Origins of host-specific populations of the blast pathogen Magnaporthe oryzae in crop domestication with subsequent expansion of pandemic clones on rice and weeds of rice.

Rice, as a widely and intensively cultivated crop, should be a target for parasite host shifts and a source for shifts to co-occurring weeds. Magnaporthe oryzae, of the M. grisea species complex, is the most important fungal pathogen of rice, with a high degree of host specificity. On the basis of 10 loci from six of its seven linkage groups, 37 multilocus haplotypes among 497 isolates of M. oryzae from rice and other grasses were identified. Phylogenetic relationships among isolates from rice (Oryza sativa), millet (Setaria spp.), cutgrass (Leersia hexandra), and torpedo grass (Panicum repens) were predominantly tree like, consistent with a lack of recombination, but from other hosts were reticulate, consistent with recombination. The single origin of rice-infecting M. oryzae followed a host shift from a Setaria millet and was closely followed by additional shifts to weeds of rice, cutgrass, and torpedo grass. Two independent estimators of divergence time indicate that these host shifts predate the Green Revolution and could be associated with rice domestication. The rice-infecting lineage is characterized by high copy number of the transposable element MGR586 (Pot3) and, except in two haplotypes, by a loss of AVR-Co39. Both mating types have been retained in ancestral, well-distributed rice-infecting haplotypes 10 (mainly temperate) and 14 (mainly tropical), but only one mating type was recovered from several derived, geographically restricted haplotypes. There is evidence of a common origin of both ACE1 virulence genotypes in haplotype 14. Host-haplotype association is evidenced by low pathogenicity on hosts associated with other haplotypes.

Applying Rice Seed-Associated Antagonistic Bacteria to Manage Rice Sheath Blight in Developing Countries.

One promising area of disease management for resource-poor farmers that emerged in recent years in developing countries is the potential of biological control. Biological control agents (BCAs) were found to be ubiquitous in the rice ecosystem. Seed bacterization with BCAs appeared to promote plant growth. BCAs showed efficacy on sheath blight (Rhizoctonia solani AG 1) but produced inconsistent results over time in the field using it alone. The control efficiency ranged from 50 to over 90%, with a high variance. To improve the efficacy, a half-dose of a commonly used fungicide, like Jingangmycin in China and Validamycin in Vietnam, was introduced to mix with BCAs and was found to be effective, and it reduced the variance of the field performance tests. To scale up the BCA technology for resource-poor farmers, a participatory approach, engaging the farmers to evaluate the product, was initiated in China and Vietnam. The BCA strain that is indigenous at a locality is mass produced at the research institution based on the total area required for application, as relayed by the farmers to extension workers. The demand by farmers would serve as the basis for the amount to be produced and for delivery to the rice farmers who were participating in the trials and, later, to those farmers who ordered the product. This process alleviates BCA storage and shelf-life problems. Data from the field performance trials also were used by the researchers to apply for registration for commercial use of BCAs. Scaling up to extend the BCA technology to more rice farmers as an integral part of their pest management scheme, in particular, and crop management practices, in general, is foreseen in the near future.