Soybean Diseases And Fungicide Considerations
DR. HEATHER MARIE KELLY
JACKSON, TENNESSEE
As we near the middle of
July and soybean fields move into pod formation, it’s time to decide whether a
fungicide application is needed. When making that decision, we need to consider
the cost of fungicide application, the crop price, and disease pressure,
meaning its potential to rob yield.
A disease becomes a PEST,
causing economic damage, when four conditions are met: the Pathogen
is present, there is a conducive Environment for infection/disease
development, a Susceptible host/variety, and all occur at the
right Time/growth stage for yield to be robbed.
Pathogens
The three major foliar
diseases that can reduce yields for growers in TN are frogeye leaf spot (FLS),
target spot, and Septoria brown spot.
Historically, Frogeye
leaf (Figure 1) spot has been the major disease growers have fought and the
primary driver of a fungicide application. Based on surveys conducted by the
“Sentinel Plot Project” over the last thirteen years in multiple counties across
the state, we observed that FLS typically first appears in July. This year has
been no different, as the first samples with FLS were consistently diagnosed in
the last two weeks among early-planted beans, although at low levels. The
disease is diagnosed based on pale brown lesions with a dark border and tiny
brown or silver hairs on the underside of the leaf.
Septoria brown (Figure 2)
spots are observed in the lower canopy, and they can be observed early on in
the season, but continuing into the season can cause premature defoliation
starting in the lower canopy and up into the middle canopy. The disease is diagnosed
by irregular, dark brown lesions that can coalesce into large blotches on the
lower leaves
Target spot (Figure 3) is
observed less frequently than FLS and Septoria brown spot, but it can be
present in TN. It is an important disease as it can infect soybeans and cotton.
The disease is identified by dark brown lesions with target-like concentric
rings and a surrounding yellow halo.
Environment
The weather conditions,
with frequent rain and high relative humidity, are creating an environment
conducive to disease development, which could yield a positive return on
investment from fungicide application.
Susceptible
host/variety
Another important factor
to consider is the level of susceptibility of the variety planted; this is
specific to each disease. The variety can restrict disease development and, in
some cases, lead to the possibility of using lower-efficacy fungicides to manage
potential yield losses from disease.
Timing/growth
stage
In general, the earlier a
disease infects and develops in the season/growth stage, the greater potential
it has to cause yield loss. But most foliar diseases don’t start until
reproductive growth stages, so it’s important to start scouting for disease at
beginning bloom (R1). While the best fungicide application timing depends on
disease scouting and environmental conditions, it is usually mid- to late-R3
(beginning pod development = at least a 3/16-inch pod in the upper 4 nodes of
the plant).
Fungicide considerations
and potential return
On average over the
years, fungicide protected yield compared with no fungicide application, as
observed in a multi-year study conducted from 2012 to 2021 across eight states
(AL, AR, IL, IA, KY, LA, MS, TN) (Figure 4). The study examined various commercial
fungicide groups available on the market and observed a positive yield response
when the end-of-season FLS disease severity exceeded 14%. This level of disease
intensity is commonly encountered in both susceptible and moderately
susceptible varieties during typical years in Tennessee.
Among the fungicide
groups studied, only the QoI (Quinone Outside Inhibitors, FRAC 11 on labels)
fungicides did not protect yield (i.e. was not statistically different from the
non-treated check). It is important to note that resistance to QoI fungicides
has been documented in all three of the previously mentioned diseases, which is
potentially reducing the efficacy of this fungicide group.
The level of yield
protection offered by the fungicide will change according to the disease
intensity of the season and the variety’s susceptibility. A study investigating
fungicides across varieties with varying levels of FLS susceptibility was
conducted over three years across two locations in TN. Results demonstrated
that in the year when disease pressure was higher, such as 2021, the use of
fungicides protecting yield varied based on the variety. Where all fungicides
protected yield over the non-treated check (NTC) in the susceptible variety,
while a few fungicides did not differ from the NTC in the moderate and
resistant variety (Figure 5a).
In moderate to lower
disease pressure environments (like 2023-2024), there were no differences in
fungicide product yield protection within each variety, but the same response
on average across all varieties – where all fungicides protected yield compared
to the non-treated check, but differed compared to each other (Figure 5b-e).
Additionally, based on
analysis of another research trial across 2018-2020 at 2 to 3 locations each
year in West TN (evaluating fungicide-treated and non-treated plots of 268
soybean varieties, 42 of which were replicated in at least two seasons) – the use
of fungicide can provide an average 3 bushels/acre yield protection, compared
to not treating. Again, this can vary by year (due to environment), variety,
specific disease, and fungicide product. More information about soybean
diseases, along with tables on fungicide efficacy and variety susceptibility,
can be found at https://utcrops.com/ and https://cropprotectionnetwork.org/.
The research referenced
in this article was supported by the Tennessee Soybean Promotion Board. ∆
DR. HEATHER MARIE KELLY
UNIVERSITY OF TENNESSEE