Magnaporthe grisea

Scientific Name Magnaporthe grisea (Hebert) Barr [teleom.], Pyricularia grisea (Cooke) Saccardo [anam.]
Synonyms Magnaporthe oryzae B.C. Couch [syn.], Pyricularia oryzae Cavara [syn.]
Common Names English: Rice blast disease; leaf blast, neck blast; German: Reisbrand; French: Pyriculariose du riz; Spanish: Añublo del arroz, piricularia
Description The rice blast fungus M. grisea,( conidial stage : Pyricularia grisea), is a member of the Ascomycetes. The pathogen attacks leaves, stem nodes, all parts of the panicle and grains.

Biology

Damage

On leaves, elliptical or spindle-shaped lesions (1 - 3 mm) appear with pointed ends and gray-whitish centres, dark-green to reddish-brown margins, sometimes with a yellow halo. Under humid conditions, abundant conidia are produced on both sides of the leaf. Leaf sheaths dry up and whole plants may be killed. Severely infected fields have a scorched appearance. On leaf sheaths, rotting may result in premature leaf fall. Nodal infection causes the culm to break; on lower nodes, rotting causes white heads. On panicles, all parts of the rachis or rachilla may be infected. Most often, the basal node of the panicle is infected, resulting in neck rot. Early infections result in white heads or partially filled kernels. Leaf blast is most severe between seedling stages and maximum tillering. Leaves become more resistant with age, and leaves on older plants acquire this property faster. At heading, disease again increases on nodes and panicles.

Lifecycle

Pyriform, often biseptate conidia (19 - 27 x 8 - 10 µm) germinate in the presence of free water and high relative humidity is required for infection. At optimal temperatures of 24 - 28 °C, plant tissue will be penetrated after 6 - 8 hours. The fungus enters the plant either through the cuticle or via stomata. Latent periods require 4 - 6 days at optimal temperatures. At RH > 93 %, conidia are produced within 6 days after germination. Blast is a polycyclic disease, with 7 - 8 cycles per year in temperate environments, compared with 10 - 15 cycles per season and 2 - 3 seasons per year in tropical environments.

Cause

Under temperate conditions, the fungus survives as conidia and mycelia in infected straw, within the glumes, endosperm and embryo of infected seeds and on alternative hosts. In tropical and subtropical regions, where rice is cropped twice a year, crop seasons often overlap because of difficulties in synchronized water delivery favoring disease transmission from one crop to the next.

Occurrence

Additional Crop Information

M. grisea has a wide host range and is known to infect about 40 species of Gramineae, including wheat, maize, millets (Eleusine coracana, Panicum repens) and weeds like Echinochloa colonum, Eleusine indica and Cyperus rotundus. Pathogenicity of isolates is largely restricted to its host species of origin; no gene flow among rice and non-rice isolates has been detected.

Control

Integrated Crop Management

Rice cultivars resistant to leaf and panicle blast are widely used for disease control. Resistance, however, may be overcome because of the genetic variability of the pathogen. Seed treatments with carpropamid, probenazole, tricyclazole and pyroquilon reduce blast development.
Excessive fertilizer use should be avoided; nitrogen should be applied in small amounts. Optimum water management reduces the probability of infection.

Chemical Control

In regions where blast is regularly an important rice disease, several effective fungicidal modes of action are mostly available for the control. Non-cross resistant modes of action should be used alternatively in a rational way in order to prevent resistance development by the blast pathogen. Compounds with systemic distribution in the young plant are often applied via the seedling or seed box or by water surface application of granules and may protect the young plant for several weeks. Such long lasting effects are available from resistance inducers such as probenazole, from MBI-D fungicides such as carpropamid and from QoI fungicides such as metominostrobin.

If in later growth stages spray applications have to be made in order to control leaf or panicle blast there is a broad choice of blasticides with different modes of action available in those countries where rice blast is regularly one of most important diseases (e.g. Japan and Korea). Examples of blasticide classes in these countries are:

- Phosphorothiolates such as edifenphos
- QoIs (“strobilurins”) e.g. metominostrobin
- Melaninbiosynthesis-Inhibitors (MBI) from two non-cross resistant classes
- MBI-D (e.g. carpropamid)
- MBI-R (e.g. pyroquilon, tricyclazole)
- kasugamicin
- blasticidin

In countries, where rice blast is occurring less regularly and where therefore specific blasticides are not registered sometimes DMI fungicides such as tebuconazole have shown some effect. There are great differences in the systemic properties, the risk of resistance, the duration of control and the spectrum of activity of the fungicide classes mentioned above. Therefore, local advisory systems should be contacted in any case for more detailed information.

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