Diseases of wheat will always be a challenge

In my previous posting I spoke about how we can select wheat plants which are resistant (or also specific resistance genes, but I won’t go into detail on that now) to make a plant, and all seed that is eventually multiplied from it, resistant or immune to stem rust (and also other wheat diseases). That is one of the best ways of combating plant diseases. The problem lies in that the fungus can, over time, change itself in a way that will allow it to infect the resistant plants. So, resistance “falls”, meaning that the once resistant wheat plant, will slowly become infected with rust, and later on no longer be resistant. One can think of stem rust as a “family of individuals” – the first individual is unable to infect certain cultivars (cultivated varieties, meaning that they belong to a group of selected seeds). As the family grows, new individuals arise that are similar to their family, but different in a way that means they can infect resistant wheat plants again.

 

So when we think of stem rust, we should remember that it is not one individual fungus, with one identity, that infects wheat plants throughout the world. It is actually a collection of different stem rust “groupings” and each group infects a fixed cluster of cultivars. Scientists use the pattern by which the stem rust infects different wheat cultivars to give the group an identity. Say group A, group B and group C - all groupings would be known as stem rust, but each group is different in that they infect a particular set different cultivars. New groups arise when fungi from old groups start infecting previously resistant cultivars (and this changes their “pattern” of which cultivars they infect – which give them a new name). These groupings are known as pathotypes or races.

Here you can see four wheat stems. The top stem is resistant to stem rust, and the bottom one is stem rust susceptible (this means it will get sick). In between you have the stems for which some resistance is still present, but the resistance is being "broken down". In the stem second from the bottom, one can see how the red fungus starts breaking through the outer tissue layers of the stem. So the progeny of those plants will eventually become as diseased as the last stem in this picture. 

A very prominent race, or pathotype, that caused great upset in wheat production globally is called Ug99 (first discovered in Uganda in 1999). The reason it is so extremely important is because it overcame a resistance gene (known as Sr31), which was very widely deployed throughout the world, almost like the “magic” ingredient to resistance in wheat, and nobody thought that stem rust will overcome that kind of resistance. Thus, unexpectedly, stem rust was able to infect multiple wheat cultivars grown in the world, which had stem rust resistance for many years. This was a massive crisis to global food production. Not long after this happened, the fungus also overcame other resistance genes (meaning that it could infect even a wider range of wheat plants), and scientist had to respond swiftly to breed new wheat plants that would be resistant to Ug99.

The efforts following the initial outbreak of Ug99 is an excellent example of how international scientists could stand together to avert a potentially catastrophic food crisis in the world. You can read more about this initiative at this link http://www.globalrust.org. Resistant cultivars have now been developed, and deployed in some countries, but not all where Ug99 has been found. Thus, although a major wheat production crisis has been slowed down, efforts to supply the world with resistant wheat cultivars, not only resistant to Ug99, but to other fungal races or pathotypes of stem rust is ongoing.