Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/2/2013
Publication Date: 12/15/2014
Citation: McClung, A.M. 2014. Rice blast research: improving our arsenal and using it. Proc. 35th Rice Tech. Work. Group Meet., New Orleans, LA, p.41. Feb. 18-21, 2014. CDROM.
Interpretive Summary: Rice blast disease is a fungal disease of rice that occurs worldwide. Research has been conducted in the US for many years to develop blast resistant rice varieties. This has helped reduce losses only there are frequently sporadic outbreaks of the disease. The organism that causes this disease on rice has variants which attack many other plant species. Recently blast disease on wheat has been found in South America. Because of the large acreage of wheat in the US, there is concern that this could become an important disease and threaten wheat production. A multi-state project entitled “Novel strategies for managing blast diseases on rice and wheat” and funded by USDA/NIFA/AFRI has been initiated and is the basis for a symposium focused on developing blast management strategies for both rice and wheat. Nine presentations were made at the symposium which laid the groundwork for the basis of the project and the approach that was being used to address the research problems. This included an overview of recent outbreaks of rice blast in the US and strategies to develop new sources of blast resistance. One method includes combining different resistance genes to provide resistance to a wide array of rice blast variants. Developing genetic markers that are linked to these genes will facilitate rice breeding methods to develop new varieties. In regards to wheat blast, little is known about the disease development cycle or sources of resistance. A multi-disciplinary approach that includes developing weather forcasting models that can be used to predict disease outbreaks and developing accurate methods to inoculate plants with the pathogen to be able to screen for resistance in wheat cultivars. In summary, this symposium provided an overview of what has been learned about rice blast disease management, what are the existing knowledge gaps, how this knowledge can help direct research to control wheat blast.
Technical Abstract: Rice blast disease is a constant threat to U.S. rice production, and there have been sporadic outbreaks of the disease for many decades. However, the U.S. southern rice growing area has been fortunate because the pathogen population has been relatively stable compared to other rice producing areas in the world. It has only been when susceptible varieties and the right environmental conditions have occurred that serious outbreaks have developed. This is in spite of the narrow U.S. rice gene pool that is essentially confined to tropical japonica germplasm and is derived from relatively few progenitor cultivars. Rice cultivars introduced to the U.S. before 1920 have been shown to be the source of the major blast resistance genes, Pi-ks, Pi-km, and Pi-z, that are still being effectively deployed today. Other broad spectrum blast resistance genes have been more recently introduced; Pi-ta2 in 1989 and Pi-b in 2004, both from indica sources. Since 2003, U.S. rice breeders have utilized genetic markers that are closely linked to major Pi-genes in their selection process. In addition, there have been on-going efforts to evaluate the USDA rice germplasm collection to identify different genetic sources possessing these known genes, as well as identify new Pi-genes through mapping efforts. It is critical for breeders to have these tools so that Pi-genes can be pyramided to provide durable resistance against existing and emerging races of blast. Wheat blast disease recently emerged in South America and could become a threat to U.S. wheat production. Our understanding of wheat blast disease is comparable to that of rice blast disease some 50 years ago. The problem is serious because of the extensive acreage of wheat production, there are no known resistance genes, and control using fungicides is unreliable. Because of the on-going threat of blast disease in rice and new concerns regarding blast in wheat, a multi-state USDA/NIFA/AFRI grant entitled “Novel strategies for managing blast diseases on rice and wheat” was developed and provided the opportunity to host this symposium focused on rice blast management strategies and the potential impact of wheat blast. Nine scientific presentations were made at the symposium with an overview of the research problems being addressed and the expected outcomes of the project being presented by Dr. Barbara Valent. Although rice Pi-genes are available and used by breeders, none of the commonly deployed Pi-genes provide resistance to all U.S. races of blast. Dr. Rick Cartwright presented an historical perspective regarding the sporadic but persistent nature of blast disease in the U.S. Drs. Don Groth and Yeshi Wamishe reported that recent outbreaks of rice blast in the U.S. have been attributed, in part, to the release of high yielding but susceptible varieties and mild winters allowing survival of the pathogen on crop residue. Dr. Fernando Correa-Victoria demonstrated the need for an aggressive breeding strategy to stack Pi-genes and that this can be facilitated through the use of linked molecular markers. Dr. Yulin Jia presented results of a survey of some 1,800 cultivars from the USDA rice core collection and found that Pi-ta and Pi-b genes were common to indica and tropical japonica germplasm whereas Pi-z was found in all but the aromatic subgroup of rice. This indicates that these genes have been globally important and, through genotyping and phenotyping efforts, novel genes can be found in genetic collections. The impact of flood depth and duration on rice blast disease development was presented by Dr. Fleet Lee and suggested that future efforts to decrease the amount of irrigation water used in rice production may have a negative effect on blast disease control. Dr. Cruz performed modeling studies to assess the likelihood of wheat blast proliferating in the southern U.S. where there is significant w