Stem rust - Catch it before it catches you

Causal organisms: Puccinia graminis f.sp. tritici- wheat, barley, triticale Puccinia graminis f.sp. avenae- oats Puccinia graminis f.sp. secalis- rye

Stem rust is a highly damaging disease of small grains which causes losses in grain yield and quality. Throughout history, and as far back as the wheat cultivated by Roman civilisations, crop losses due to stem rust epidemics have been recorded. To this day, it remains one of the greatest challenges to successful small grain production, particularly of wheat, around the world.

Growth and proliferation of the stem rust fungus happens optimally at higher temperatures of 20-24°C, while development slows at temperatures below 15°C. Thus, severe epidemics of the disease may occur in warm and humid climates that favor the development of the disease. 

In the growing season, the spores survive within infected wheat fields and out of season it survives on volunteer plants and on wild grasses. Fungal spores are spread by plant-to-plant contact and by air dispersal. A small proportion of the spores reaches the atmosphere and may be dispersed over vast distances, even over continents, by wind. 

Disease symptoms, which are rust brown pockets of fungal spores, may occur on leaf blades, stems and ears of susceptible cultivars. In time, these symptoms turn black, from which the name "black rust" for this same disease originates.

Globally, stem rust remains a common disease of small grains despite the application of fungicides and planting of resistant cultivars. The successful survival of this fungus over centuries relies on its ability to produce masses of spores in short periods of time, which is spread over wide geographical areas through wind. Moreover, the fungus also has the ability to constantly regenerate in to specific strains that overcome the resistance of host plants. Thus, it is improbable that stem rust will ever be a totally overcome.

During the past growing season, here in South Africa, I have noticed that timely intervention on stem rust remains critical. As with all plant diseases, a preventative fungicide application approach is better than a curative approach, and more so for susceptible cultivars. Stem rust, under suitable conditions, will easily catch you before you can catch it, so the best plan is to break the life cycle and resultant proliferation of the fungus early in the season, to get the upper hand.

In some countries, fungicides are not routinely used for the control of stem rust, but they rely on the planting of resistant cultivars. If you grow wheat in such a region, it is imperative that you adopt new, more resistant cultivars. It remains an environmentally friendly, reliable and affordable option for stem rust control.