Cereals: Fusarium Head Blight (Scab)

Excerpt from Agronomy Guide for Field Crops (Chapter 6)

Order OMAFRA Publication 811: Agronomy Guide for Field Crop

Table of Contents

1. Incidence
2. Appearance
3. Disease Cycle
4. Management Strategies
5. Updates on Cereals: Fusarium Head Blight (Scab)
6. Related links...

Incidence

Fusarium head blight (scab) is one of the most important diseases of small grains in Ontario. In recent years, severe outbreaks have occurred when the weather is warm and wet at the flowering to soft dough stages. Besides the potential for significant yield losses, mycotoxins that are harmful to livestock can be produced.

Appearance

Symptoms of scab become noticeable soon after flowering. Diseased spikelets (glumes and florets) appear to have ripened prematurely (bleached) in contrast to healthy, green heads. The fungus may attack all or only part of the head. Bleaching of the heads or head blight appears 3-5 days after infection. The entire head may be killed when the neck (the stem immediately below the head) is infected.

Plate 120. Fusarium head blight bleaches all or part of the head. Typically, the stem remains green.

During warm, humid weather, the fungus produces a salmon-orange-to-pink ring of spores at the base of the spikelet or in the crease of the kernel. If conditions continue, the infection may spread to adjacent kernels. Infected kernels are usually shrunken, wrinkled and light in weight. These kernels have a rough, scabby appearance and range in colour from light brown to pink to greyish-white. The amount of scab on the seed depends on the time of infection and the weather conditions at the time of infection.

The planting of infected seed can result in the development of the seedling blight phase of the disease, which is separate from scab. Infected kernels may not germinate and can result in poor stands. Infected plants that emerge may lack vigour and will often die before they become established. Infected seedlings can appear light-to-reddish-brown and may be covered with a white or pink mould. As the plants mature, they are usually smaller with few tillers and small heads. If the root or crown is cut, a light-to-reddish-brown root rot can be observed.

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Disease Cycle

Although several species of Fusarium can cause scab, the principal pathogen is Fusarium graminearum, which can infect corn, wheat, barley, oats and rye. All species overwinter in infected kernels, chaff, stubble or straw/stalk residues left on the soil surface. They survive between crops as asexual spores (conidia), fungal strands (mycelium) and within dark purplish-black fruiting bodies (perithecia), which the sexual spores (ascospores) are borne in. The fungi will continue to grow and produce spores from harvest until the crop residues have decomposed in the soil.

Both types of spores can be carried from infected residues of the previous crop by wind or rain splash onto the wheat head. The conidia are produced during warm, moist weather on corn and small grain residues while the ascospores are released during wet and dry cycles. By doing so, the fungus is able to spread spores into the air for a longer period of time. Spores that land on the head require rainfall or heavy dew to germinate and invade flower parts (anthers), glumes and other portions of the head. The potential for disease increases substantially when these spores land during an extended warm period at temperatures between 22°C-27°C (70°F-80°F) with wet, humid weather. The longer it stays wet during flowering, the greater the chance of infection and therefore increased disease severity. If warm, moist weather continues, the salmon-pink spore masses produced on the spikelets will be air-borne and can act as another source of infection.

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Management Strategies

Avoid planting wheat following wheat or corn. When residues of either of these crops are left on the surface and wheat is subsequently planted, the chances of Fusarium head blight infections are greatly increased. Clean plowing of infected residues reduces the risk of infection from spores originating from within the field. However, Fusarium head blight may still develop from spores blown in from surrounding fields under weather conditions favourable to disease development. As many of the infected kernels are small, shrunken and lighter than sound kernels, it is possible to blow a large proportion of these kernels out the back of the combine by increasing the air blast above normal ranges. This may cause some additional loss of good kernels (up to 0.13 t/ha or 3 bu/ac). Proper storage and drying will limit further Fusarium head blight development after harvest. The availability of resistant varieties is presently limited, but new varieties are being developed.

Research done at Ridgetown College, University of Guelph, on Fusarium head blight (FHB) management has led to a mycotoxin prediction model (DON or vomitoxin). The model was developed over many years (1996-2000) and in 399 Southern Ontario wheat fields. It continues to be modified and is quite innovative since it relates DON accumulation in the wheat grain to the environmental conditions surrounding heading and how it relates to inoculum production, wheat head infection and subsequent fungal growth within the head. Visit the Fusarium Web site at www.ridgetownc.on.ca/OWN/Fusarium/Fusarium_Frame.htm. to view prediction maps for Ontario.

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Updates on Cereals: Fusarium Head Blight

• Field Crop Protection Guide - Publication 812
  • Cereals: Wheat: Fusarium Head Blight

Related links...

• Diseases
• Cereals

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