Roundup of key soybean diseases from 2013 season
The soybean diseases of most concern this year in Wisconsin were white mold (also known as Sclerotinia stem rot or SSR) and Sudden Death Syndrome (SDS), according to UW-Madison plant pathologist Damon Smith. In terms of the most acres affected, this pair led the way, with white mold most prevalent in southern Wisconsin. Northerly counties experienced more drought.Septoria leaf spot (otherwise known as brown spot) showed up this year, which was kind of unusual, Smith says. Some viruses also appeared in 2013. Soybean vein necrosis disease (SVND), a recent discovery in Wisconsin, was previously reported on in Agri-View, and tobacco streak virus was picked up late in the season by the industry. Smith says SVND appears to be present in 10 percent of acres in the state, gaining a little larger foothold than it had last year.SDS, according to Smith (who addressed soybean diseases recently at the UW-Extension Pest Management Update meetings), is reported throughout most soybean growing areas in the U.S. It was first documented in Wisconsin in 2005 and has become more common and severe since then. It’s most severe when soybeans are planted into cool wet soils and when mid-summer rains saturate the soil.The first noticeable symptoms of SDS are yellow blotches that form between the veins of the leaflets (while veins stay green). They expand, and the tissue later turns brown and dies. Soon after, entire leaflets shrivel and die. Leaflets will also drop off, leaving petioles attached. Taproots and below-ground portions of the stems of plants with SDS when split open, show no internal stem discoloration. In plants with advanced foliar symptoms of SDS, small, light blue patches form on taproots and stems below the soil line. These are spore masses of the fungus that causes this disease.Smith says once symptoms are evident, yield loss is inevitable, and can range from not much all the way up to 100 percent, depending on variety grown, growth stage at time of infection and whether SCN is present in a field (See related story this week on SDS and SCN.). If SDS occurs at flowering, yield loss can be substantial. When SCN is also present, the combined damage from both is substantially more than the sum of the damage expected from the two individual diseases.SDS is a soil-borne fungus (Fusarium virguliforme), which overwinters in soil, crop residue and the cysts of SCN. It infects soybean roots, possibly as early as one week after crop emergence. It does not invade leaves, flowers, pods or seeds, but does produce toxins in the roots that move to the leaves, causing the SDS foliar symptoms.The disturbing thing is that SDS cannot be controlled once plants are infected. Foliar fungicides and fungicide seed treatments have no effect on the disease. Use SDS-resistant varieties, which might be somewhat difficult to find, in suitable maturity groups. Rotating to other crops does not appear to significantly impact SDS either, and corn has been shown to harbor this pathogen.Smith says foliar symptoms of SDS can be confused with leaf symptoms of brown stem rot (BSR). However, internal stems generally appear normal with SDS, versus BSR that has brown discoloration of the root pitch, especially at and between nodes near the soil line.In addition, SDS is usually seen at R3 to R4, while BSR turns up at R5 to R6. According to Smith, BSR also cannot be controlled once plants have been infected. Foliar fungicides and fungicides seed treatments have no effect on this one either. Use crop rotations of two to three years away from soybeans for BSR, as well as tillage that incorporates residue. Keep fertility up, and grow varieties with BSR resistance.White mold is, unfortunately, all too familiar to growers. Smith reports that this year, it was worse in soybeans planted later (i.e. early June). Earlier planted soybeans flowered during hot, dry weather and escaped. As noted, it was more of a problem in southern Wisconsin (which got more rain) this year, than in the northern portion of the state.White mold is caused by Sclerotinia sclerotiorium, which thrives in cool, humid conditions. For every 1 percent increment of plant mortality at R6 or R7, yield loss is a quarter to a half bushel per acre, Smith reports.Management is a function of field history (keep good records of field history and disease incidence), variety selection, increasing row width and reducing plant population, crop rotation (though one year is not enough as survival structures can last up to 10 years), and chemical and/or biological control. Where irrigation is used, reduce the frequency during flowering. Use cover crops to reduce inoculums density.Smith says fungicides may be warranted in fields with a history of white mold and where risk is high. They should be applied at R1 for best results. Timing is important, and applying after R3 is ineffective.Septoria leaf spot (or brown spot) was in soybean fields, too, this year. The Septoria fungus is common in soybean residue and is spread by splashing rain. Infection occurs as early as V2 on lower leaves. Under warm, wet weather, the disease may move up through the plant. Late in the growing season, infected leaves turn rusty brown or yellow and drop prematurely. The spread of the fungus is usually halted during hot, dry weather.Septoria leaf spot does not usually affect soybean productivity in Wisconsin. In some years it can cause premature defoliation, which can be confused with normal maturity. Yield loss occurs most often in high- yield environments and is related to timing and rate of defoliation. Disease severity at the R6 growth stage is predictive of yield. Severe brown spot results in smaller seed size.Look for irregular, dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches. Leaves become rusty brown. Symptoms of Septoria leaf spot can also develop on stems and pods of plants approaching maturity.Smith says tobacco streak virus was a possible cause of “green stem syndrome” in 2013. It results in bud proliferation and underdeveloped pods. He says there will be a lot of pods all at the terminal end of the plant with not a lot down the stem. Not much is understood about this disease in soybeans, according to Smith. It’s possibly transmitted by thrips. The other virus that is a newcomer to Wisconsin is soybean vein necrosis disease. Plants exhibit vein clearing (lighter vein color) and yellowing, as well as mosaic patterns (blotchy light and dark areas) on affected leaves. Initially, symptoms develop around the veins of leaves and eventually expand outward. As the disease progresses, vein and leaf browning and death occur.This is a tospovirus and belongs to a group of common vegetable viruses, including tomato spotted wilt virus and viruses on iris and impatiens. This one also appears to be thrips related. State soybean checkoff funds are being utilized to look closer at SVND. Thrips on soybeans will be captured again next year.Source - http://www.agriview.com/