*2.3.3 Virulence of US reference isolates of M. oryzae on IRRI rice NILs*

Among the blast reference isolates, IB33, 49D, and TM2 were the most virulent isolates, whereby only 8–13 (15.7–21.6%) of the NILs were resistant. Isolates IB54, ID13, and #24 were the least virulent isolates with 33–38 (64.7–74.5%) of the NILs being resistant. Other isolates were intermediate in virulence (**Table 3**).

NILs containing gene *Pi3(t)* were susceptible to all reference isolates tested. NILs containing *R* genes *Pia*, *Pi19(t)*, *Pii*, *Piks*, and *Pizt*, were only resistant to one isolate. Those lines containing *Pikh*, *Pikp*, and *Pita* were resistant to two isolates. NILs containing the *R* genes, *Pi1*, *Pi7(t)*, *Pik*, *Pikp*, *Pikm*, *Pit*, and *Piz*, were resistant to three isolates. NILs containing *Pi9(t)* or *Pi12(t)* were resistant to all isolates (**Table 4**). Lines containing genes *Pib*, *Pi11(t)*, and *Pita-2/Ptr* [29] were resistant to 9 or 11 isolates.


**Table 3.**

*Number and percentage of rice NILs resistant or susceptible to blast isolates.*


**59**

**Table 4.**

*Disease responses of rice NILs to US Magnaporthe oryzae reference isolates.*

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

**ID Gene 49D TM2 24 A119 A264 A598 IB33 IB54 ID13 ZN7 ZN15 ZN46**

IRBL 1-CL *Pi1* S S R S S S S R R S S S IRBL 3-CP 4 *Pi3* S S S S S S S S S S S S IRBL 5-M *Pi5(t)* S S R R R S S R R S R S IRBL 7-M *Pi7(t)* S S R R R S S S R S S S IRBL 9-W *Pi9* R R R R R R R R R R R R IRBL 12-M *Pi12(t)* R R R R R R R R R R R R IRBL 19-A *Pi19(t)* S S S S R S S S S S S S IRBLKM TS *Pikm* S S R S S S S R R S S S

**IB49 IE1K IG1 IB49 IC17 IB49 IB33 IB54 ID13 IE1 IB1 IC1**

*Pi20* S S S R R R S R R S R R

*Piz-5* R R S R R S R R R S R S

IRBLTA 2-PI *Pita-2* R S R R R R S R R R R R IRBLTA 2-RE *Pita-2* R S R R R R S R R R R R IRBLTA CP 1 *Pita* S S R S S S S R S S S S IRBL 11-ZH *Pi11(t)* S R R R R R S R R R R R

LJXTHG *LTH* S S S R R S S R S S S S IR 85430 *Pish* S R R R R S S R R S R R IR 85424 *Pish* S R R R R S S R R S R R IR 93322 *Pish* S R R R R S S R R S R R IR 85417 *Pib* S R R R R R S R R R R R IR 85427 *Piz-5* R S S R R R R R R S R R IR 85429 *Piz-t* S R R R R R S R R R R R IR 85413 *Pi5(t)* S R S R R S S R R S S S IR 85423 *Piks* S S S S S S S R S S S S IR 85420 *Pik* S S R S S S S R R S S S IR 85419 *Pik* S S R S S S S R R S S S IR 85421 *Pikm* S S R S S S S R R S S S IR 85422 *Pikp* S S R S S S S R R S S S IR 85411 *Pi1* S S R S S S S R R S S S IR 85414 *Pi7(t)* S S R S R S S R R S S S IR 85426 *Pita* S S R R R S S R S R S S IR 93324 *Pita* S S R R R S S R S R S S IR 93323 *Pita-2* R S R R R R R R R R R R IR 85425 *Pita-2* R S R R R R S R R R R R IR 93325 *Pita-2* R S R R R R S R R R R R CO39 *Pia* S S S S S S S S S S S S

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

IRBL 20-IR 24

IRBLZ 5-CA (R)



*Protecting Rice Grains in the Post-Genomic Era*

inoculation, the plants were incubated at 100% relative humidity in a mist chamber at approximately 22°C for 24 h, allowed to dry for 2–3 h before being moved to the greenhouse. The inoculated plants were incubated in the greenhouse for 6 days. On the 7th day after inoculation, 15–20 plants of each line were scored according to a standard 0–9 disease rating scale developed by IRRI [22]. Lines rated 0 to 3 were considered resistant, whereas those rated 4–9 were considered susceptible.

Among the blast reference isolates, IB33, 49D, and TM2 were the most virulent isolates, whereby only 8–13 (15.7–21.6%) of the NILs were resistant. Isolates IB54, ID13, and #24 were the least virulent isolates with 33–38 (64.7–74.5%) of the NILs

NILs containing gene *Pi3(t)* were susceptible to all reference isolates tested. NILs containing *R* genes *Pia*, *Pi19(t)*, *Pii*, *Piks*, and *Pizt*, were only resistant to one isolate. Those lines containing *Pikh*, *Pikp*, and *Pita* were resistant to two isolates. NILs containing the *R* genes, *Pi1*, *Pi7(t)*, *Pik*, *Pikp*, *Pikm*, *Pit*, and *Piz*, were resistant to three isolates. NILs containing *Pi9(t)* or *Pi12(t)* were resistant to all isolates (**Table 4**). Lines containing genes *Pib*, *Pi11(t)*, and *Pita-2/Ptr* [29] were resistant to 9 or 11 isolates.

**Line A598 ZN15 ZN46 24 A264 ZN7 TM2 49D A119 IB33 IB54 ID13** R 16 21 17 33 31 15 13 11 27 8 38 34 % 31.4 41.2 33.3 64.7 60.8 29.4 25.5 21.6 52.9 15.7 74.5 66.7 S 35 30 34 18 20 36 38 40 24 43 13 17 % 68.6 58.8 66.7 35.3 39.2 70.6 74.5 78.4 47.1 84.3 25.5 33.3

**ID Gene 49D TM2 24 A119 A264 A598 IB33 IB54 ID13 ZN7 ZN15 ZN46**

IRBLA-A *Pia* S S S S S S S R S S S S IRBLA-C *Pia* S S S S S S S R S S S S IRBLI-F5 *Pii* S S S S S S R R S S S S IRBLKS-F5 *Piks* S S S S S S S R S S S S IRBLKS-S *Piks* S S S S S S R R S S S S IRBLK KA *Pik* S S R S S S S R R S S S IRBLKP-K60 *Pikp* S S R S S S S S R S S S IRBLKH-K3 *Pikh* S S R S S S S S R S S S IRBLZ FU *Piz* S R S S R S S S R S S S IRBLZ 5-CA *Piz-5* R S S R R R R R R R R S IRBLZT-T *Piz-t* S S S S S S S R S S S S IRBLTA CT 2 *Pita* S S R S R S S S S S S S IRBLB-B *Pib* S R R R R R S S R R R R IRBLT-K59 *Pit* S S S R R S S R S S S S IRBLSH-S *Pish* R R R R R R R R R R R R IRBLSH-B *Pish* S S R R R R S R S S R S

*Number and percentage of rice NILs resistant or susceptible to blast isolates.*

**IB49 IE1K IG1 IB49 IC17 IB49 IB33 IB54 ID13 IE1 IB1 IC1**

*2.3.3 Virulence of US reference isolates of M. oryzae on IRRI rice NILs*

being resistant. Other isolates were intermediate in virulence (**Table 3**).

**58**

**Table 3.**

### **Table 4.**

*Disease responses of rice NILs to US Magnaporthe oryzae reference isolates.*

Four loci provided resistance to reference isolate 49D (race IB49) or IB33 (race IB33), 7 loci were resistant to isolate TM2 (race k), and 14, 16, and 17 loci were resistant to isolate IB54, isolate #24 (race IG1), and isolate ID13, respectively.

Discrepancy in disease reactions was observed among lines putatively containing the same target *R* genes. One NIL (IRBLSH-S), containing *Pish*, was resistant to all blast reference isolates, while four *Pish* containing lines were only resistant to six to eight isolates. Both NILs IRBLZT-T (in LTH background) and IR 85429 (in CO39 background) contain *R* gene *Pizt*. NIL IRBLZT-T was resistant to one isolate, while IR 85429 was resistant to 10 isolates. These discrepancies may have resulted due to a number of reasons including linkage drag from different donor parents. The *R* genes in the NILs also need to be confirmed with specific molecular markers.

Thus, NILs containing *Pia*, *Pi3*, *Pi19(t)*, and *Pi12(t)* were not useful for differentiating races of the US reference isolates tested. Resistance loci *Pi9(t)*, *Pi12(t)*, *Pib*, *Pi11(t)*, and *Pita-2* were the most effective *R* genes to the panel of US reference isolates evaluated and could be exploited to improve resistance to rice blast disease in the USA.
