Download Technical Bulletin

Technical Bulletin
A publication of the LG Seeds Agronomy department
Issue 285: June 2016
Soybean Foliar Diseases and Fungicide Timing
Cody Kerr, Agronomist – LG Seeds
Soon enough soybean fields across the region will begin reaching the critical reproductive growth stage.
During this stage foliar diseases can begin to impact yield. This technical update will focus on the main
soybean foliar diseases and management considerations for each disease.
Fungicide Timing
With fungicide applications on soybeans, timing and proper disease identification are keys to a successful
application. Proper identification of the disease is important to determine if the disease is a fungus that can
be controlled by a fungicide (although some fungal diseases don’t respond to a fungicide) or a bacterial
disease that cannot be controlled with a fungicide.
For example, bacterial blight and bacterial pustule are caused by bacteria and will not be controlled with a
fungicide whereas Septoria brown spot can be.
Sudden death syndrome and brown stem rot are both foliar diseases that are caused by a fungus but cannot
be controlled with a foliar fungicide.
The optimal growth stage for a foliar fungicide application in soybeans is from the R2-R4 growth stage. The
chart below details what plant characteristics to look for to determine when to make the fungicide
application. This chart was developed for Priaxor fungicide, but most fungicides need to be applied during
this same growth stage window. (Chart courtesy of aganytime.com)
lgseeds.com
©2015 LG Seeds
6 Common Soybean Foliar Diseases (Pictures courtesy of Iowa State University)
Septoria Brown Spot
 Brown spot is extremely common on seed leaves
(unifoliates) of young plants (as early as V2)
 Dark Brown, angular spots or brown “dots” starting on
lower leaves and eventually yellowing and falling off.
 Favored by warm, moist conditions
 Fungicides are recommended for control. Apply at of near
R3 to protect upper canopy.
Bacterial Blight
 Usually one of the first to appear, starting at V2
 Small, angular, yellow-to-brown spots surrounded by yellow
halos that eventually grow together
 Disease is seen on the leaves at the top of the plant
 Infection is often caused through a wound. Common after a
heavy rain event and if temperatures remain cool
 Fungicides will not help
Bacterial Pustule
 Mid-late season infection that occurs on the upper canopy
 Small yellow-green spots with angular red-brown centers.
The spots may join together and form large irregular dead
areas that can tear away
 Pustules can be seen on the underside of the leaf
 Favored by warm/humid weather
 Fungicides will not help
Frogeye Leaf Spot
 Mid-late season infection that occurs on the upper canopy
 Small angular spots with grey centers and a distinct purple
to red-brown margins
 Favored by warm/humid weather
 Fungicides can help. There is some resistance with
strobilurin fungicides so a dual mode with a triazole
Cercospora Leaf Blight
 Mid-late season infection that occurs on the topmost three
to four trifoliates
 Starts as a orange-purple mottled look that turns to a
bronze color and has a leathery feel
 Favors warm wet weather
 Same fungus that can cause Purple Seed Stain
 Fungicides can help to prevent the disease. Spraying during
pod fill will prevent seed stain, but will not affect yield.
lgseeds.com
©2015 LG Seeds
Downy Mildew
 Mid-late season that usually occurs on the top three to four
trifoliates
 Light green lesions appear on the upper surface of the
leaves and eventually become yellow spots. The underside
of the leaf may have grayish, fuzzy tufts in humid weather
 Favored by warm/humid temperatures
 Fungicides are labeled for control, but yield loss in unlikely
from this disease
Sources
http://extension.agron.iastate.edu/soybean/documents/PM1989.pdf
http://cropdisease.cropsci.illinois.edu/soybeans/
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the
information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
Piaxor is a registered trademark of BASF. LG Seeds and design are trademarks of SCA Limagrain.
lgseeds.com
©2015 LG Seeds