Managing frogeye leaf spot on susceptible varieties

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Several reports and samples this past week with frogeye leaf spot on leaves in the upper/mid canopy. You will be able to find pictures and a detailed factsheet of this soybean disease at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/t01_pageview3/Soybean_images.htm

And for those that prefer facebook  https://www.facebook.com/OsuSoybeanPathology.

Frogeye leaf spot is a fungal disease that is caused by Cercospora sojina. This pathogen is typically pretty rare in the northern states, but due to the widespread planting of some highly susceptible varieties and milder winters, we now have more inoculum in the spring.One of my previous graduate students (Christian Cruz) did the tedious work to examine soybean residue to find the viable conidia (spores). At the end of the 2012 season, there was quite a bit of frogeye late in the season in our fungicide trials. At present there is enough there (almost R1) to begin to plan sprays.

There has been another development with this fungus that is a bit troubling. Numerous populations of this pathogen have been identified that are resistant to azoxystrobin (Quadris) and pyraclostrobin (Headline). These were first found in Kentucky in 2010 and since then have also been found in states up and down the Mississippi. To date, none have been detected in Ohio, but the truth is we have done very little sampling.

In 2007, we had frogeye develop at two of OARDC Branches on Seed Consultants line SC 9384, a susceptible line that they have very generously donated to our field studies. Disease levels in the top canopy at the end of the season were 28.5% and 47% at Northwest (Wood County) and Western (Clark county) Branch research stations. The disease was only present in the top canopy at Hotyeville, so fungicide applications were made at the R5, but had very little impact if any. There were no yield differences. Western had a different story. At this location, the mid canopy foliage also was more than 40% of the leaf area affected and yield loss was approximately 19% when the best fungicide treated plots (65 to 69 bu/A) were compared to the nontreated (Mean 54 bu/A). 

Fungicides applied at R1 on these indetermimant soybeans were not significantly different than the nontreated. I think that this is due to the fact the plant still has a lot of growing to do and the later foliage does contribute to yield.  Some other examples from the trial include:

Another thing is that rate of the fungicide makes a difference. For example Evito at 3.1 fl oz yielded 59 bu/A while Evito at 5.7 fl oz (both R3 only applications) yielded 65.9 bu/A. Domark (3 fl oz/A) at R3 followed by Domark (3 fl oz/A) at R5) yield 64.9 bu/A while Domark plus Headline (3 fl oz/A) and again at R5 only yield 67.7 bu/A. 

There are several things to learn from this study:

1. If the strobilurins are effective, use them at the higher rates. This will diminish the chances of fungicide resistance developing in the population.

2. The triazoles are equally effective, if applied at the right rates and timings.

3. Through the management of many, many pathogens with fungicides, it is always best to rotate classes of chemistries rather than combining them. The only reason to combine chemistries is if you have multiple pathogens to control and you need more than one active ingredient in the tank.

Finally – the best approach to managing this disease though is to use a resistant variety, they are the same price as the susceptible and you won’t have to worry about this.


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