Novel Strategies for Managing Blast Diseases on Rice and Wheat

Abstract: Novel Strategies for Managing Blast Diseases on Rice and Wheat Rice and wheat are the top two sources of calories in the human diet, and both are important export crop for the U.S. Rice blast, caused by the Magnaporthe oryzae Oryza pathotype (Moo) remains an explosive threat to U.S. rice production. Host resistance is one of the most effective means of controlling blast, but there is a critical need for strategies to move resistance (R) genes into elite rice varieties while maintaining yield and quality. R-genes are generally lacking for the newly-emerged wheat blast disease in South America (S.A.), caused by the M. oryzae Triticum pathotype (Mot) and fungicide treatments are unreliable. It is paramount to prepare for incursion and prevent establishment of the S.A. blast fungus on U.S. wheat. It is also essential to determine the potential for wheat blast to develop in the U.S. from native strains of the Lolium pathotype (Mol), which causes gray leaf spot disease on turf grasses. Overall Hypothesis or Goal: Pathogenicity factors and avirulence (Avr) effectors have been extensively characterized from Moo, and 18 blast R-genes have been cloned from rice. Our goal is to apply this knowledge to develop new genetic tools for controlling Moo, Mot and Mol. New management methods, including rice and wheat blast forecasting models, will also be developed and integrated with current practices. The specific objectives of this proposal are: (1) Enhance blast resistance in elite U.S. rice varieties. We will determine the population structure of Avr-effectors for cloned R-genes and deploy R-genes with maximum potential for disease control. To simplify the regulatory process, cisgenic approaches (only native rice sequences are retained) will be used to introduce R-genes into elite varieties. (2) Develop novel resistance strategies based on understanding of genes required for pathogenicity in Moo. We will develop Host Induced Gene Silencing in rice and identify effectors that are conserved in Moo, Mot, and Mol populations, as well as Avr-effectors responsible for rice and wheat specificity. (3) Enable rapid detection of wheat blast in the U.S. Understanding wheat blast field biology, and providing diagnostics, workshops and web-based training resources will establish an effective first-responder network for preventing establishment of wheat blast in the U.S. by understanding the population structure, ecology and epidemiology of the wheat blast fungus in S.A.; determining the potential risk to wheat posed by native Mol strains; validating, refining and deploying PCR-based diagnostic protocol(s); educating first responders about wheat blast and training a new generation of plant pathologists to work on international crop biosecurity. (4) Incorporate blast resistance in wheat. With current USDA support, we have identified U.S. wheat varieties with high levels of resistance in BSL-3 containment growth chamber/greenhouse studies. From these, broadly useful R-genes will be identified through rigorous field testing in S.A. We will develop molecular markers for incorporating effective R-genes into wheat. Additionally, we will test if cloned rice R-genes corresponding to conserved Mot Avr-effectors (e.g. Piz-t and PiCO39) function in wheat to control blast. (5) Develop prediction models for wheat and rice blast that can be used in regions where these diseases are currently causing problems. We will test prediction models of wheat blast in North America using diseases with similar epidemiology as model systems. (6) Prepare for stakeholder acceptance of new technologies by assessing economic, social, and environmental impacts of new technologies and communicating these impacts to stakeholders. We will also assess stakeholder attitudes on cisgenic versus transgenic crops and develop educational materials and strategies to promote acceptance. Objectives 1 and 2 will be immediately developed for rice blast control, but these strategies will also be relevant for control of wheat blast. Objectives 3 and 4 will wheat blast control in South America and rapid detection and eradication of wheat blast if it appears in the U.S. Objectives 5 and 6 apply equally to rice and wheat blast control. Use of the rice blast forecasting model and new resistant varieties will reduce fungicide application and enhance sustainability. Understanding wheat blast biology and training a first-responder network will enable quick response if wheat blast occurs in the U.S. In the longer term our results should also aid control of the turf grass disease caused by Mol, and other diseases caused by host-adapted populations of M. oryzae.