canker is brown, while the tissue below is green.
The disease can be either seed-borne or air-borne. In the southern United States, wet weather and wind favor disease development. Since the fungus survives on old, infected soybean debris, crop rotation and clean tillage help control its spread. Planting tolerant varieties and using fungicide seed treatment are also effective.
Yellowing or chlorosis, caused by iron deficiency (IDC), is associated with higher pH soils commonly found west of the upper Mississippi River. IDC is most common in the calcareous soils of the eastern Dakotas, Minnesota and northern Iowa. Plants with IDC are pale green or yellow and stunted, with veins remaining green. Dry soils accentuate the problem.
The effects of IDC are reduced by delaying planting until mid- or late May, early-season cultivating and ridge-tilling. The best control method is genetic tolerance to IDC. Many CROPLAN® soybean varieties have excellent IDC tolerance.
Usually, sclerotinia white mold (SWM) symptoms do not appear until two or more weeks after flowering. The disease is associated with cool, moist weather. It causes wilting and death of upper leaves, similar to other soybean fungal diseases. Infected plants exhibit lesions with white, cottony, thick fungal growth just above the soil surface.
For control, avoid continuous cropping and rotation with sunflowers, dry beans and other susceptible broadleaf host crops. Varieties do exhibit varying levels of field tolerance to SWM, but no varieties are immune. Several CROPLAN® soybean varieties have acceptable levels of field tolerance.
Phytophthora root rot (PRR) can infect plants at any stage of growth, either killing them or significantly reducing productivity. The disease favors cool temperatures and high soil moisture. PRR frequently develops in fields with poor internal drainage and also occurs in normally well-drained fields that are saturated for 10 to 14 days. Symptoms include wilting and yellowing of upper leaves with a dark-brown discoloration of roots that eventually moves up to the stem. The PRR fungus survives and overwinters in soil or buried crop debris.
The most effective method of control for PRR is planting varieties with genetic resistance. Specific genes provide resistance to the many different PRR races. Minor gene combinations provide general field tolerance to PRR without specific race resistance. Many CROPLAN® soybean varieties carry both forms of resistance.
Soybean aphids can impact plant health in two ways: causing yield reduction due to physical damage from aphid feeding and transmitting soybean mosaic virus (SMV). SMV carries a double punch, causing both yield reduction and seed staining that can result in dockage, particularly in specialty soybeans.
Your local agronomist can help with strategies for effective scouting, weed management and crop protection that, when partnered with appropriate insecticide choices, will help hold soybean aphid infestations below the economic threshold.