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…