**4. Discussion**

Growing resistant cultivars has been demonstrated to be the most economical and effective way to manage rice blast disease. During 2005 to 2016, 2377 breeding lines were evaluated for disease resistance to the 12 reference isolates. Breeding lines resistant to all isolates have been found in each year of the period except 2005. Some lines were only susceptible to the most virulent isolate IB33. The use of the lines that have the broadest level of resistance to the spectrum of reference isolates would reduce the loss due to rice blast disease.

Based on the international differential cultivars and nomenclature, isolates A119, A598, and 49D are classified as race IB49 [18]. The disease reactions of many NILs tested to these three isolates were identical. However, these three isolates can be differentiated by some NILs (*R* genes), as *Pib*, *Pi11(t)*, and *Pi20* containing lines were resistant to A119 and A598 but susceptible to 49D; *Pi5(t)* and *Pit* containing lines were resistant to A119 but susceptible to 49D and A598. These results indicated that a set of differential cultivars should be chosen to more clearly demarcate races within the US rice blast pathogen population.

Any mutation, insertion, or deletion of the avirulence genes in the pathogen could cause the changes in its pathogenicity, thus resulting in the loss of function of the corresponding *R* gene and disease development. The coding region of *AVR-Pib* was found in all 12 reference isolates, but the promoter region was not amplified from isolates 49D, IB33, and IB54, and this may explain why the *Pib* gene containing line IRBLB-B cannot provide resistance to these three isolates. Some of the avirulence genes in the US population of *M. oryzae* have been studied [17, 18, 25]. However, the variation of other avirulence genes in the US population of *M. oryzae* needs to be evaluated.

Specific primers were used to detect the presence/absence of seven avirulence genes. Three avirulence genes, *AVR1-CO39*, *AVR-Pi9*, *AVR-Pikz*, *AVR-Pizt*, were present in all 12 reference isolates. According to the gene-for-gene concept [9], the corresponding *R* genes *Pi-CO39* line, *Pi9*, *Pikz*, and *Piz-t* would interact with these avirulence genes and initiate the defense response. It is unknown how many avirulence gene/*R* gene pairs could be involved in the resistance recognition process. When *AVR-Pita*1 was introduced into strains that were virulent on *Pita* containing cultivars, those transformed strains lost their pathogenicity on *Pita* containing cultivars [30], suggesting that one *R* gene recognized one corresponding avirulence gene to initiate the resistance response. If this is the case, then cultivar CO39 and lines carrying *Pi9*, *Pikz*, and *Piz-t*, would have broad-spectrum resistance to the US isolates. It has been shown that Pi9 containing line IRBL 9-W had resistance to all 12 reference isolates, but this is in contrast to the results of the NILs carrying *Pi-CO39*, *Pikz*, and *Piz-t* based on the reference isolates. If these avirulence genes in the reference isolates are functional, then the critical avirulence gene or combination of avirulence genes needs to be further evaluated for managing the disease.

A number of *R* genes to the blast pathogen disease have been identified from rice [4, 6–8]. Although more than 20 *R* genes were incorporated into the NILs, only *Pi9*, *Pi11(t)*, *Pi12(t)*, *Pib*, and *Pita-2* showed broad spectrum of resistance to the reference isolates of *M. oryzae* found in the southern USA. The *R* gene *Pita-2* has been widely used in US rice breeding programs, and has been effective, but incorporation of other *R* genes to develop more durable resistant cultivars will help to reduce the impact of rice blast disease.

### **5. Conclusions**

The population of *M. oryzae* in the southern USA is very diverse. Breeding lines with broad spectrum of resistance to the reference isolates have been developed,

**63**

**Author details**

provided the original work is properly cited.

Chunda Feng and James C. Correll

University of Arkansas, Fayetteville, AR, USA

\*Address all correspondence to: cfeng@uark.edu

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Evaluation of Resistance of US Rice Breeding Lines to the Rice Blast Pathogen*

tered by the Arkansas Rice Research and Promotion Board.

and incorporation of other *R* genes to develop more durable resistant cultivars will

This research has financial support through the rice checkoff funds adminis-

*DOI: http://dx.doi.org/10.5772/intechopen.84980*

**Acknowledgements**

**Conflict of interest**

No conflict of interest.

help to reduce the impact of rice blast disease.

*Evaluation of Resistance of US Rice Breeding Lines to the Rice Blast Pathogen DOI: http://dx.doi.org/10.5772/intechopen.84980*

and incorporation of other *R* genes to develop more durable resistant cultivars will help to reduce the impact of rice blast disease.
