Agriculture Reference
In-Depth Information
of blast resistance genes (
Pi-1
,
Pi-2
,
Pi-33
,
Pi-b
,
Pi-9
, and
Pi-ta
2
) for which
their corresponding avirulence genes are highly conserved in blast
pathogen populations in Colombia as the basis for the development
of durable blast resistance for the irrigated ecosystem in Colombia.
Molecular markers highly linked to these blast resistance genes have
been found to be facilitating the introgression and pyramiding of these
genes into Latin American rice cultivars and elite lines derived from
rice breeding programs aiming at developing rice cultivars with durable
blast resistance.
Rainfed conditions prevalent in LAC are more conducive for blast
development than irrigated lowland environments. In general, high
blast pressure as well as high diversity of the pathogen found under
these conditions makes resistance less stable and durable. Although
resistance normally breaks down in 1
3 years after cultivar release, there
are examples of cultivars such as Oryzica Llanos 5 in Colombia whose
resistance lasted for
-
15 years, and it had a virtually unmatched degree
of resistance when evaluated at several blast-conducive sites in Asia.
The genetic basis of the high durable resistance to rice blast in this
cultivar was found to be due to a combination of quantitative and
qualitative resistance genes (Lopez-Gerena 2006). The growing number
of mapped minor and major resistance genes and development of
marker-aided selection suggest that quantitative blast resistance genes
can be ef
>
ciently combined with major genes.
Despite all the efforts made to improve the ef
ciency for selecting and
developing rice cultivars with durable blast resistance for the favored
upland rice ecosystem, blast resistance is continuously being lost in
breeding lines after the fourth or later generations. Speci
c studies
conducted by CIAT researchers demonstrated that for the uplands
more stable blast resistance in advanced generations would be obtained
by selecting resistant plants within F
2
families that show a higher
number of blast-resistant plants, and that show a higher number of
resistant sister lines. The reasoning behind this resistance stability is
that F
2
populations exhibiting a predominant number of resistant plants
carry a larger number of different resistance genes, including major
and minor genes (Correa-Victoria and Martinez 2009). Advanced resist-
ant lines originating in these populations have a greater probability of
carrying a larger number of different resistance genes and therefore they
would be more stable. Researchers observed during their studies that
some parents are better than others for potentially originating stable
blast resistance and recommended that before incorporating new par-
ents as sources of stable blast resistance in a breeding program, a careful
evaluation of these lines over time and under high blast pressure should
