Pyricularia grisea (Magnaporthe grisea) - Current interest. Rice and other infection gramináceas (wheat, rye, barley, millet, and grass): Molecular diagnosis.

 

Pyricularia grisea (syn. Magnaporthe grisea, Dactylaria grisea) is a fungus that affects the rice plant (Oryza sativa), and other grasses such as rye, barley, millet, or corn. In addition, it can affect the grass plants (Lolium multiflorum and Lolium perenne). In the rice plant (Oryza sativa) causes the disease known as rice blast (rice blast).

Until 2002 this fungus has been called Magnaporthe grisea, but since this denomination would correspond to the sexual form (telomorfa), not found in nature, would be more appropriate to call Pyricularia grisea, which is the name assigned to your asexual form (anamorph) which it is found in nature. Some authors have proposed the name Pyricularia oryzae, but this species is considered to correspond to that infects other types of different grasses rice.

It is thought that its origin is in Southeast Asia, but is currently distributed worldwide in temperate and tropical regions, affecting important one of most food crops importance, as is the way rice.

fungal features

It is a fungus of the Ascomycetes group, because it has been shown that it can reproduce sexually (with ascosporas) and asexually (with conidias). This fungus is characterized by generating septate mycelia hyaline (colorless), multiple conidiophores in which conidia (spores) appear. Conidia are hyaline or slightly greenish, spindle-shaped or racquet, with a baseline appendix implantation (basal hilum) by which joins the conidiophore and a pointed end. They have one or two partitions (septa) thereby presenting 2 to 3 cells (cells).

To infect plants, the fungus generates penetration bodies called "aprensorios" with that infects aerial tissues of these plants, but not infect the roots. To infect aerial tissues causes the manifestations of the disease. After infection, sporulated in plant tissues, and generates asexual propagation elements, conidia (conidiospores). Conidia are spread elements, which will scatter and continue the cycle of infection between plants. The cycle of infection-conidiogenesis is completed in a week on favorable terms, and from an injury can be released thousands of conidia in one night, conidia can be generated for about 20 days so it spreads quickly in the environment. The cycle is maintained during hibernation.

Manifestations of the disease in infected plants

From a conidia, the plant infection may occur at several locations, the leaves, the leaf collar, the base sheet, the space stem between nodes, the nodes of the stem, the base of the bouquet, or posy itself.

Initial symptoms are whitish or gray-green lesions or spots with dark edges, while older lesions are elliptical or fusiform, whitish or greyish with necrotic edges. The lesions may enlarge and merge with each other to end the full sheet, or break the stem to develop an injury rot.

These infections affect seed formation, and therefore constitute significant damage to crops.

Dispersion in nature

Conidia, source of infection, disperse through the air or through sprinkler irrigation. The fungus may persist in the environment and hibernate or plant residues (residues).

Rice blast (rice blast) is a major problem in temperate regions and can be found in irrigated areas. Favorable for release, conditions include extended periods of high relative humidity and temperatures of 25 to 28 ° C, humidifying the leaves favoring the fungus conidia to multiply and generate new infections.

Control

This fungus is favorable conditions for multiplication and extension when ambient temperature is suitable, high humidity, sufficient supply of nitrogen, and the plant is weakened.

Plants are weakened when they have a higher contribution than necessary nitrogen, as with the overuse of nitrogenous fertilizers, because although the plant grow, is weaker. Drought-induced stress in plants that increase their sensitivity. Drought periods also favor infection because more aerate the soil, making the ammonium nitrate, and thus higher nitrogen content.

This fungus has developed resistance to chemical treatments through genetic mutations. For this reason, it is necessary to avoid excessive use of a single chemical method for controlling infections, because when one method is used it facilitates the development of resistance. However, the use of fungicides is not economical, and only considered an alternative. If used, they should alternate fungicides with different modes of action to prevent the development of resistance to them. By required free moisture, irrigation be controlled.

To avoid generating conidias during hibernation should be properly disposed of all previous stubbles plantations, since in these, during the hibernations conidias that help keep infection occur between seasons.

Where possible it is recommended crop rotation, to introduce other crops that are not susceptible to infection by this fungus, thus perpetuating the infection is decreased in an area.

Today they have developed varieties of plants resistant to infection by Pyricularia grisea rice.

Studies have been conducted biological control by introducing other competitive fungus, Trichoderma spp.

  • Samples for the detection of Pyricularia spp. :
    • Seeds, leaves or stems of affected plants with lesions or necrotic spot.
  • Containers to submit samples:
    • Wrappings of paper, no plastic bags to prevent excessive moisture that encourages the development of other fungi during storage.
  • Storage and transport of samples:
    • Refrigerated or at room temperature.
  • Tests:
    • Cultures and microscopic morphological identification.
    • PCR detection with molecular identification by sequencing.
  • Cost of the test:

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