About Rice Blast

Rice blast is one of the most important diseases of rice worldwide. It caused by a fungus that can attack the aerial parts of the rice plant 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-shaped--wide in the center and pointed toward either end. Large lesions usually develop a greyish center, with a brown margin on older lesions. Under conducive conditions, lesions on the leaves of susceptible lines 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 (anamorph Pyricularia grisea [=P. oryzae]). Aside from rice, this fungus can also attack more than fifty other species of grasses and sedges. Despite this apparently broad host range, any particular strain is only able to infect a few host species. Most strains isolated from rice can only infect a limited number of cultivars.

Epidemiology

Conidia (asexual spores) infect the plant under conditions of high humidity. They germinate by rapid growth of a hyphal element called a germ tube. The tip of the elongating germ tube enlarges and forms a dome-shaped, melanized infection structure called the appressorium. Enormous turgor pressure builds within the appressorium leading to penetration of the plant cuticle by the penetration peg, which enters the underlying epidermal cell. Once inside, invading hyphae swell and fill the cell within twenty-four hours. Penetration of neighboring epidermal or parenchymal 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.

On susceptible host genotypes, lesions are gray or whitish and rapidly enlarge. Eventually a brown margin is produced and lesion growth stops. On resistant cultivars, only very small brown specks may be observed. Sporulation occurs in the gray area of the lesion, under conditions of high relative humidity. Conidia are produced on conidiophores that usually project through the stomata, though extrusion through the epidermal cell wall can also occur. Conidia are usually released at night or early morning. Conidium formation reaches a peak three to eight days after appearance of lesions and may continue for as long as twenty days. Dispersal of the inoculum is primarily by air.

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.