Rice blast is caused by a
fungus that can attack the
aerial parts of the rice plant (i.e., any part above ground) at any stage of growth.
Symptoms
Rice blast is characterized by the appearance of lesions on the leaves, nodes, and panicles. On the leaves, lesions are typically spindle- or diamond-shaped, grayish with a brown margin. Lesions expand rapidly and tend to coalesce, leading to complete drying of infected leaves. Resistant plants may develop minute brown specks, indicative of a hypersensitive reaction.
Besides attacking the leaves, the fungus may also attack the stem at the nodes, causing neck rot, or at the panicle, causing panicle blast. When a node is infected all parts above the infected node may die. When this occurs, yield losses may be large because few seeds in the panicle develop.
Causal Organism
Rice blast is caused by the fungus Magnaporthe grisea. Aside from rice, this fungus can also attack more than fifty other species of grasses and sedges. However, any particular strain is only able to infect a few host species. Disease losses can be very high in susceptible varieties.
Epidemiology
Conidia (asexual spores) infect the plant under conditions of high humidity. They germinate by rapid growth of a germ tube. The tip of the elongating germ tube enlarges and forms a dome-shaped infection structure called the appressorium. Enormous pressure builds within the appressorium leading to penetration of the plant cuticle by the penetration peg. Once inside, invading hyphae swell and fill the cell within twenty-four hours. Penetration of neighboring epidermal cells occurs within forty-eight hours. The colony then grows rapidly. Approximately five days after inoculation, the first visible symptoms of infection may be observed.
Control
Varietal resistance is generally agreed upon to be the most economical way to control disease. Resistance to blast in the plant is effective against attack at all stages of growth. However, the fungus is highly variable--new races can appear which attack resistant varieties. Some fungicides control neck rot or panicle blast. Even though they are expensive, they may be economical to use at this stage.