14.8 Black stem rust (Puccinia graminis f. sp. tritici) threatens global wheat harvest

Wheat stem rust (Puccinia graminis f. sp. tritici) is historically the most damaging disease of wheat (Triticum aestivum). In the worst cases, yield losses of 70% or more are possible. It was at one time the most feared disease of the wheat crop. Feared because what looked like a healthy crop about three weeks before harvest could be reduced by stem rust to a black tangle of broken stems and shrivelled grain by harvest time. The first detailed reports of crop loss to wheat stem rust appeared in Italy in the 18th century and two devastating stem rust epidemics in North America in 1904 and 1916 led directly to research that showed that the stem rust pathogen had various races that differed in their ability to infect wheat varieties because the latter carried different combinations of resistance genes.

This was followed by a concentration of research on identifying resistance to stem rust and breeding resistant wheat cultivars, and on understanding rust epidemiology and evolution. Another important point is that these efforts also initiated global collaboration between wheat geneticists and plant pathologists.

Understanding the biology of the disease revealed the relationship between the common barberry (Berberis vulgaris) and Puccinia graminis. Barberry is the alternate host of P. graminis and provides the sexual stage of the pathogen’s life cycle so is the source of new combinations of virulence genes in the pathogen population. In addition, barberry enables the pathogen to survive through the winter and therefore serves as the source of new stem rust epidemics on wheat in the spring. Common barberry had been planted widely as an ornamental bush, but eradication of the alternate host in North America and Europe was an important step in controlling stem rust epidemics. Between 1918 and 1980, over 500 million barberry bushes were destroyed in the major wheat-producing states in the United States.

The barberry eradication programme was accompanied by efforts to devise genetic control strategies. The International Spring Wheat Rust Nursery Program was initiated in 1950 by the United States Department of Agriculture Agricultural Research Service and operated continuously until the mid-1980s. Its objectives were to:

  • find new genes or combinations of genes in wheat for field resistance to rusts throughout the world, and
  • test new varieties and selections of wheat developed by plant breeders and pathologists for resistance to rusts (Dubin & Brennan, 2009).

About 50 resistance genes in wheat have been catalogued; all but one of which are race-specific, and therefore derive from the gene-for-gene relationship between the host plant resistance gene and corresponding pathogen virulence genes (see Section 14.17).

All these efforts stabilised the stem rust situation in many countries where modern wheat cultivars were adopted. For example, changes in stem rust races have not been observed in Mexico for almost 40 years and natural infections are non-existent. Similarly, a resistant wheat cultivar released in 1960 in the Indian subcontinent and subsequently grown on millions of hectares, remains resistant to stem rust today. The incidence of stem rust decreased throughout most of the world to almost non-significant levels by the mid-1990s. Unfortunately, as disease incidence declined, there was a corresponding decline in emphasis on research on stem rust and wheat breeding against this disease; and in 1999 the fungus got ahead of us.

First identified in Uganda during 1999, Puccinia graminis tritici Race Ug99 is the only known race group that has virulence for a wheat resistance gene (Sr31, derived from rye, Secale cereale) which is incorporated (as a chromosomal translocation) in many of the most widely use wheat cultivars. Race Ug99 (in North American scientific nomenclature, Ug99 is known as race TTKSK) also carries virulence for most of the resistance genes of wheat that have been introduced over the years by selective breeding. This is the headline: Ug99 carries a unique combination of virulence to known and unknown resistance genes of wheat. Consequently, the unique virulence associated with Ug99, or its race group variants, has rendered a large proportion of global wheat cultivars susceptible to disease, irrespective of where they were developed. It is now believed that 80-90% of all wheat varieties grown in developing countries are susceptible to the new race of stem rust and infection can cause yield losses up to 80%.

Since the first detection of race Ug99 in Uganda in 1999, 10 variants in its race group have been detected in 13 countries Uganda, Kenya, Ethiopia, Sudan, Tanzania, Eritrea, Rwanda, South Africa, Zimbabwe, Mozambique, Yemen, Iran and Egypt (Patpour et al. 2016). Plants recognise pathogens through receptors that identify pathogen effectors secreted by the pathogen into the plant. Wheat genes that confer resistance to Ug99 and other virulent races of Puccinia graminis are being identified (Babiker et al., 2016, 2017; Zhang et al., 2017), and two effectors from Puccinia graminis f. sp. tritici were identified recently that are expected to aid developments of approaches to track the spread of the rusts of wheat and improve understanding of these pathogens (Moscou & van Esse, 2017; and see http://rusttracker.cimmyt.org/ and http://www.fao.org/agriculture/crops/rust/stem/en/).

Resources Box 14.1

Where to find more information about crop diseases, crop losses, plant pathogens and food and agriculture statistics

We have a page giving references to scientific papers and hyperlinks to online resources.

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Updated July, 2019