Scabby Wheat Showing Up
DR. HEATHER YOUNG KELLY
JACKSON, TENN.
Wheat in Tennessee has entered the ripening stage (Feekes 11 – kernel
development) and symptoms of Head Scab can be seen in some fields.
Wheat field with Head Scab
Symptoms of Head Scab (Fusarium Head Blight) may be visible in fields
before wheat starts to turn golden and fully mature. There is nothing
to be done about scabby wheat now, as the window to apply fungicides has
passed. Fungicides applied to manage Head Scab (Fusarium Head Blight)
have a Pre-Harvest Interval (PHI) of 30 days, which roughly equates to 6
days after flowering. While wheat can be infected from head emergence
until harvest, infections initiated at and soon after flowering have the
greatest destructive potential and that is why the recommended window
for fungicide application is at bloom to manage scab.
I’ve gotten calls and visited fields in Tennessee that have varying
levels of scab infection this past week (from little to none, up to
40-50 percent heads affected). Factors that affect scab infection and
severity include:
• variety (some may have more tolerance than others)
• plant date/time of flowering
• weather (temperature and moisture before, during and after flowering)
• previous crop (higher severity is likely in wheat planted behind corn)
• fungicide application (product, timing, and coverage)
In west Tennessee from May 5 through May 13 warmer temperature (highs
in the upper 70s and 80s and lows above 60 °F) were in the range for
Head Scab infection to occur if wheat was flowering and received
rain/irrigation. Even fields that did have a fungicide application (only
solo triazole products should be applied at flowering) may have Head
Scab infection because even in an ideal situation (i.e. all the wheat in
a field is flowering at the same time) an application is not going to
give 100 percent control. Additionally, timing and coverage also effect
efficacy of fungicides to manage scab. The best way to know how much a
fungicide managed a disease is to have some unsprayed strips left in the
field to compare the sprayed area with. Appropriate fungicides applied
at the right timing have reduced scab severity levels by 50-60 percent
across multi-state trials.
Symptoms of scabby wheat include bleached spikelets or entire heads.
On bleached spikelets the fungus appears as pink or orange growths,
usually at the base of a disease spikelet (image below). Diseased
spikelets are often sterile or have discolored or shriveled seed, that
is chalky in appearance and can be referred to as “tombstones”. Yield
reduction is due the sterile florets and poor seed filling. If saving
seed from a scabby wheat field, combine can be set where the diseased
kernels will be blown out when harvesting, although some symptomless
seed may be affected and have poor germination the following year and a
seed treatment is recommended.
Fusarium fungus on scabby wheat head
In addition to reduction in yield, scabby wheat may contain
mycotoxins, usually deoxynivalenol (DON or vomitoxin), and grain may be
docked or rejected based on the level present. The consumption of
mycotoxin-contaminated grain has been associated with several acute and
chronic diseases in humans and animals. The level of DON in grain,
flour, and finished products derived from wheat is regulated, with
various limits set by different countries (for more info on U.S.
guidelines for DON levels visit: http://www.fda.gov/Food/GuidanceRegulation/…).
Scab is not usually problematic in Tennessee every year, only when the
right conditions occur (temperature between 59-86 °F, ≥ 90 percent
relative humidity/rain event) during flowering time period and
unfortunately it looks as if this year the right conditions have
occurred in some areas of Tennessee. ∆
DR. HEATHER YOUNG KELLY: Extension Plant Pathologist, University of Tennessee