Rice blast, caused by Magnaporthe oryzae, is the most destructive rice disease worldwide. The disease symptoms are usually expressed on the leaf and panicle. The leaf disease intensity in controlled environmental conditions is frequently quantified using a 0-5 scale, where 0 represents the absence of symptoms and 5 represents large eyespot lesions. However, this scale restricts the qualitative classification of the varieties into intermediate resistant and susceptible categories. Here we develop a 0-6 scale for blast disease that allows proper assignment of rice breeding lines and varieties into six resistance levels (highly resistant, resistant, moderate resistant, moderate susceptible, susceptible, and highly susceptible). We evaluated 41 common rice varieties against four major blast races (IB1, IB17, IB49, and IE1-K). Varieties carrying the Pi-ta gene were either highly resistant, resistant, or moderate resistant to IB17. The IE1-K race was able to break Pi-ta-mediate resistance of the rice varieties. The Pi-z gene conferred resistance to the IB17 and IE1-K races. The varieties M201, Cheniere, and Frontier were highly susceptible (score 6; 100% disease) to the race IE1-K. Moreover, varieties that were resistant or susceptible to all four blast races also showed similar levels of resistance/susceptibility to blast disease in the field. Taken together, our data proved that the 0-6 blast scale can efficiently determine the resistance levels of rice varieties against major blast races. This robust method will assist rice breeding programs to incorporate durable resistance against major and emerging blast races.
Keywords: Magnaporthe oryzae; geographically restricted pathogens; pathogen races; resistance breakage; resistance genes; rice breeding lines.