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Weather, fertility key components: How you can slow soybean diseases

Soybean disease pressure in the Delta was higher in 2002 than it has been for several years, but that won't help growers know what's to come in 2003. While some of the disease pressure can be blamed on weather conditions, Mississippi State University's Alan Blaine says some disease problems were directly correlated with fertility.

“More than 50 percent of the participants in our SMART program needed a maintenance fertilizer application in their soybean field,” said Blaine, Extension soybean specialist with MSU.

Blaine highly encourages growers to use a broad-based seed treatment that covers Pythium, and an inoculant treatment for soybeans planted following cotton.

Among the mid- to late-season diseases that have the potential to infect the Delta's soybean crop, Blaine says stem canker is considered the Delta's most problematic disease. “The disease has been very sporadic and losses minor because of an increased number of resistant, high-yielding varieties, but yield losses of 30 bushels per acre were attributed to the disease in state variety trials in 1991,” he says. “Some stem canker occurs in Mississippi every year, and soybean producers should keep stem canker resistance high on their list of desired varietal characteristics.”

Caused by the fungus Diaporthe phaseolorum var. caulibora, stem canker is identified by a canker on the main stem. Another symptom of the disease is a yellowing between the veins of leaflets of trifoliate leaves in the upper part of the soybean plant.

Blaine suggests examining the main stem closely to find a canker either extending below, above, or up and down, from the attachment of a leaf petiole. “If you cannot find a canker in association with a leaf petiole on the main stem, examine the main stem from the soil line up for several inches on the plant. The lesion will generally be light to dark brown with a definite margin between the healthy and diseased tissue.”

No seed treatments are generally recommended for the control of stem canker. Instead, Gabe Sciumbato, plant pathologist at Delta Research and Extension Center in Stoneville, Miss., suggests planting resistant varieties to control potential outbreaks of the disease. Because environmental conditions favorable for the disease often occur early in the season, disease specialists recommended growers plant soybeans after May 15 if planting a moderately susceptible variety.

In addition to stem canker, there are several foliar diseases that Mid-South soybean growers should keep on their radar screens, according to Sciumbato. These include aerial web blight, cercospora, frogeye leaf spot, and purple leaf stain.

Since it is caused by Rhizoctonia, the same organism that causes sheath blight in rice, the potential for an aerial web blight infestation in soybeans can be much worse following rice.

Normally a south Louisiana problem, aerial web blight can flourish in the central and northern Delta if provided with the unseasonably high temperatures and high humidity levels.

The disease can often be identified by simply looking down into the canopy to check for leaves that appear to wilt and stick together. A spider web-like mass is often also visible. Varietal differences are evident, but if growers do not experience extremely hot, humid weather, it may not be a big concern, Blaine says.

In cases of severe infestation, Sciumbato recommends treating with the fungicide, Quadris. “If you have a susceptible variety planted, and you have a history of aerial web blight in that field you will likely need to spray a fungicide. That's especially true if you are planting soybeans behind rice,” he says.

While frogeye leaf spot caused severe soybean yield losses in the Mid-South region in 1989, the disease is not very common in most years because many varieties are resistant.

Several soybean varieties released in the last five to 10 years are susceptible, however, and extended periods of wet weather during the growing season can favor disease development. In fact, Sciumbato says, infestations of the disease were severe enough in some Delta locations in 2002 to warrant treatment.

Frogeye leaf spot, as the name indicates, is primarily a foliar disease of soybean caused by the fungus, Cercospora sojina. It's often seen in fields where infected seed is planted, or where the disease occurred in the previous year's soybean crop and the land has not since been rotated to another crop.

Symptoms of the disease include lesions that first appear on the upper surface of leaves, are circular, and vary in size from less than 1 millimeter to 5 millimeters in diameter. These lesions are distinctive in that the brown spots are surrounded by a narrow red or dark reddish-brown margin. In extreme cases of infection, the affected leaves may die causing early defoliation of the soybean plant.

Managing frogeye leaf spot most often equates to avoidance by planting resistant varieties. However, if your soybean field was infected with the disease last year, that step alone may not be enough. Disease specialists suggest rotating that field to a different crop for at least one year, and preferably two.

Cercospora leaf spot can be identified at the beginning of the pod-filling stage as a mottled purple to orange discoloration of the uppermost leaves. Infected leaves often turn dark purple and appear leather-like as the season progresses, eventually turning an orange color when the plant approaches maturity.

“Most of the time, Delta growers will not treat their soybean crops for cercospora or purple leaf stain, either for economic reasons, or because the disease will not seriously limit yield potential,” says Dan Poston, soybean specialist at Delta Research and Extension Center in Stoneville, Miss.

When scouting soybean fields for weeds and insects, disease specialists suggest taking notes on the severity and prevalence of diseases infestations. It's best, though, to avoid these diseases altogether by planting resistant varieties, practicing crop rotation, and planting treated seed.

The use of a foliar fungicide may be beneficial if the disease is present and environmental conditions that favor disease development occur.

Fungicides, however, are not generally recommended since disease pressure is rarely sufficient to reduce soybean yield below the cost of treatment,” Sciumbato says.

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