Ida Paul
Plant Pathologist
Small Grain Institute
Stellenbosch
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Stem rust, caused by Puccinia graminis Pers. f. sp. tritici Eriks. & E. Henn., is a highly damaging disease of wheat that primarily occurs in warm weather and can cause great damage to susceptible wheat crops.  Symptoms include raised orange to red pustules that occur on leaves, leaf sheaths, stems and ears of susceptible cultivars.  The masses of urediniospores appear similar to rust on a weathered iron surface.  In time these pustules turn black. 

Today, for most countries, the continued breeding of varieties resistant to stem rust has become a low priority and stem rust incidence is increasing.  This is exacerbated by the ill-advised use of susceptible varieties.  Susceptible varieties are popular because it may have better quality characteristics than the resistant lines.  However, susceptible lines encourage stem rust epidemics which basically require susceptible material, a favourable climate and inoculum of the pathogen. 

Fig. 1. Stem rust pustules on the stem and leaf of Morocco, a highly susceptible wheat cultivar.

The stem rust pathogen develops optimally near 24°C and development slows at temperatures below 15°C.  Thus, severe epidemics of the disease can occur in warm and humid climates that favour its development.  The spores survive within infected wheat fields and on volunteer plants.  Spores are spread by plant-to-plant contact and by air dispersal.  A small proportion of the spores reaches the atmosphere and is dispersed over vast distances.  So for instance, have spores from Southern Africa have been reported to cause infection to wheat in Australia.  The ability of rust spores to travel these incredible distances, means it is only a matter of time before the new pathotype Ug99 spreads throughout the world.

Fig.3. Pustules of the Ug99 pathotype on a susceptible cultivar (photo: Global Rust Initiative)
 

A pathotype of yellow rust that emerged in eastern Africa in the late 1980’s managed to spread via Yemen to South Asia.  As the disease spread, major losses were recorded in Egypt, Syria, Turkey, Iran, Iraq, Afghanistan and Pakistan. Should the stem rust pathotype Ug99 follow the same path, far greater losses are expected. Countries in the predicted pathway grow more than 65 million hectares of wheat, accounting for 25% of the global wheat harvest. This is a major threat to food production, as wheat is a staple food and the demand is already currently higher than the supply.

At this point in time plant breeders and pathologists are screening for resistant genotypes – and a plan of action has been put together by the Global Rust Initiative (http://www.globalrust.org).  Luckily, some genes still confer resistance to Ug99 and may have immediate value, although others will need further research.  Global Rust Initiative scientists have already identified promising experimental wheat materials with resistance to Ug99.  However, the process of breeding resistant lines will take time and effort, and it will be a few years before these lines would become commercially available.

The prospect of a harsh stem rust epidemic is real and global research efforts are focused at stopping the potential damage to crops and resultant human suffering it may cause.  The current crisis highlights the continuing and devastating impact stem rust has and might still have around the world.  In the words of Norman Borlaug “If we fail to contain Ug99 it could bring calamity to tens of millions of farmers and hundreds of millions of consumers…we know what to do and how to do it.  All we need are the financial resources, scientific cooperation and political will to contain this threat to world food security.”

It is not the first time that stem rust has been the cause of alarm.  In 1951, Rodenhiser and Moore published an article entitled “The new stem rust threat” in Science.  That threat has now long been forgotten and the potential devastation of a new stem rust pathotype lurks on the horizon…

References

• Boshoff, W.H.P., van Niekerk, B.D. and Pretorius, Z.A. (2000).  Pathotypes of Puccinia graminis f. sp. tritici detected in South Africa during 1991-1997.  Suid Afrikaanse Tydskrif vir Plant en Grond, 17(2), 60-63.
  CIMMYT (2007).  Dangerous wheat disease jumps red sea http://www.globalrust.org/images/IR2007_002_GRI.pdf
• Expert Panel (2005).  Sounding the alarm on Global Stem Rust.  An assessment of race Ug99 in Kenya and Ethiopia and the potential for impact in neighbouring regions and beyond.  http://www.cimmyt.org/english/wps/news/2005/aug/pdf/Expert_Panel_Report.pdf
• Leonard, K.J. and Szabo L.J. (2005). Stem rust of small grains and grasses caused by Puccinia graminis. Molecular plant pathology, 6 (2), 99–111.
• Murray, T.D., Parry, D.W. and Cattlin, N.D. (1998).  A color handbook of diseases of small grain cereal crops. Iowa State University Press.
• Rodenhiser, H. A. and Moore, E. G. (1951). The new stem rust threat. Science, 113 (2941) p 3a. http://www.sciencemag.org/cgi/issue_pdf/edboard_pdf/113/2941.pdf
• Watson, I.A. and de Sousa, C.N.A. (1983). Long distance transport of spores of Puccinia graminis tritici in the Southern Hemisphere. In Proc. Linn. Soc. N.S.W., 106:311-321.