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Magnaporthe grisea, also known as rice blast fungus, rice rotten neck, rice seedling blight, blast of rice, oval leaf spot of graminea, pitting disease, ryegrass blast, and johnson spot, [1] is a plant-pathogenic fungus that causes an important disease affecting rice. It can also infect a number of other agriculturally important cereals including wheat, rye, barley, and pearl millet causing diseases called blast disease or blight disease. M. grisea causes economically significant crop losses annually, each year it is estimated to destroy enough rice to feed more than 60 million people. The fungus is known to occur in 85 countries worldwide. It is sometimes thought of as a model organism in the study of phytopathogenic fungi.
[edit] Biology and pathology M. grisea is an ascomycete fungus. It is an extremely effective plant pathogen as it can reproduce both sexually and asexually to produce specialized infectious structures known as appressoria that infect aerial tissues and hyphae that can infect root tissues. In 2004 it was shown that in addition to infecting plants through the leaf, M. grisea can also infect the plant roots. The mode of root infection is the same as most root infecting fungi: it grows long hyphae that form an infection pad to gain entry to the root's interior. Once embedded in the root the fungus can produce resting structures. The blast fungus can also invade the plant's vascular system, growing inside the xylem and phloem and blocking the transport of nutrients and water from the roots, and produce lesions on aerial plant parts. Infection of root and vascular tissues has the potential to kill the plant by cutting off the supply of water and nutrients to the root. In 2005 the complete genome of M. grisea was sequenced. The organism is predicted to have over 11,000 genes. It is expected that the genome will reveal the mechanisms of fungal pathogen-plant interaction, in both aerial and root infection. [edit] DistributionRice blast was probably first recorded as rice fever disease in China in 1637. It was later described as imochi-byo in Japan in 1704, and as brusone in Italy in 1828. The fungus is currently reported to be present in at least 85 countries. In 1996 rice blast was found in rice in California, and has since been found in grasses on golf courses in the midwestern United States. Strains of the fungus can infect domesticated grasses such as barley, wheat, rye, pearl millet, and turf grasses in addition to rice. Thus, even when crops are burned to destroy fungal infection, grass weeds can act as a disease reservoir. The disease may be called different names depending on the crop infected: in rice it is called rice blast, in wheat it is called wheat blast, in rye it is called rye blast and so on. [edit] ControlThe fungus has been able to develop resistance to both chemical treatments and genetic resistance developed by plant breeders in some types of rice. It is thought that the fungus can achieve this by genetic change through mutation. Researchers hope that by having the full genome sequence of the fungus the development of effective control methods will be possible. For example, a mutant M. grisea unable to produce a penetration peg has been shown to lack the ability to infect rice. [edit] Biological weaponM. grisea spores were prepared as an anti-plant biological weapon independently by the United States and the USSR during World War II.[citation needed] The United States is known to have researched the agent for use against Japan’s rice crop during World War II.[2] The United States Chemical Warfare Service worked with Canadian and British scientists to weaponize rice blast but as World War II ended in Europe it was not ready for use in battle.[3] Because the spores did not fare well in warm weather, the agent was not seen as having much potential as a biological weapon.[2] U.S. biological warfare research showed renewed interest in M. grisea as a weapon during the 1960s.[4] From 1962–1969 M. grisea spores were produced by Charles Pfizer and Company and shipped to Fort Detrick. The entire U.S. stockpile of M. grisea was destroyed during the United States' biological weapons demilitarization process, which was completed in 1973.[5] There are also concerns that M. grisea may be used as a biological weapon by a terrorist organization.[6] [edit] Notes
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