Scouting for Sudden Death Syndrome in Soybean

With soybean in the R4 to R5 growth stage in many areas of the state, we are starting get reports of plants expressing symptoms that for many (given the longer history in the state) would be identified as Brown stem rot (BSR) (Figure 1). A sample received into the lab earlier this week, however, showed symptoms of Sudden death syndrome (SDS) (Figure 2) and a check of our field trials under inoculated conditions within the past day is also showing symptoms of SDS. Weather conditions in 2010 have been very favorable for the development of SDS in the state with wet soil conditions during periods of planting followed by extensive rainfall during the flowering period.


Figure 1. Soybean plants expressing symptoms of Brown stem rot.


Figure 2. Early symptoms of Sudden death syndrome observed on July 29 at the West Madison ARS. The timing of scouting in soybean can help determine if symptoms are due to SDS or BSR.

What is SDS? Sudden death syndrome is caused by the fungus, Fusarium virguliforme. Foliar symptoms of SDS are similar to BSR so careful examination of plants is needed to differentiate the two diseases. It is also possible that both can occur in a soybean plant. Symptoms of SDS include a yellow to brown discoloration of the leaves around veins. Initially, these begin as small, circular spots. Examine the roots also since SDS can lead to a root root and these may be black in color. Also, there may be evidence of the pathogen on the root if you see a blue coloration (this is growth of the fungus). SDS does not lead to a brown discoloration of the vascular and pith tissues that is typical with BSR.

What are the risk factors that lead to SDS? The pathogen overwinters in soybean debris as chlamydospores, which as resistant fungal structures. Disease is favored by high soil moisture during vegetative growth and wet and cooler conditions around flowering.

What if I have SDS...what should I do? First of all, make sure to get a proper diagnosis. If you see evidence of SDS in the field, take a sample (including roots) and send it to the Plant Disease Diagnostic Clinic. After proper identification and if yield was impacted by SDS, consider the use of cultivars with increased resistance to SDS. Also, monitor conditions at planting to avoid cool soil temperatures that are favorable for infection by the pathogen and consider tillage to help increase soil temperature and drainage.

Further information about SDS is available here.

Reports of White Mold - Soybean Too Late to Spray

We are starting to receive reports of white mold in soybean. With the exception of later planted soybean, most of the soybean crop is at the R4 into R5 growth stage. As you scout the soybean crop, areas of wilted soybean plants may be indicative of white mold (Figure 1). Closer inspection of the wilted area will often lead to the "white mold" symptom that is diagnostic, which is the fluffy white mycelium (Figure 2). Seeing symptoms of white mold means that the plants were infected weeks earlier and the application of a foliar fungicide for control of white mold is not recommended. Also, as earlier stated, with most of the soybean crop past the R3 growth stage, foliar fungicides are not recommended even if scouting does not show evidence of white mold.

For further information about scouting for white mold, please consult:

White Mold of Soybean in Wisconsin

and

White Mold in Soybeans



Figure 1. Areas of soybean where plants are wilted. This is often indicative of white mold.



Figure 2. White mycelium on the soybean stem are a good diagnostic indicator of white mold.

When it Comes to Soybean Aphid Saving a Pass Doesn't Necessarily Save you Money

Many growers are confronted with the following question.....

It is R1/R2 soybean and I am making my last glyphosate application. Should I tankmix an insecticide with my herbicide to clean up the few aphids I have in the field?

The answer to this question is rather simple....NO!!! Below Eileen Cullen our State Field Crops Extension Specialist explains why.

UW Entomology and IPM/entomology programs throughout the region do not recommend tank mixing insecticides for preventative aphid control in the absence of soybean aphid, or at soybean aphid numbers below the economic threshold of 250 aphids/plant. Insecticide premix products containing pyrethroid + nicotinoid active ingredients are asked about as potential 'insurance' treatment to use in a tank mix with fungicide before soybean aphids colonize a field or increase to economic threshold level. The foliar application of a nicotinoid has some translaminar systemic activity, but data on 30 days residual and 6 bu/acre yield assurances in the absence of economic threshold aphid populations have not been documented to my knowledge. Moreover, there is no assurance that a field treated preventatively would reach economic threshold if left untreated. There are many factors that determine this (timing of aphid colonization, weather, temperature, natural enemy (predator and parasitoid) suppression). We can expect some systemic activity with the nicotinoid class (whether applied as seed treatment, foliar, or in-furrow - depending on crop, soybean, potato, etc.). However, as the plant grows, active ingredient is translocated through the plant, but not all new growth retains that translocation throughout the life of the plant.

Regular field scouting and timing insecticide treatment to soybean aphid threshold is the recommendation in terms of efficacy, reliable yield protection, and cost to the grower. Systemic activity of a nicotinoid applied foliar would logically be best placed at a threshold-timed treatment in which the pyrethroid portion of the premix suppressed the aphid population at economic threshold of 250 aphids/plant, and any benefits of systemic activity from the nicotinoid could be retained following treatment. The pyrethroid broad spectrum activity will also kill natural enemies in the field, another reason the recommendation is to wait until aphid economic threshold to give natural enemies a chance to suppress populations. All the labeled insecticides will go a good job of controlling soybean aphid when applied at threshold. Regardless of insecticide material used, field scouting should continue through the R5 growth stage to monitor populations and effects of any previous insecticide treatments.

Communication Methods Survey to Help Us More Effectively Reach You

Drs. Shawn Conley (University of Wisconsin) and Vince Davis (University of Illinois) are conducting a communication methods survey and are asking for your participation. The objective of this survey is to investigate the technology that soybean growers and agronomic consultants use to find and share soybean production and marketing information. This survey is being conducted through the mail via post cards and through an on-line survey program. The research is sponsored by the Illinois and Wisconsin Soybean Marketing Boards through the soybean checkoff. The survey should take less than 10 minutes of your time but will be extremely valuable to us. This survey is completely voluntary and the information you provide will remain anonymous. Please visit www.surveymonkey.com/s/soybeangrowercommunications to take part in this research to help us serve you better.

 

We sincerely thank you in advance for your participation. – Shawn P. Conley and Vince M. Davis