Fusarium Head Blight (FHB) or Head Scab
- Figure 1. Wheat showing FHB-bleached kernels
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The extensive rain and warm conditions during flowering of the crop in 2010 resulted in the first widespread significant occurrence of Fusarium Head Blight (FHB) in wheat, durum and barley crops in southern Queensland. This resulted in significant downgrading of some crops. Until recently FHB has been reported irregularly and in isolated areas of Australia´s northern region. The most significant infections in the past occurred in durum crops on the Liverpool Plains in the late 1990s.
FHB is a fungal disease that can occur on many grass species, including both crop and weeds. Where it occurs in crops it is most commonly in wheat, durum and barley. It is a frequent and widespread disease of major wheat production areas of North America, Asia and Europe where a large amount of research into its control has been ongoing for more than 20 years.
FHB can cause significant yield losses and quality reductions. Major yield losses occur mainly from floret sterility. Additional yield and quality losses can occur when damaged and shrivelled lightweight grains are produced as a result of infection. Quality reductions may also occur from seed discolouration varying from whitish-grey, pink to brown. Fungal infection can sometimes be associated with the production of a toxin (mycotoxins).
If fungal toxins are produced in infected seed the grain is often unacceptable for certain end uses and downgraded in the marketplace depending on the concentration of toxin present. Toxin levels and fungal infection can not be accurately estimated from visual appearance.
- Figure 2. Salmon-orange masses of spores on glume
Bread wheat and durum
In wheat and durum, any part or all of the head may appear bleached (Figure 1). Heads that are partly white and partly green are one of the diagnostic symptoms in wheat for the disease, but can easily be confused with ´white grain disorder´.
A brown/purple discolouration on the stem tissue or on the peduncle (immediately below the head) in infected heads is another distinguishing factor that can be seen in heavily FHB-infected crops. Discolouration on the stem tissue or peduncle without the bleaching may be due to other causes such as physiological melanism.
Additional symptoms that occur during prolonged wet weather and heavy infection of FHB are pin-head sized, pink to salmon-orange spore masses on infected spikelets and glumes (Figure 2).
Depending on how soon after flowering bread wheat and durum is infected, grain can have different severities of aborted kernels, shrivelled seed, low test weight and grain discolouration. If disease infection occurs later in grain development, Fusarium-infected seed may be normal in size but it may have lost its amber translucence and will appear chalky or opaque or pink (Figure 3).
- Figure 3. Whitish-grey shrivelled Fusarium-infected grain; pink shrivelled Fusarium-infected grain; healthy grain (all from the same sample)
In barley, infected seed may show a bleached appearance or a browning or water-soaked appearance (Figure 4). Severely infected barley grain at harvest may show a pinkish discolouration in the sample and although rare, salmon-orange spore masses of the fungus can be seen on the infected spikelet and glumes during prolonged wet weather.
FHB can be caused by several species of Fusarium. The most common species causing FHB is Fusarium graminearum. This fungus can also cause stalk and cob rot of corn.
How does this relate to crown rot
The crown rot fungus is a closely related species, Fusarium pseudograminearum. The crown rot fungi can occasionally affect heads but this is rare. The infections in Queensland during the 2010 season have been almost 100% Fusarium graminearum.
Survival and spread
The fungi persist and produce spores on previous crop residues of wheat, barley and corn (Figure 5 and Figure 6) although other grass weeds and crops can also be a source of inoculum. During moist weather, sexual spores are produced in microscopic black flask-shaped structures on old crop and weed debris on the soil surface.
The most favourable conditions for spore production and infection are 48 to 72 hours of high humidity and temperatures of 23-29ºC. Longer periods of high humidity can compensate for lower temperatures if optimum temperatures are not experienced. These conditions do not have to be continuous and spore production will still take place if one or two dry days punctuate the humid periods.
With continued high humidity the spores are windblown or splashed onto the heads of cereal crops where they germinate in the humid conditions and infect the plant. If prolonged favourable conditions persist, asexual spores are then produced on the head and result in even more spores and secondary infections. Spores from within a crop are the major inoculum source, but spores blown from surrounding crops, sometimes long distances away, can also be a source of infection.
Wheat and durum crops are susceptible to infection from the flowering (pollination) period up to hard dough stage of kernel development, but the flowering period is when most infection occurs. Spores landing on the extruded anthers at flowering time grow into the developing kernels. The anthers are a major source of infection in wheat as they exude chemicals that attract the fungus.
Infection by spores landing on glumes or other parts of the head is also possible. Once a floret is infected the fungus can grow into the rachis and then grow up and down in the rachis infecting adjacent kernels. Infection of adjacent kernels can also occur through the fungus growing over the surface of the glume to an adjacent floret.
Barley flowers when the head is in the boot. Often the anthers are not extruded, so in barley infection is most common after the head emerges from the leaf sheath and through penetration of the glume. Barley is resistant to growth in the rachis but infection of adjacent kernels can occur through the fungus growing over the surface of the glume to an adjacent floret. Under favourable environmental conditions for the disease, infection can continue in barley until grain maturity.
- Figure 4. Fusarium-infected barley showing tan to brown discoloured kernels. Even under high disease only one or two adjacent kernels show symptoms
FHB is best managed by integrating multiple management strategies. Use of a single strategy often fails when the environment favours severe disease. Management strategies to reduce FHB should include a combination of as many of the following practices as possible.
A disease will develop into a serious epidemic when three important factors all occur at once. These are:
- a susceptible host
- adequate inoculum
- conducive weather conditions.
As there is no demonstrated resistance in Australian cultivars and weather can not be controlled, management of the disease should revolve around reducing inoculum.
All current varieties of wheat, barley and durum grown commercially in Australia are likely to be susceptible to FHB. Although some level of resistance does exist in germplasm around the world, resistance to FHB is a very complex and not easily inherited trait. Even in countries where FHB occurs more extensively and frequently, after many years of breeding, commercial varieties have only moderate levels of resistance.
Durum is more susceptible to the disease than bread wheat and barley. Durum should be avoided in areas where there is a likelihood of the disease developing.
There has been no evidence the fungus can grow from infected seed up through the stem and into the developing head to produce head blight. However, infected seed can result in seedling blight and dead seedlings when the seed is planted.
Currently there are no seed dressings registered for control of seedling blight caused by the FHB pathogens. Research conducted by the Department of Industry and Investment in New South Wales in the 90s showed that the most effective seed treatment to prevent seedling blight was thiram + carboxin. Tests are currently underway to determine the effectiveness of more modern fungicides as seed dressings.
If grain from an infected source is to be used as seed, it should be cleaned and only used if it has high germination and vigour.
Because the fungus survives on residue left on the soil surface, any tillage practices that bury, destroy or promote faster decomposition of residue from a host crop will reduce the potential inoculum for future host crops. Before removing or destroying residues, consideration should be given to any effect of these practices on ground cover, water infiltration and soil organic matter.
Crop rotation is effective in reducing FHB levels. Sowing a susceptible crop after one or more years of non-host crops will reduce inoculum levels. The greatest risk of FHB infection is when small grains are planted on last year´s FHB-infected wheat, barley or corn.
Corn in a rotation is a significant risk as Fusarium graminearum - the major cause of FHB - also attacks corn, causing stalk, root and cob rot, and the fungus can survive for more than one season in corn residue. Although it does not appear to exhibit any significant disease symptoms, sorghum can also host Fusarium graminearum which may infect following winter cereal crops. However, the major species of Fusarium that attack sorghum, Fusarium thapsinum and Fusarium andiyazi, are not pathogens of winter cereals.
The best rotational crops for reducing the inoculum level include any non-grass species (e.g. sunflower, cotton, soybean, chickpea, mungbean, faba bean, canola, field peas).
As infection requires moisture during the flowering or head emergence period, staggering the planting period or planting varieties with varying maturity should spread the risk in years where there are not continuous or repeated periods of high humidity.
Currently there are no fungicide sprays registered in Queensland for control of FHB. However, use of fungicides has proven a useful tool in reducing infections overseas where reductions in FHB severity of 50-60% can be achieved when the most effective fungicides are applied at early flowering for wheat and durum, and at early head emergence in barley. If conditions are favourable for FHB infection in coming seasons, emergency registration for chemicals should be able to be obtained prior to the crop flowering.
When applying fungicide to control FHB, the target is the vertical head rather than the more horizontal flag leaf. Modify application techniques for best effectiveness. Spray coverage and disease control for FHB is improved when the sprays are directed at a 30º angle from horizontal either both forward and backward, or less optimally with single nozzles directed toward the grain head. Also a higher water volume is recommended for good head coverage and ground application is more effective than aerial application.
Where FHB is expected, fields should be inspected prior to harvest and heavily infected fields or sections of fields should be harvested and stored separately from less affected areas.
During harvest of wheat, some of the lighter FHB-infected grain can be removed by turning up the air and blowing it out the back of the header with the straw. This will not remove all FHB kernels because some FHB infections occur late in the development of the kernel, and these infected kernels may still be plump.
After harvest of wheat, gravity table grain separation may be effective in removing lightweight, FHB-damaged kernels. A test should be conducted before committing to this costly strategy. Size grading is not usually as effective.
Harvest and post-harvest management is less effective in barley where shrivelled grain and light grain due to FHB infection is not a common symptom.
FHB-infected grain may contain fungal toxins also known as mycotoxins. The most common mycotoxin associated with FHB-infected grain is deoxynivalenol or DON (vomitoxin), which may cause vomiting and feed refusal in animals.
Infection of grain by FHB does not automatically mean significant levels of mycotoxins are present. The occurrence, amount and kind of mycotoxins depend on several factors, including environment, species of Fusarium present, severity of infection and the variety or crop class.
Testing for DON and other mycotoxins can be done at certified laboratories. In Queensland testing for DON can be performed by SGS Australia Pty Ltd. If testing is done for marketing it is important to negotiate this beforehand with any potential buyer.
It is important to ensure a representative sample of the grain is taken which will involve taking numerous probe samples from various sections of the storage, or stream sampling at numerous times as the storage is filled. The buyer and/or laboratory doing the analysis should be consulted about appropriate sampling.
A veterinarian or feed specialist can provide information on safe livestock feeding levels. FHB-affected and mouldy grain can cause allergy and breathing problems. Producers and grain handlers should wear a good-quality dust mask when working around grain with high amounts of FHB or other moulds.
- Figure 5. Fusarium-infected corn cob from a field harvested in 2009 and sown to wheat in 2010. Pink coloration is due to Fusarium
For human consumption, levels of DON are measured in the final product not the grain, as the processing of the grain can remove some of the toxin. For milling, the white, shrunken grains result in a loss of milling yield because they are removed from the sample.
Kernels infected late in their development by FHB may show little or no visual signs of damage, but may still have elevated levels of DON. Some of this mycotoxin will be removed with the seed coat during the milling process. The fungal infection process can break down protein and reduce gluten strength which adversely affects the bread and pasta making properties of the flour.
DON has adverse impacts for malt used for brewing. The toxin is not significantly denatured by the malting or brewing process and can be carried through to the finished product. It can also cause low malt yields due to disruption of the endosperm and the existence of proteins that cause gushing in beer.
DON does not generally cause death of livestock; however at elevated levels it can cause feed refusal and vomiting. This may result in lower weight gain or reduced milk and egg production and failure to thrive. Monogastrics (pigs) are more susceptible than chickens or ruminants.
Currently Australia does not have any regulatory levels for DON in either finished grain products for human consumption or feed for animal consumption. However some trade contracts have toxin levels stipulated. The United States Food and Drug Administration (FDA) has established some advisory levels for DON in food products for human consumption and feed grain feed to livestock in the US.
- Finished grain products for human consumption: 1 ppm (no standard for raw grain going into the milling process, individual millers set their own acceptable levels)
- Cattle over 4 months old: 10 ppm (providing grain at that level is not more than 50% of the total diet)
- Poultry: 10 ppm (providing grain at that level is not more than 50% of the total diet)
- Pigs: 5 ppm (providing grain at that level is not more than 20% of the total diet)
- All other animals: 5 ppm (providing grain at that level is not more than 40% of the total diet).
- Figure 6. Fusarium infected corn stalk from a field harvested in 2009 and sown to wheat in 2010. The black dots are the sexual stage of the fungus which produces spores that infect following susceptible crops
Principal Plant Pathologist & Winter Cereals Pathology Team Leader
Phone: +61 7 4639 8888
Grains Biosecurity Officer
Phone: +61 7 4639 8864
Mobile: 0429 727 690