Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2012
Publication Date: 10/1/2012
Citation: Cruz, C.D., Bockus, W.W., Stack, J.P., Tang, X., Valent, B.S., Pedley, K.F., Peterson, G.L. 2012. Preliminary assessment of resistance among U.S. wheat cultivars to the Triticum pathotype of Magnaporthe oryzae. Plant Disease. 96:1501-1505. Interpretive Summary: In 1986, a new wheat disease was reported in the state of Parana, Brazil. Wheat blast, caused by the fungal plant pathogen Magnaporthe oryzae, (a wheat pathotype that differs from the rice blast pathotype) has since spread to all wheat production areas of Brazil and more recently into Bolivia, Paraguay and Argentina. Studies in South America have found few regional wheat cultivars with resistance to the disease and chemical control has proven to be ineffective. The disease is seed transmissible. In 2010, disease losses in South America were reported nationally as 30-50 percent with some individual field showing 100 percent loss. In anticipation of the eventual movement of wheat blast into the United States, 85 selected U.S. wheat cultivars were tested for resistance to seedling and head infection by a Brazilian isolate of wheat blast. Testing of cultivars was performed in a bio-containment, bio-safety level-3 laboratory. In initial testing, severity of seedling infection was not reliable in predicting the severity of head infection in a cultivar. Because of the importance of disease development at the head stage in the field, assessment of all 85 cultivars occurred at the head stage. When inoculated separately with 3 different isolates a broad range of cultivar susceptibility was observed with cultivars Everest and Karl 92 showing more than 90 percent disease severity while cultivars Postrock, JackPot, Overley, Jagalene, Jagger, and Santa Fe showed less than 3 percent infection. No differences in susceptibility were observed among the 3 different isolates of blast. In the event this disease becomes established in the U.S., these studies will save breeders many years of preliminary field screening to identify potential disease resistance for incorporation into U.S. cultivars.
Technical Abstract: Magnaporthe oryzae is the causal agent of blast disease on several graminaceous plants. The M. oryzae population causing wheat blast has not been officially reported outside South America. U.S. wheat production is at risk to this pathogen if it is introduced and established. Proactive testing of U.S. wheat cultivars for their reaction to blast and identification of resistance resources is crucial due to the national and global importance of the U.S. wheat industry. In this study, the phenotypic reaction of 85 U.S. wheat cultivars to M. oryzae (Triticum pathotype) was determined. Testing of cultivars was performed in a bio-containment, bio-safety level-3 laboratory. Although there was a significant correlation in the reaction to blast at the seedling and adult plant stages, only 57 percent of the seedling reaction was explained by the head reaction. Because of the importance of disease development at the head stage in the field, assessment of all 85 cultivars occurred at the head stage. Among cultivars tested, a continuum in severity to head blast was observed; cultivars Everest and Karl 92 were highly susceptible with more than 90 percent disease severity while cultivars Postrock, JackPot, Overley, Jagalene, Jagger, and Santa Fe showed less than 3 percent infection. No evidence of the presence of physiological races among isolates T-7, T-12, and T-22 was found.