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, Johnson spot, neck blast, wheat blast Complex members isolated from Digitaria have been more narrowly defined as M. grisea. The remaining members of the complex isolated from rice and a variety of other hosts have been renamed Magnaporthe oryzae, within the same M. grisea complex. Initial symptoms are white to gray-green lesions or spots with darker borders produced on all parts of the shoot, while older lesions are elliptical or spindle-shaped and whitish to gray with necrotic borders. Lesions may enlarge and coalesce to kill the entire leaf. Symptoms are observed on all above-ground parts of the plant. Lesions can be seen on the leaf collar, culm, culm nodes, and panicle neck node. Internodal infection of the culm occurs in a banded pattern. Nodal infection causes the culm to break at the infected node (rotten neck). It also affects reproduction by causing the host to produce fewer seeds. This is caused by the disease preventing maturation of the actual grain. The pathogen is able to move between the plant cells using its invasive hyphae to enter through plasmodesmata. M. grisea then sporulates from the diseased rice tissue to be dispersed as conidiospores. After overwintering in sources such as rice straw and stubble, the cycle repeats.

A single cycle can be completed in about a week under favorable conditions where one lesion can generate up to thousands of spores in a single night. Disease lesions, however, can appear in three to four days after infection. With the ability to continue to produce the spores for over 20 days, rice blast lesions can be devastating to susceptible rice crops.

Infection of rice induces phosphorylation of the light-harvesting complex II protein .

Environment

Rice blast is a significant problem in temperate regions and can be found in areas such as irrigated lowland and upland. Conditions conducive for rice blast include long periods of free moisture and/or high humidity, because leaf wetness is required for infection.

In February 2016 a devastating wheat epidemic struck Bangladesh.

Management

thumb|250px|[[J. Sendra rice]]

This fungus faces both fungicides and genetic resistance in some types of rice developed by plant breeders. It is able to establish both resistance to those chemical treatments and virulence to crop resistance by genetic change through mutation. In order to most effectively control infection by M. grisea, an integrated management program should be implemented to avoid overuse of a single control method and fight against genetic resistance. For example, eliminating crop residue could reduce the occurrence of overwintering and discourage inoculation in subsequent seasons. Another strategy would be to plant resistant rice varieties that are not as susceptible to infection by M. grisea. Papajani et al. 2015 finds the essential oils of both Origanum vulgare and Rosmarinus officinalis to be effective in vitro, and provides treatment thresholds.

The wheat blast strain can be diagnosed by sequencing.

Importance

Rice blast is the most important disease concerning rice crops in the world. Since rice is an important food source for much of the world, its effects have a broad range. It has been found in over 85 countries across the world and reached the United States in 1996. Every year the amount of crops lost to rice blast could feed 60 million people. Although there are some resistant strains of rice, the disease persists wherever rice is grown. The disease has never been eradicated from a region.

Strains

Three strains, albino (defined by a mutation at the ALB1 locus), buff (BUF1), and rosy (RSY1), have been extensively studied because they are nonpathogenic. This has been found to be due to their inability to synthesize melanin, which is a virulence factor in some fungi. The strain (M. o. pv. triticum) causes the wheat blast disease. Export of Magnaporthe from the US is restricted.

Genetics

Whole-genome sequences were just becoming possible, and being made available, in 2003.

M. grisea mutants lacking the AGT1 gene were observed to be nonpathogenic through their inability to penetrate host surface membranes. This indicates the possibility of impaired lipid utilization in M. grisea appressoria in the absence of the AGT1 gene.

Biochemistry of host-pathogen interactions

A 2010 review reported clones for quantitative disease resistance in plants. The rice plant responds to the blast pathogen by releasing jasmonic acid, which cascades into the activation of further downstream metabolic pathways which produce the defense response. is a rice protein that acts as an immunoreceptor. Some rice cultivars carry resistance alleles of the OsSWEET13 gene, which produces the molecular target of the X. oryzae pv. oryzae effector PthXo2.

See also

  • Corn grey leaf spot, a similar disease in maize/corn
  • Gray leaf spot, a similar disease in other grasses

References

Further reading

  • California EPA. Rice Crop Infestation in Three Counties Leads To Emergency Burn Agreement , February 11, 1998
  • CIMMYT. What is wheat blast?, 2019.
  • Kadlec, RP. Biological Weapons for Waging Economic Warfare, Air & Space Power Chronicles
  • NSF. Microbial Genome Helps Blast Devastating Rice Disease , April 21, 2005
  • United States Congress. Testimony of Dr. Kenneth Alibek, 1999
  • GROMO - Genomic Resources of Magnaporthe oryzae
  • Magnaporthe grisea Genome
  • The official Website of the International Rice Blast Genome Consortium
  • Index Fungorum
  • Magnaporthe grisea at MetaPathogen: stages, tissues, mating types, strains, references