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Issue No. 21, Article 5/August 14, 2009

Soybean Disease Update

Soybean rust in Arkansas and Mississippi. Soybean rust has been confirmed in Holmes County, Mississippi (August 6) and in Chicot County, Arkansas (August 10). These were the first soybean rust observations for these states in 2009. Given that the Mississippi River Valley seems to be the pathway that soybean rust has taken to reach Illinois the past three years, it is important that we watch this disease's northward progression in the next few weeks (soybean rust observation maps are available at www.sbrusa.net). Most research has shown that soybean rust infections that take place when plants are past the R5 growth stage (beginning seed stage) do not cause economic yield losses.


Soybean rust observations in the United States as of August 10, 2009. Most recent soybean rust observation maps are available at www.sbrusa.net.

White mold. White mold (aka Sclerotinia stem rot) is being observed in several fields in central and northern Illinois. In issue 18 of the Bulletin (July 24, 2009), I described this disease and the available management practices. Once white mold symptoms are observed on plants, there are no in-season management practices available. Most research has shown that foliar fungicides must be applied between the R1 and R2 growth stages to achieve the best protection against white mold (see the article in issue 18). The warmer weather that has moved into the state will slow down or stop white mold progression within infected plants. When temperatures move into the upper 80s and 90s, the white mold fungus shuts down.


Soybean stem affected by white mold (aka Sclerotinia stem rot).

Sudden death syndrome. Symptoms of sudden death syndrome (SDS) are being observed in different areas in the state. SDS is caused by the fungus Fusarium virguliforme. Initial symptoms appear as light-yellow flecking on the leaves. The yellow areas enlarge to cause interveinal chlorosis (yellow leaves with the veins remaining green) and eventually interveinal necrosis (dead leaves with the veins remaining green). The foliar symptoms generally do not appear until soybean plants are into the reproductive growth stages. Foliar symptoms of SDS are identical to those caused by brown stem rot, another soybean disease. To properly diagnose SDS, split the stem with a knife and look for internal discoloration in the pith. If the pith is discolored, then it is likely that brown stem rot is the causal disease; if the pith is not discolored, SDS is the likely cause.

Although SDS symptoms appear on plant foliage, the Fusarium fungus that causes SDS actually infects the soybean roots early in the growing season. The foliar symptoms of SDS are caused by a toxin that the fungus produces, which then moves upward in the vascular system of the plant.


Foliar symptoms of sudden death syndrome.

Unfortunately, there are no silver bullets available to control SDS completely, so using multiple management practices is encouraged to help limit its damage:

  • Variety choice: One of the most important SDS management decisions can be made before the growing season begins. Although no soybean varieties with complete resistance to SDS exist, there are differences in susceptibility. Many seed companies provide SDS-resistance ratings for their soybean varieties. Additionally, many varieties are rated for SDS by University of Illinois, Southern Illinois University, and USDA-ARS personnel in field and greenhouse trials as part of the Illinois VIPS (Varietal Information Program for Soybeans) program. The results of these trials are available on the VIPS website.
  • Planting date: Early planting may predispose soybean plants to infection by the SDS fungus. Plant fields with no history of SDS first and those with a history of SDS last.
  • Soil compaction and drainage: Soils with compaction and/or drainage problems may lead to bigger problems with SDS. Using management practices that alleviate soil compaction and drainage problems in a field may also help limit losses from SDS.
  • Interaction with soybean cyst nematode: Although it is not always been easy to prove in research trials, there appears to be an interaction between SDS and the soybean cyst nematode. If both are present in a field, then yield losses may be more dramatic than if either one were present alone in a field. Using good management practices for SCN will certainly reduce losses due to SCN and may also provide some benefits with SDS management.

--Carl A. Bradley

Author:
Carl A. Bradley

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