Wilmarie Kriel

Lecturer
Department of Plant Sciences
University of the Free State

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Because of the nature and pathogenic ability of Fusarium, this fungal genus is intensively studied by plant pathologists.  One of the most serious diseases caused by a Fusarium, is Fusarium head blight (FHB) of wheat.  It is mainly caused by Fusarium graminearum (Teleomorph:  Gibberella zeae), but F. crookwellense, F. culmorum, F. avenaceum and F. poae may also be involved.

Macroconidia of Fusarium graminearum, causal fungus of FHB.

Under favourable environmental conditions symptoms may develop within three days after infection. The critical period for infection is the two week window during flowering.  The first symptoms may usually appear on a single floret, but the disease soon spreads to cause necrosis and bleaching of the entire wheat head.  Under moist conditions, bright orange sporodochia of the fungus develop on the glumes.  The kernels become shrivelled (also referred to as tombstone kernels) and may be covered with a layer of fungal mycelium.

The disease is of importance in the high rainfall and irrigation areas of South Africa .  It is responsible for losses in grain yield and quality.  If infected seed is planted, it can also result in reduced seedling emergence.  It has been estimated that an average 25% infected seed result in about 67% reduction in stand.  Yield losses according to an ARC-SGI assessment may reach 10-30%, although severe levels of the disease may lead to losses of up to 70%.  The pathogens are also able to produce a number of mycotoxins deleterious to human and animal health.  The most important mycotoxins produced by FHB fungi are deoxynivalenol (DON), nivalenol (NIV) and zearalenone.

Infection by Fusarium graminearum and disease development are favoured by warm conditions in the range of 15-30 C, with an optimum temperature of 25 C and a moist period of longer than 16 h.  Fusarium crookwellense, on the other hand, is favoured by cooler conditions. Overhead irrigation is highly advantageous to disease development and symptoms frequently occur under centre pivot irrigation.  Disease severity is also optimal around the centre of the pivot. High humidity enhances ascospore production and thus dissemination.

Perithecia of the teleomorph, G. zeae, are produced on residue of the previous crop, in most cases maize, wheat or grasses.  These ascospores serve as the primary inoculum for infection.  Asexual macroconidia and hyphal fragments on residue or seed can also serve as sources of inoculum.

The disease cycle of the pathogen is strongly influenced by the crop production system, first and foremost, the cropping sequence.  The presence of residue on the soil surface is a vital factor.  Fusarium is able to survive saprophytically between seasons and crops on residues, therefore the tillage system is significant.  Research in Canada has shown survival of up to 8 months on buried residue and up to 3 yr on residue above ground.  In Mexico the trend tend to be 14 months on the soil surface as opposed to 9 months if buried 15 cm deep. Increasing FHB epidemics in Argentina have been attributed to an increase in conservation tillage (“no-till”) practices (personal communication, S. Pereyra, 2006).  No-till enhances soil health and structure, but the crop rotation system is critical.  Maize followed by wheat is a high risk practice with regard to FHB and other diseases.

Farmers should plant high quality, treated seed. Chemical seed treatments should control most of the seedborne Fusaria.  Foliar chemical sprays for the control of FHB, has so far not proven its efficacy.  No chemicals are currently registered in South Africa for the control of FHB on wheat, although two fungicides are scheduled for registration in the US in 2007 (personal communication, McMullen, 2005).  A further constraint on chemical spray efficacy in SA is the inability to obtain a good cover with the aerial application necessitated in irrigation crops.  Effective timing and residual activity of chemical application also need optimisation and further investigation.
 

Proper crop rotation and tillage practices have so far been the only option to farmers to reduce their risk of FHB.  Since their most practical cropping sequence comprises wheat and maize, farmers have to rely on the burning and ploughing of residue to reduce inoculum levels however, this practice is not sustainable in the long run.   Farmers can also change the environment by adjusting their irrigation schedules to reduce the leaf wetness period and thus escape favourable conditions for infection.

Cultivar resistance seems to be the only reliable control method against FHB.  Resistance against FHB can be divided into five types:
Type I: Resistance against initial infection of the head
Type II: Resistance against the spread of disease within the head
Type III: Resistance to kernel infection
Type IV: Tolerance
Type V:  Resistance against toxin accumulation
Resistance breeding mainly targets Types I, II and to a lesser extent, V.  The two major genetic sources of resistance, are Sumai 3 from China, and Frontana from Brasil.  Resistance is controlled by complex QTL’s, leading to variable levels of expression in breeding lines and subsequent slow breeding progress.  Breeders have to rely on marker assisted selection of resistant material.

References

• Bai, G. and Shaner, G.  2004.  Management and resistance in wheat and barley to Fusarium head blight.  Annu. Rev. Phytopathol.  42:135-161.
• Boshoff, W.H.P., Pretorius, Z.A. and Swart, W.J.  1999a.  A comparison of head infection and blight development caused by Fusarium graminearum and Fusarium crookwellense in wheat.  S. Afr. J. Plant Soil 16:79-84.
• Desjardins, A.E.  2006.  Fusarium Mycotoxins. Chemistry, Genetics, and Biology.  APS Press, St. Paul.  260 pp.
• Dill-Macky, R. and Jones, R.  2000.  The effect of previous crop residues and tillage on Fusarium head blight of wheat.  Plant Dis. 84:71-76.
• Gilbert, J. and Tekauz, A.  2000.  Review: Recent developments in research on Fusarium head blight of wheat in Canada.  Can. J. Plant Pathol.  22:1-8.
• Goswami, R.S. and Kistler, H.C.  2004.  Heading for disaster: Fusarium graminearum on cereal crops.  Mol. Plant Pathol. 5:515-525.
• Kriel, W.M. and Minnaar-Ontong, A.  2006.  Chemical control of seedborne Fusarium species in wheat.  44th Annual Congress of the Southern African Society for Plant Pathology, 22-25 January 2006, Magalies Park Country Club.
• Leonard, K.J. and Bushnell, W.R.  2003.  Fusarium Head Blight of Wheat and Barley.  APS Press, St. Paul.  512 pp.
• Scott, D.B., De Jager, E.J.H. and Van Wyk, P.S.  1988.  Head blight of irrigated wheat in South Africa.  Phytophylactica 20:317-319.
• Scott, D.B. and Smith, J. 1995.  Aarskroei – 'n nuwe bedreiging vir koring onder spilpunt besproeiing langs die Oranjerivier (Head blight – a new threat for wheat under centre pivot irrigation adjacent to the Orange river).  In: Course in small grain production for agricultural advisors.  Small Grain Institute, Bethlehem, South Africa. 109-110.