NDSU Extension - Griggs County

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October 23

NDSU Extension Griggs County Column by Jeff Stachler

Soybean and dry bean harvest is completed in the county!  Yields were ok.  Corn harvest is progressing nicely with the great weather we have been having.  As of Friday at least 60% of the corn was harvested in the county!  Yields have been across the board like soybean.  Yields will be average. 

While making my Friday drive around the county I thought it would be interesting to record the amount of fall tillage that has been accomplished so far.  About 70% of wheat fields have had some type of tillage done to them.  About 35% of soybean fields have been worked with some type of tillage.

Let’s discuss sudden death syndrome in soybean (SDS).  I’m not sure how much is present in the county, but I know it has been found within the state.

Identifying symptomology of SDS includes leaves turning yellow between the veins followed by brown lesions.  The upper leaves of the plant are affected first.  The veins in the leaf remain a dark green.  The yellow and brown can be found together between the veins in a mottled pattern in the early stages of symptomology followed by mostly brown.  Next the leaves shrivel up and fall off the plant.  In most cases the petiole, the structure that attaches the leaf to the stem, stays on the plant.  If you dig up a plant when the soil is moist to wet it is possible to find blue spores on the root which is unique to SDS.  You will not always find the blue spores even when the soil is moist to wet.

Sudden death syndrome is caused by the soil fungus Fusarium solani f. sp. glycines.  The fungus over winters on plant residue and as chlamydospores in the soil.  The chlamydospores are very resistant to the soil environment.  The fungus can also survive on the cysts of soybean cyst nematode.  As soil temperatures rise in the spring, chlamydospores near the soybean roots are stimulated to germinate, then infect the soybean root.

Roots may be invaded by the pathogen as early as one week after crop emergence and some say as late as right before or up to flowering.  The fungus grows very slowly in the plant and no symptoms can be seen shortly after infection.  The earliest symptoms show up is in mid-July at the R3 stage which is when pods are ¾ inch in length and as late as just prior to maturation.  The leaf symptoms can appear during periods of heavy rain in July and August.  The leaf symptoms are caused when the fungus produces a toxin in the plant.  The fungus does not invade the stem beyond the first few inches above the soil line.  The pathogen does not invade seeds, pods, flowers, or leaves.

Sudden death syndrome is most severe when soybean are planted early into cool and wet soils and when heavy midsummer rains saturate the soils.  The more soybean cyst nematode you have the more SDS is likely to be present due to the soybean cyst nematode providing an entry point for the fungus.

There are no management practices that will PREVENT the disease from occurring, however there are some practices available to reduce the risk of SDS.  There are no foliar fungicides to control the disease since it occurs in the root.  One management strategy is to delay planting until the soils have thoroughly warmed and there is little risk for saturated soil conditions after plant emergence.  The greater the water infiltration the more limited the infection should be.  Crop rotation is another management practice, although its effectiveness is limited.  Research shows that multiple years of corn in a rotation will not reduce the fungal pathogen.  Planting a resistant variety is another management option.  However, there are no highly resistant soybean varieties available.  Plant breeders are still working on improving varieties.  Find all sources of information regarding the level of resistance of a variety and ask how much research has gone into that rating.  Reduce the amount of soybean cyst nematode.

Keep good records as to whether a field has SDS and how frequent it is in the field in order to plan your management practices for the future.

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