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Sponsored: Be on the Lookout for Soybean Diseases

Hot, humid conditions conducive to disease pressures

As the 2016 growing season winds down, growers should be on the lookout for yield-robbing mid- to late-season soybean stem diseases, such as sudden death syndrome and stem canker, and taproot decline.

With each of these diseases, infection may have occurred early in the crop year, but symptoms of infection often aren’t observed until soybeans reach the R5 growth stage or later. However, properly diagnosing and managing these diseases now could improve your potential yield in 2016 and in future years.

Sudden death syndrome (SDS). Soybeans planted early into cool, wet soils followed by heavy summer rains are at high risk for SDS, a serious yield-robbing disease. SDS thrives in wet soils and cool temperatures. After overwintering in crop residue or the soil, the fungus can grow and infect next year’s crop.

Early symptoms of SDS include yellowing and defoliation of the upper leaves in a small area of the soybean field. As the infection grows, the number of areas showing symptoms increases and the areas enlarge. A close look at the leaves show a mottling or mosaic pattern. The leaf tissue between the major veins turns yellow, dies and then turns brown.

To diagnose SDS, examine the stem to see if the interior is slightly tan to light brown. If the interior of the stem is a dark brown color, that can indicate brown stem rot, another soybean disease.

While there is not much a grower can do to control SDS during the growing season, disease management can be improved by selecting a soybean variety with some resistance to SDS and using a seed treatment to reduce the threat. Management practices such as crop rotation, planting date and wider row spacing can also help to reduce the impact of this detrimental disease in future seasons.

Stem canker. Often triggered by hot, humid and wet weather conditions, stem canker is caused by the fungus Diaporthe phaseolorum. While heat and excessive moisture can activate the dormant soil-borne fungal bacteria, hail damage and reduced tillage production systems may intensify infection risk.

The disease can be identified by its reddish-brown cankers that prevent needed nutrients from freely traveling through the soybean plant. The cankers, which have dark reddish-purple margins, are located on the plant’s lower stems, petioles and nodes. Like sudden death syndrome, stem canker infections can cause leaves to yellow and exhibit green veins.

Fields should be scouted for stem canker beginning at pod development and continuing until harvest.

To reduce the risk of future infection, it is recommend growers with a history of stem canker plant resistant varieties in a conventional tillage system. Crop rotation and fall tillage also are risk reduction strategies.

Taproot decline. A root-associated disease, taproot decline was only recently identified in southern soybeans. It is often confused with sudden death syndrome.

While symptoms of SDS and other diseases often appear widespread, taproot decline can be identified by interveinal chlorosis scattered in small clumps throughout the field or even affecting a single plant. A thick black growth at the plant stem base and/or along the soil line is an additional symptom of taproot decline.

The disease does not appear to be soil-type- or varietal-type-dependent. Researchers are studying the newly named disease to help determine management recommendations.

Other soybean diseases with the potential to reduce harvested yield include white mold and frogeye leaf spot. If a fungicide treatment option is available, time your treatment applications at early disease onset for maximum efficacy.

White mold. Cool summer temperatures (less than 85 F) and excess moisture from rain, dew or high humidity favor the growth of white mold, a serious yield-robbing disease.

When scouting for white mold, look for water-soaked stem lesions above or below nodes that eventually cover the entire stem. Late-season stems infected with white mold will look bleached and stringy. The lesions also can infect pods, petioles and leaves. In severe cases, plants wilt, lodge and eventually die.

Control white mold before it starts by planting a resistant soybean variety. Crop rotation, wider row spacing and lower plant populations that can help to open up the canopy also may reduce the severity of future infections. 

In-season, broadleaf weed control is important because weeds can serve as hosts for white mold. Foliar fungicides also can help control white mold infections if applied immediately before infection.

Frogeye leaf spot. A foliar disease, frogeye leaf spot occurs more frequently in southern areas under hot, humid weather conditions. While the disease can strike at any growth stage, it most frequently appears after flowering.

The earliest signs of infection are small, yellow spots on the leaves that can reach ¼ inch in size. The centers of the spots become gray to brown and have reddish-purple margins. Eventually, the spots grow together in an irregular pattern. Stems and pods also can be infected with lesions. Early in the infection, the disease may be mistakenly identified as herbicide drift.

Frogeye leaf spot lives in crop residue from a previously infected soybean crop. The fungus produces long, narrow spores that rise from the lesions and move with the wind to other fields. As the disease advances, it reduces green leaf area and infects pods and seed formation, causing significant yield losses.

Foliar fungicide application during late flowering and from early pod set to pod-filling stages can reduce the impact of frogeye leaf spot.

Diagnose diseases. Proper diagnosis is important to controlling future soybean disease outbreaks. When scouting for diseases, pay close attention to no-till fields where more crop residue can allow for increased overwintering of disease organisms.

While seed treatments can control some diseases, such as Phytophthora root rot and pythium blight, management practices can further assist with disease management.

Contact your local seed adviser or agronomist for help making the proper diagnosis. Also work with your agronomist and sales team to pick the best soybean varieties for the pathogens in your area. More information on these diseases are available at Mycogen.com/Agronomy.  

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