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Over the past week, we have had reports of Frogeye leaf spot in soybean. Frogeye leaf spot is caused by the fungus Cercospora sojina. DATCP noted finds of this disease in some fields in the southern part of the state. While Frogeye leaf spot has been documented in Wisconsin, it is still a disease that for many is a relative unknown. As you scout soybean fields late in the growing season, symptoms of Frogeye leaf spot can be recognized as angular, brown to reddish brown spots that are irregularly shaped and have a light brown to gray center. While lesions on stems and pod can occur later in the season they are less common and distinctive than lesions on the leaves. If there are pod infections, seeds near those lesions can be infected and develop conspcuous light to dark gray or brown areas.

Why 2010? The prolonged warmer, more humid and rainy periods we have seen this year are very favorable to development of this disease. Management recommendations for Frogeye Leaf Spot include the use of resistant soybean varieties, crop rotation that is 2 years or longer (the pathogen overwinters in soybean debris). Foliar fungicides can be effective for control of this disease, but timing of application is important.

For more information about Frogeye Leaf Spot, there are several good fact sheets like:

http://www.ces.purdue.edu/extmedia/BP/BP-131-W.pdf

http://www.soydiseases.illinois.edu/index.cfm?category=diseases&disease=119

http://www.planthealth.info/frogeye_basics.htm

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.

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.

Across many areas of Wisconsin the soybean crop is just starting to bloom. As we enter the soybean reproductive growth phase there are a few things to keep in mind. The first is that soybean will produce flowers for ~3 to five weeks, depending upon planting date and environment. Soybean will abort anywhere from 20 to 80% of the flowers that they produce. Generally it is the first and last flush of flowers produced that are most likely to be aborted.

Second of all the timing window for our glyphosate applications in soybean are quickly closing. Chris Boerboom, Former UW Extension Weed Scientist stated that:

“Monsanto has clarified the definition of “throughout flowering” in regard to the latest stage that glyphosate can be applied to glyphosate-resistant soybean. Soybean flowering is defined as the R2 growth stage. The R3 growth stage begins when one of the four top nodes with a fully developed leaf has a 3/16 inch long pod. With this definition, glyphosate can be applied through the R2 growth stage, but not after the R3 stage begins. Applications made after the R3 stage are off-label applications.”

On average it takes ~ 4 days to move from R1 (beginning flower) to R2 (full flower) and ~10 days from R2 to the start of R3 (beginning pod). That means we have ~2 weeks for glyphosate applications to occur.

Next, wheel track damage made from ground applications may start to reduce yield. Sprayer wheel traffic from first flower (R1) through harvest can damage soybean plants and reduce yield (Hanna et al. 2008). Our research suggests that an adequate soybean stand (more than 100,000 plants per acre) planted in late April though mid-May can compensate for wheel tracks made when a field is sprayed at R1. Yield loss can occur, however, when wheel tracks are made at R1 or later in thin soybean stands (less than 100,000 plants per acre) or late planted soybeans. Regardless of stand, plants could not compensate for wheel tracks made at R3 (early pod development) or R5 (early seed development). The average yield loss per acre is based on sprayer boom width (distance between wheel track passes). In our trials yield losses averaged 2.5, 1.9, and 1.3% when sprayer boom widths measured 60, 90, and 120 foot, respectively. Multiple trips along the same wheel tracks did not increase yield loss over the first trip.

Lastly, for white mold questions please refer to the following information: White Mold in Soybean in 2010: Factors to Consider

Image 1. Soybean crop beginning to bloom.

As we move further into soybean development, we have started to receive an increase in questions about white mold (syn., Sclerotinia stem rot). During 2009, we made a video that is available on the UWEX YouTube webpage that discussed many of the factors we need to consider regarding the risk of white mold in soybean as well as scouting. We will provide further updates over the next few weeks regarding early risk and management for white mold.

Fig. 1. White mold in Iowa County in 2009.