Fusarium Head Blight (Scab) of Small Grains
PP-804 (Revised), December 1999
Marcia P. McMullen, Extension Plant Pathologist
Robert W. Stack, Professor of Plant Pathology, Agricultural Experiment Station
Fusarium head blight (FHB) or scab is a fungus disease that can occur on all small
grain crops but is most commonly seen in North Dakota on spring wheat, durum, and barley.
Yield losses from FHB are due to sterility of the florets (flowers) and to formation of
shriveled, light test-weight kernels. FHB infected grain may be downgraded at the market.
FHB not only causes yield and quality losses, but may also be associated with fungal
toxins (mycotoxins) that are hazardous to animals.
In wheat, any part or all of the head may appear bleached (Figure 1). These white heads
are very conspicuous in a green field (Figure 2). Frequently only part of the head (often
the upper half) is affected by FHB. These partly white and partly green heads are
diagnostic. The fungus also may infect the stem (peduncle) immediately below the head,
causing a brown/purplish discoloration of the stem tissue. Additional indications of FHB
infection are pink to salmon-orange spore masses of the fungus often seen on the infected
spikelet and glumes during prolonged wet weather (Figure 3).
Figure 1. (29KB color photo) Scabbed
wheat heads showing partly infected white and green heads (left and center) and entire
head infected (right). (Photo by Vernyl Pederson)
Figure 2. (33KB color photo) Severe
scab in field. (Photo by Marcia McMullen)
Figure 3. (28KB color photo) Salmon-orange
spores visible on glumes of wheat. (Photo by Marcia McMullen)
Many infected wheat kernels are shriveled, lightweight, and are a dull grayish or
pinkish in color (Figure 4). These kernels are sometimes called "tombstones"
because of their chalky, lifeless appearance. Other fusarium-infected kernels may be more
normal in size, if infection occurred late in kernel development. These kernels, however,
may have a dull appearance or a pink discoloration. Infected kernels of durum often lose
their amber translucence and appear chalky or opaque.
Figure 4. Fusarium-infected hard red spring kernels [click
here (18KB color photo)] and durum kernels [click here (15KB color photo)].
(Photos by Jim Miller)
In barley, FHB infections are not always readily apparent in the field. Infected
spikelets may show a browning or water soaked appearance (Figure 5). Infected barley
kernels show a brown discoloration (Figure 6) similar to that caused by other kernel
blighting organisms.
Figure 5. (28KB color photo)
Symptoms of scab on barley heads. (Photo by Robert Stack)
Figure 6. (20KB color photo)
Fusarium infected and healthy barley kernels. (Photo by Brian Steffenson)
Estimates of FHB levels in a field are based on actual counts of blighted and healthy
heads. A colored visual scale to estimate severity of FHB in wheat is available from the
NDSU Extension Service (publication PP-1095) Samples of entire diseased heads are helpful
for confirmation of FHB. Grain suspected to contain scabby kernels also may be examined.
Samples should be submitted to county extension offices or to the Plant Diagnostic
Laboratory at North Dakota State University. There is a nominal fee for processing the
samples.
Seedling blights, poor stands and poor vigor are symptoms frequently associated with
the planting of scabby grain. Infected seedlings may exhibit a brownish-red discoloration
of the roots and crown and may rot at the soil line.
FHB is caused by fungal species in the genus Fusarium. The most common species
causing scab is Fusarium graminearum (sexual stage—Gibberella zeae).
This fungus is the same one that is frequently associated with stalk rot of corn. Another
Fusarium species that causes FHB is Fusarium culmorum. Both F. graminearum
and F. culmorum also may cause root rot of small grains. On barley, two other
Fusarium species, F. poae and F. avenaceum, can also cause kernel blight.
These fungi persist and multiply on infected crop residues of small grains and corn.
During moist weather, spores of the fungi are windblown or splashed onto the heads of
cereal crops.
The small grain crops are susceptible to infection from the flowering (pollination)
period up to soft dough stage of kernel development for wheat and durum. Spores of the
causal fungus may land on the exposed anthers at flowering time and then grow into the
kernels, glumes, or other parts of the head. For barley, which flowers when the head is in
the boot, infection may be most common after the flowering period, once the head breaks
through the leaf sheath.
The most favorable conditions for infection are prolonged periods (48-72 hours) of high
humidity and warm temperatures of 75-85 degrees Fahrenheit. However, infection does occur
at cooler temperatures when high humidities persist for longer than 72 hours. Early
infections subsequently may produce air-borne spores which are responsible for secondary
spread of the disease, especially if the crop has uneven flowering due to late tillers.
Since scab development depends on favorable environmental conditions from flowering
through kernel development, disease occurrence and severity varies from year to year. A
combination of factors leads to the severest yield and quality losses: abundant inoculum
is present, prolonged or repeated wet periods during flowering through kernel development
occur, and a very susceptible cultivar is grown.
Resistance
Repeated scab epidemics and large economic losses have resulted in increased emphasis
on developing resistant cultivars. Breeding and pathology programs are actively screening
and developing genetic material with improved resistance to FHB. None of the currently
available commercial cultivars are immune to Fusarium infection, but differences in
partial resistance or tolerance to FHB do occur among different hard red spring wheats.
Recent cultivars such as AC Barrie, Gunner and Parshall show less infection than other
varieties in many cases. Improving FHB resistance is a high priority in spring wheat
breeding programs at NDSU and elsewhere in the region, and lines with very good resistance
to FHB are under development. Differences in durum and barley cultivars also occur, but
the level of FHB tolerance in durum and barley, at present, is less than in bread wheat.
Producers in areas of high risk for scab should select cultivars that have shown more
tolerance to scab. Information on cultivar response to scab is available in NDSU variety
trial publications and from county extension offices.
Seed Treatment
Seed treatment and the use of quality seed will help reduce seedling blight due to
infected seed but will not protect against subsequent head blight. If scabby
grain is to be used as a seed source, it should be thoroughly cleaned and conditioned to
remove the majority of scabby kernels and to improve test-weight. A germination test
should be run to indicate germ and vigor, and seed treatment fungicides commonly used for
small grains should be considered to improve stand and vigor.
Other crop management techniques should also be used to reduce the risk of FHB.
Although environmental conditions play a key role in disease development, certain cultural
practices will reduce survival and carryover of the fungal pathogen and lower risk of
severe FHB infection.
Tillage
Tillage practices that bury residue from small grains or corn reduce the inoculum
potential of the fungus, since the fungus survives best on residue left on or above the
soil surface. In minimum or no-till practices, effective spreading and distribution of
chaff and other residue may allow faster decomposition of the chaff, reducing inoculum
potential. Chopping or grinding corn residue to reduce size may also favor more rapid
breakdown.
Crop Rotation
Crop rotation is effective in reducing FHB levels. Crop rotation to a non-host crop or
planting of a small grain on last year's broad-leafed crop ground should be considered.
The greatest risk of FHB infection is when small grains are planted on last year's
FHB-infected small grain residue or on last year's corn residue. Species of Fusarium
that cause FHB (and in particular F. graminearum) also attack corn, causing stalk,
root, and ear rot, and survive for several years in corn residue.
Planting Date
Staggered planting of the small grain crop or planting of cultivars differing in days
to maturity is advised, so a producer's entire crop is not at risk of flowering during a
period favorable for FHB infection.
Fungicide
A fungicide spray program may reduce FHB damage. Reductions in FHB severity of 50 to 60
percent can be achieved when fungicides are applied at early flowering for wheat and
durum, and at early heading in barley. In the early to mid-1990s, protectant mancozeb
fungicides were registered for heading application to wheat and barley, and the systemic
fungicide benomyl was registered for heading application in wheat. In the late 1990s,
other fungicides became available for producers. The systemic fungicide Tilt received a
North Dakota state label (24C) for heading application to wheat, and the systemic
fungicide Folicur received a Special Exemption (Sec. 18) label for heading application to
wheat and barley. In 1999, the fungicide Quadris was federally registered for heading
application to wheat. Other effective fungicides may become registered in the near future.
Application studies have shown that spray coverage and disease control with these
fungicides is improved when the sprays are directed at an angle both forward and backward
toward the grain head.
Harvest
At harvest, the combine may be adjusted so that light-weight, FHB kernels are removed
along with the chaff. However, this will not remove all FHB kernels, since
some FHB infections occur late in the development of the kernel, and these infected
kernels may still be fairly plump. Infected barley and oat kernels are not so easily
removed in the combining process. Visible FHB damage is considered a part of total kernel
damage by the Federal Grain Inspection Service and, if excessive, will lower the market
grade. Severely affected grain may be graded "feed" rather than
"milling," or rather than "malting" in the case of barley.
FHB-infected grain may contain fungus-produced toxic substances called mycotoxins. The
most common mycotoxin associated with FHB infected grain in the Northern Great Plains is
deoxynivalenol or DON (vomitoxin), a mycotoxin that may cause vomiting and feed refusal in
non-ruminant animals. Grain with DON would have to be ingested in very high amounts to
pose a health risk to humans.
The presence of FHB-infected grain does not automatically mean mycotoxins are present.
The occurrence, amount and kind of mycotoxins may depend on several factors, including
environment, species of fungus present, severity of infection, and the variety or kind of
crop. FHB-infected grain may be tested for DON and other mycotoxins at properly equipped
laboratories. In North Dakota, quantitative analysis for several mycotoxins, including
DON, is provided by the Veterinary Science Diagnostic Laboratory, Van Es Hall, NDSU,
Fargo, ND 58105, phone (701) 231-8307. A one-half to one pound representative sample of
the harvested grain should be submitted. A fee is charged.
The Food and Drug Administration (FDA) established the following advisory levels for
DON in food and feed in 1993:
- 1 part per million (ppm) for finished grain products for human consumption
- No standard for raw grain going into milling process
- Cattle, over four months old: 10 ppm (providing grain at that level doesn't exceed 50
percent of diet)
- Poultry: 10 ppm (providing grain at that level doesn't exceed 50 percent of diet)
- Swine: 5 ppm (not to exceed 20 percent of ration)
- All other animals: 5 ppm (providing grains don't exceed 40 percent of diet)
A veterinarian or feed specialist should be contacted for further information on safe
livestock feeding levels. The risk of human exposure to DON ingestion is minimal under the
FDA guidelines, but producers and elevator operators need to be aware that moldy grain can
cause allergy and breathing problems. A good quality dust mask should be worn when working
around grain with high amounts of scab or other molds.
PP-804 (Revised), December 1999
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