Agriculture Reference
In-Depth Information
these diseases are related to the great number of pathogenic races in the
blast pathogen and its capacity to develop new pathotypes, as well as to
the quantitative nature of the resistance to rhizoctonia and RHBV. Most
recent approaches include the use of marker-aided selection in disease
resistance breeding although just a few national programs have been
able to implement and maintain the necessary infrastructure and
nan-
cial resources on a permanent basis.
A. Breeding for Rice Host Plant Resistance
1. Rice Blast. Rice blast disease (caused by P. oryzae ) is the most
important rice problem in Latin America. Rice is normally grown under
irrigation or
flooded conditions characterized by low blast pressure or
under rainfed upland conditions characterized by high blast pressure
(Bonman and Mackill 1988; Bonman et al. 1991). The development of
durable blast resistance for these two environments should be possible
if breeding programs are based on a complete understanding of pathogen
diversity in the target area (Correa-Victoria et al. 2004). The great patho-
genic diversity observed in P. oryzae is considered the main cause of
resistance loss in newly released cultivars. One strategy to improve the
durability of blast resistance is to pyramid resistance genes (Jacobs and
Parlevliet 1993; Parlevliet 1993). Rice researchers at CIAT in Colombia
have conducted extensive studies on the genetic structure of blast
pathogen populations in Colombia and other countries in Latin America
focused on determining the composition, distribution, and frequency
of the avirulence genes that underline race variation (Correa-Victoria
et al. 2008). The blast pathogen population has been characterized in
genetic lineages using RFLP- and PCR-based markers (Correa-Victoria
et al. 2000, 2004). Based on this information, blast resistance gene
combinations have been identi
ed and incorporated into commercial
rice cultivars using genetic markers and continuously evaluating and
selecting breeding lines at
sites characterized by high disease
pressure and pathogen diversity (Correa-Victoria et al. 2008). High
disease pressure and pathogen diversity (hot spots) are maintained in
the breeding plots during the entire crop cycle using spreader rows
composed of a mixture of commercial rice cultivars susceptible to
different genetic lineages of the pathogen.
Rice blast differentials or NILs (near isogenic lines) developed at
IRRI with known blast resistance genes were used to study avirulence
gene compositions and their frequency in the blast pathogen and to
identify relevant blast resistance genes. Using these differentials as well
as
hot spot
field studies, the rice researchers at CIAT suggested a combination
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