Mid-Season Soybean Diseases – What can we Predict?

Frogeye Leaf Spot: Top of leaf; classic purple border around the lesion. Below, Right: The underside of the leaf showing dusty whiskers on the bottom of lesions.

This year, there is a very high degree of variability across the state with regards to soybean height, canopy coverage, and overall crop condition.  Accordingly, the USDA planting estimates reached 85 million acres of soybean and the prices of soybean, corn, and wheat were highlighted in the Wall Street Journal last week.  What to watch for now…

1.       Brown spot.  In all of our field plots this year, only one was no-till. This is the only one with high levels of brown spot, the unifoliates are plastered.  We have shown through 3 years of studies that brown spot typically only contributes to 3 to 4 bushels of yield loss in todays “workhorse” varieties.  If the unifoliates and lower canopy are clean as we move through flowering, then these fields do not need an application.  If you have brown spot on later planted fields, it is likely that the added costs of the application combined with the yield loss from later planting are going to hurt the overall budget for that field.  Pencil it out to be sure it is going to work for you.  Secondly – LEAVE AT LEAST 2, BETTER 3 UNTREATED strips.  At harvest, combine these 3 strips first and separately– take the average, then go through the rest of the field.  This is the best information of any if these mid-season applications are bringing a return to your farm.

2.       Frogeye leaf spot.  We have a couple of locations that we have been monitoring in the state where we know frogeye has overwintered in the past.  I was almost fooled last week as when we got to the site, lots of necrotic spots with purple borders were everywhere – note I said everywhere.  Many of the herbicides that the weed group is recommending to combat weed resistance will also cause the characteristic purple border.  The difference – if it is frogeye leafspot, caused by the fungusCercospora sojina, the spots will have “whiskers” on the underside of the leaf in the spot (technically called conidia).  In Kentucky and in Indiana there are populations of this fungus that are resistant to the strobilurins (Headline, Quadris etc); we are participating in a regional study to monitor for the presence of these insensitive strains, so again, please send me leaves.  Place them in a plastic bag without any extra paper.  Put them in a padded mailer and they should arrive just fine.

3.       White mold.  This is targeted to those producers who have been dealing with white mold for a long time.  This cool wet weather we are having definitely fits the conditions necessary for white mold development.  So for highly susceptible varieties it is time to put the first spray on which should occur at the first flower.  Remember, white mold does not suddenly appear, a field is infested with the sclerotia, hard black fruiting bodies which serve as the survival structure.  Under the types of conditions we are having and with canopy closure they are perfectly timed to produce a very small mushroom which can produce hundreds of spores.  These land on the flowers, they need the sugar to begin to germinate and infect the plant at the node.  Last year in the northeastern part of the state, we had our fungicide trial using a moderately resistant cultivar.  Even though conditions were perfect for disease, we only had approximately 20% disease severity in our non-treated plots and over 65 bu yields.  The fungicide applications did not help.  Admittedly, I think we were 3 days late in our applications, but I was also pleasantly surprised at how little disease actually developed.  We have 2 studies out this year to examine a variety x fungicide effect, but  I do think the seed companies have done a very good job in incorporating resistance into the current cultivars.  Again, if you have some questions, leave 2 to 3 strips to see for yourself what the variety resistance is going to do for your overall management.  Fungicides, we have had success in some trials with the fungicides Aproach and Topsin M.  Headline is not recommended for management of white mold as several University trials, including at least one of mine we have doubled the disease severity with applications of pyraclostrobin.  Herbicides are even more challenging this year since the soybeans are so delayed, again, leave the strips.

4.       Soybean Rust.  The impact of the last winter can readily be seen with how little soybean rust survived in the south.  Last week was the first report from Alabama in 2014 (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi).  Remember this is the first pustule identified in county turns a county red on the USDA IPM Pipe map.  We are a long way from build-up at this time.  Again, many thanks and gratitude to my colleagues in the southern states for their efforts to keep monitoring for this pathogen. 

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.

Author(s):