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.   

A closer look at stem rust of wheat

Stem rust, which is known as a “disease” of wheat, has been with humans from the early days of wheat cultivation. Throughout history, and documented as far back as the wheat cultivated by Roman civilizations, crop losses due to stem rust epidemics have been recorded.  It is hugely important to the population of the world, since it threatens the production of wheat. Wheat flour is a major component of food stuffs eaten around the world, and most notably bread, which is a staple food in many countries.

Disease symptoms occur on leaf blades, stems, and ears of susceptible cultivars.

The outer layer of the plant tissue is ruptured by the growing fungus. The fungus reproduces by spores. Much like plants have seeds, which we plant to grow new plants, fungi have spores, and each spore can give rise to a new fungal body (a new individual fungus, which can infect other plants). The spores have a rust brown colour, and from there the name of stem “rust” was given.

Although stem rust is seen as a very important disease of wheat, similar rust diseases also occur on barley, triticale and oats.PStem rust grows and proliferates well at temperatures of about 24°C while development slows at temperatures below 15°C.  Thus, severe epidemics of the disease may occur in warm and humid climates that favour the development of the disease.  In the wheat growing season, the spores survive within infected wheat fields. Once the wheat has been harvested, out of season volunteer wheat plants allows the fungus to survive until the next wheat season.  

The infective spores are spread mainly spread by air dispersal.  A small proportion of the spores may reach the atmosphere and from there it could be dispersed over vast distances by wind.  So for instance, spores from Southern Africa have been reported to be the source of stem rust infection to wheat in Australia.  The ability of rust spores to travel incredible distances, means the fungus can fairly easily spread to areas where it did not occur before.

A major stem rust epidemic in North America in the 1950’s destroyed up to 40% of the spring wheat crop and, for the first time, urged worldwide collaboration of scientists to find a solution to the problem. A resistant cultivar is a wheat plant which is immune to certain diseases. This means that even if the fungal spores are in the air, the plant will not get sick.  A very great man, and Nobel prize Laureate Dr Norman Borlaug, managed to breed wheat plants that were resistant to stem rust. These wheat plants, which would escape stem rust infection, because of their natural resistance to stem rust, were widely planted over the world, and saved millions of people of sure starvation. Since then breeding new wheat plants resistant to stem rust (and other diseases) became one of the most important ways of controlling the disease.

That is as much of the stem rust story I can tell you today, but I will be back with more on stem rust and other important diseases of crop plants again soon…