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Infected panicle (IRRI)
Narrow brown leaf spot
Cercospora janseana (Racib.) O. Const. (anamorph)
Cercospora oryzae Miyake
Sphaerulina oryzina K. Hara (teleomorph)
Linear brown lesions on leaf (IRRI).
The linear form of the lesions of narrow brown leaf spot makes the disease distinct from other leaf diseases.
The symptoms are similar to white leaf streak, which is caused by Mycovellosiella oryza and early stage of bacterial leaf streak caused by Xanthomonas oryzae pv. oryzicola.
The disease is observed on rice crops grown on soil deficient in potassium.
Temperature ranging from 25-28° C is favorable for the optimum growth of the disease. Susceptibility of the variety to the fungus and the growth stage of the rice crop are other factors that affect the development of the disease. Although rice plants are susceptible to the fungus at all stages of growth, they are more susceptible from panicle emergence to maturity, thus, becoming more severe as rice approaches maturity.
The narrow brown leaf spot is caused by a fungus, which is commonly found in its sclerotial state.
Its conidiophores appear singly or in fascicles of 1-7 and rarely up to 15. They are pale brown to medium brown. Conidiophores are unbranched, geniculate, and multiseptate. They measure 80-140 x 4-9 µm. Geniculations may be absent. Conidiophores often emerge from the host stomata.
The conidia are hyaline, cylindrical to obclavate, and straight to mildly curved. They have 3-10 septa. The conidia measure 15-60 x 4 µm. A conidial scar is present.
The teleomorph has perithecia, globose or subglobose, black, with a papiliform mouth (minute, rounded, blunt projection through which spores escape), immersed in the epidermal tissues of the host plant and which measure 60-100 µm in diameter. Asci cylindrical or club-shaped, round at the top, stipitate (having a stipe or stem) with ascospores biseriate (in two series) spindle-shaped, straight or slightly curved, 3-septate, hyaline, 20-23 x 4-5 µm.
Aside from the rice plant, the fungus can survive on Panicum maximum Jacq. (guinea grass), P. repens L. (torpedo grass), and Pennisetum purpureum K. Schum. (elephant grass).
The fungus penetrates the host tissue through the stomata. It becomes stable in the parenchyma where it stays beneath the stomata and spreads longitudinally in the epidermal cells.
The fungal disease is important during the tillering and stem elongation stages of the rice crop.
Premature death of leaves and leaf sheaths, premature ripening of kernels and lodging of plants are observed during severe infection. It decreases the market value of the grains because it causes grain discoloration and chalkiness, and reduces the milling recovery.
A 40% loss in yield was reported in Suriname during the 1953 and 1954. The disease has been reported in several countries in Asia, Africa, Australia, and Papua Guinea.
Cultural practices, such as the use of potassium and phosphorus fertilizers, and planting of early maturing cultivars early in the growing season, are recommended to manage the narrow brown leaf spot.
The use of resistant varieties is the most effective approach to manage the disease. However, the resistant varieties and lines are only grown in United States and India.
Spraying of fungicides such benomyl, propicanazole, carbendazim, propiconazole, and iprodione, when the disease is observed in the field is effective.
Suparyono, JLA Catindig, NP Castilla, and FA Elazequi