Magnaporthe oryzae is spread naturally from plant to plant within a field and from field to field within a region through the forces of wind and rain. This is how most epidemics develop. M. oryzae can also be spread from field to field within a region and from region to region as a result of human activities, including the movement of farm machinery or seeds infected/infested with the wheat blast pathogen from an infested field to uninfected fields. Movement of infected/infested seeds is the most likely mechanism for the spread of wheat blast from the original outbreak site in Brazil to other parts of Brazil and to Bolivia and Paraguay. Wheat blast could be introduced into the U.S. as a result of increased trade and travel between the U.S. and Brazil.
Wheat blast could also emerge in the U.S. as a result of the evolution and establishment of new strains of M. oryzae from indigenous populations on grass species. Wheat blast pathogens may have already emerged in U.S. on other hosts but environmental factors haven’t yet favored outbreaks/epidemics.
Host Plant Resistance
Wheat blast is currently managed by planting wheat varieties selected for some level of resistance to M. oryzae. Although an important component of a blast management plan, to date host plant resistance has not been adequate as a stand alone strategy. Even with "resistant" varieties, the application of fungicides is necessary during epidemic years in order to prevent substantial yield loss. The search for effective and durable resistance is underway in Brazil, Bolivia, Paraguay and at CIMMYT (the International Maize and Wheat Improvement Center in Mexico). Plant genotypes with immunity have not yet been identified. However, research at Kansas State university and at the USDA ARS Lab at Fort Detrick MD has identified wheat varieties with very high levels of resistance. The durability of that resistance needs to be determined.
In Bolivia and Brazil, wheat blast outbreaks can be significantly reduced by delaying the planting date to avoid infection. This has been effective except in El Nino years when temperatures and rainfall favor spread of the pathogen and disease development.
As with any plant disease, successful management of wheat blast with fungicides requires the application of an effective chemistry at the right dose, at the right time, and using the right equipment (e.g., spray nozzles). Wheat blast is most effectively managed with a combination of timely applications of preventative fungicides and the planting of wheat varieties with some level of resistance.