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.
Highlights
Rice (Oryza sativa L.) is an important agricultural commodity that supplies approximately 30% of the nutritional intake of the world’s population [1]
Exact cultivar identifications could not be made from the specific fields sampled, both cultivars have no resistance to rice blast (M-202 was created to replace cultivar M9 in cooler areas where M-202 threshed harder than M-201)
The mating-type PCR assay identified a single mating type (MAT1-2) in the entire collection of M. oryzae isolates. This is the first study to investigate diversity of the M. oryzae population in California using a large sample size of isolates and including isolates collected over a decade of time
Summary
Rice (Oryza sativa L.) is an important agricultural commodity that supplies approximately 30% of the nutritional intake of the world’s population [1]. One of the most important diseases of rice globally is rice blast, caused by the ascomycete fungus Magnaporthe oryzae B.C. Couch Pyricularia oryzae Cavara), which causes up to 20% yield loss in many production zones annually and up to 80–100% yield loss when significant epidemics occur [2, 3]. M. oryzae can infect most parts of the plant, but infections of the panicle neck node or the panicle are the most damaging phases of the disease [2] with reports of the disease occurring in more than 85 countries [4]. When M. oryzae infects rice and produces symptoms of neck rot or panicle blast, normal panicle and seed development is inhibited. Additional closely related species, such as Magnaporthe grisea (anamorph Pyricularia grisea), cause similar diseases in other graminaceous species including crop plants such as wheat and millet as well as cause significant damage to various turf grass species [2, 5–7]
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