Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2003
Publication Date: 6/1/2004
Citation: McClung, A.M., Shank, A.R., Bormans, C., Park, W.D., Fjellstrom, R.G. 2004. Are agronomic traits impacted by the presence of Pi- genes when blast disease is absent? In: Rice Technical Working Group Meeting Proceedings, February 29-Mach 4, 2004, New Orleans, LA. 2004 CDROM Interpretive Summary:
Technical Abstract: Developing improved cultivars that possess disease resistance genes is an effective way of enhancing and stabilizing yield potential while decreasing producer input costs through decreased use of pesticides. However, some researchers are concerned that the presence of disease resistance genes may actually reduce yield when disease is not present as a result of a metabolic 'cost' to the plant for maintenance of resistance. The objective of this study was to determine the affect of Pi- blast resistance genes on yield and agronomic traits when development of disease from Pyricularia grisea was prevented by the use of fungicides. Ten genetic populations were developed that were segregating for the major blast resistance genes Pi-ta2, Pi-b, Pi-z, or Pi-ks/kh. The populations were advanced for several generations without selection and then were screened for the presence of blast resistance genes using DNA markers closely linked to each of the Pi- genes. Standard replicated yield trials were conducted at Beaumont, TX during 2002 and 2003 using progeny that had been selected only on the basis of possessing resistant or susceptible homozygous alleles of the Pi- locus present in each population. The populations ranged from the F4 to F7 level of inbreeding, although two populations consisted of near-isogenic lines identified from F12 and F17 headrow populations. Thirty-eight to 48 lines were evaluated in each population except for one near-isogenic population where only five resistant and five susceptible lines were compared. The trials were sprayed with Quadris and Tilt fungicides according to recommendations to prevent development of diseases. Stand ratings, days to heading, plant height, grain moisture at harvest, test weight, and yield were measured in the trials. In two of the four trials where Pi-ta2 was segregating, there was no difference in agronomic traits among progeny possessing susceptible or resistant alleles. For the other two trials, the presence of susceptible Pi-ta2 alleles, resulted in a small but significant increase in yield of 664 kg/ha (p> 0.086) in one population and 498 kg/ha (p>0.014) in the other. In one population where progeny possessed either the Pi-ks or Pi-kh allele, there was no significant impact on agronomic traits. However, in the study which evaluated 38 near-isogenic lines from a Madison headrow population, the presence of the Pi-kh allele resulted in a small but significant increase in stand establishment and yield (278 kg/ha, p> 0.106) but a slight decrease in days to heading (0.56 days, p>0.037) as compared to the presence of the Pi-ks allele. Of the two populations that were segregating for the presence of Pi-b alleles, only one demonstrated an impact on agronomic traits. The presence of the Pi-b resistant allele resulted in an increase in days to heading (1.47 days p>0.09) and an increase in plant height (6.1 cm, p>0.0001). In another population where both Pi-ta2 and Pi-b were segregating, no combination of resistant or susceptible alleles resulted in a significant impact on agronomic traits. Using a population of 10 near-isogenic lines derived from the cultivar Cala, it was determined that there was no impact on agronomic traits due to the presence of resistant or susceptible Pi-z alleles. Thus, in only four of the 10 populations, the presence of Pi-ta2, Pi-b, Pi-z, and Pi-ks/kh blast resistance alleles had a statistically significant impact on agronomic traits when disease was not present, however the magnitude of this affect does not appear to have significance for cultivar improvement programs.