Now is the Time to Consider Soybean Disease Management
July 27, 2016
Depending on planting date and variety most of
the soybeans in our area are at stage R2 (full bloom) or R3 (beginning
pod). Unlike corn, which has a
relatively short reproductive period, and cease vegetative growth once tassels
emerge, soybeans continue to put on a lot of vegetative growth after blooming
begins. In fact, soybeans will double in
plant height after the R2 stage. Flowering
occurs throughout this growth period and can last up to six weeks.
It is estimated that in any given year, soybeans
can abort up to 75% of their blossoms.
This is significant because fewer blossoms mean fewer pods. Fewer pods mean fewer seeds which is a key
factor in determining potential soybean yield.
Any management practice that can help retain blossoms and pods will
likely increase soybean yield.
Over the last ten years, we have seen that the
application of foliar fungicides is one of the most effective ways to increase soybean
yields by preventing diseases and improving overall plant health.
While foliar soybean diseases usually do not develop into full blown
epidemics that cause wide spread death, some are common every year and do
contribute to yield loss. A good example
is Septoria Brown Spot. This fungal
disease is present in most, if not all, Iowa soybean fields every year. Since it is low in the canopy it usually goes
unnoticed but it you open up the canopy now you will likely see numerous
diseased leaves than have already fallen to the ground. While this may not look like a big deal, it
is estimated that this disease is responsible for 8 – 15% yield loss nationally.
Other foliar diseases to watch for are Frogeye
Leaf Spot and Cercospora Leaf Blight.
Both of these diseases usually occur later in the growing season and can
cause significant yield loss. In
addition, quality loss can be a concern with Cercospora as this is the disease
that causes purple seed stain. You have
probably seen Cercospora infected soybeans in the grain tank at some point.
All of these diseases can be prevented with
applications of foliar fungicides like Priaxor® fungicide. Since 60% of the
soybean plant’s yield normally comes from the middle part of the canopy, applications
beginning at the R2 through the R4 stage offer the greatest benefit. In fact yield increases in the 4 – 6 bushel per acre range are typical and can be greater than that
in fields with heavy disease pressure.
Knowing how important it is to help soybean
plants retain pods, think about the yield impact of saving 1 or more pods per
Assume you have 155,000 plants per acre and all
pods contain 3 beans per pod
1 pod per plant saved = 3 bushels per acre
2 pods per plant saved = 6 bushels per acre
3 pods per plant saved = 9 bushels per acre