Genetic Structure of the Rice Blast Pathogen (Magnaporthe oryzae) over a Decade in North Central California Rice Fields

Abstract

Rice blast, caused by the ascomycete Magnaporthe oryzae, is one of the most destructive rice diseases worldwide. Even though the disease has been present in California since 1996, there is no data for the pathogen population biology in the state. Using amplified fragment length polymorphisms and mating-type markers, the M. oryzae population diversity was investigated using isolates collected when the disease was first established in California and isolates collected a decade later. While in the 1990 samples, a single multilocus genotype (MLG) was identified (MLG1), over a decade later, we found 14 additional MLGs in the 2000 isolates. Some of these MLGs were found to infect the only rice blast-resistant cultivar (M-208) available for commercial production in California. The same samples also had a significant decrease of MLG1. MLG1 was found infecting the resistant rice cultivar M-208 on one occasion whereas MLG7 was the most common genotype infecting the M-208. MLG7 was identified in the 2000 samples, and it was not present in the M. oryzae population a decade earlier. Our results demonstrate a significant increase in genotypic diversity over time with no evidence of sexual reproduction and suggest a recent introduction of new virulent race(s) of the pathogen. In addition, our data could provide information regarding the durability of the Pi-z resistance gene of the M-208. This information will be critical to plant breeders in developing strategies for deployment of other rice blast resistance genes/cultivars in the future.

Keywords: AFLP; California; Pi-z gene; Population structure; Pyricularia oryzae; Rice blast disease.

Fig. 1

Distribution of sampled Magnaporthe oryzae isolates collected from 28 Oryza sativa fields used in this study. The map of California shows the four counties sampled which are expanded to show the spatial distribution among the sampled locations. Note that the San Joaquin county is not in proportion to the scale bar in the north by south direction and the distance between the southernmost sampled locations in the Sutter county to the single sampled location in the San Joaquin county is approximately 100 km. The cultivars sampled in 1997, 1998, and 2007 were all susceptible and were likely M-201 and M-202 (exact cultivar identifications could not be made). In 2010, all of the fields sampled were from the resistant cultivar M-208 except for the single field in the San Joaquin county which was sampled from the susceptible cultivar M-104

Fig. 2

Dendrogram of multilocus genotypes (MLGs) of M. oryzae (MLGs 1–15) and Pyricularia grisea (MLG16) based on UPGMA cluster analysis using PAUP* with genetic distances calculated from 37 AFLP loci. Bootstrap values are based on 10,000 replicates with values over 50% shown (see Table 1 for information regarding the distribution of the MLG from the sampled Oryza sativa fields)