*2.6.1 Identification symptoms*

Narrow brown leaf spot symptoms and disease cycle Cercospora janseana causes narrow brown leaf spot of rice [74]. Symptoms of narrow brown leaf spot (NBLS) disease include long cylindrical dark brown spots with dark margins and grayish centers with or without chlorosis. Lesions range from 1 to 10 mm x 1–1.5 mm on leaves and 15–45 x 1–2 mm on mid-ribs and leaf sheaths [74]. Morphology of symptoms varies with the susceptibility of the cultivar. On resistant cultivars, symptoms are long, narrow lesions that sometimes do not develop fully. In susceptible cultivars, spots are broad and necrotic. Initially, dark spots develop on the leaf lamina and later on the leaf mid-vein, leaf sheath, panicle, seed coat and glumes. Symptoms appear late in the season on all leaves regardless of age. NBLS causes a premature ripening of the grains, reduces yield quantity, and grain milling quality [75]. The disease cycle begins when *C. janseana* enters the plant tissues through

stomata, establishes beneath the stomata in the parenchyma cells, and spreads longitudinally in intercellular. Upon development, conidiophores emerge through the stomata. Preliminary studies have shown that 30 or more days are required to develop symptoms after inoculation [76]. This long latent period may be the probable reason of late appearance of symptoms during the season even though infection occurs at early plant developmental stages. The initial source of inoculum appears to be from *C. janseana* that has survived on residues of previous rice crops, infected seeds, and seasonal weeds [77].

The disease is emerging as a major problem in wet periods. The disease causes severe necrosis of the leaf tissue and drying and death of the leaves. Initially, short, linear, brown lesions appears on leaf blades later extends to leaf sheath, pedicels and glumes. The lesions are 2–10 mm long and 1 mm wide in size (**Figures 19** and **20**). The size of the lesions depends on varietal susceptibility. The resistant cultivars have narrower and darker lesions whereas; the susceptible cultivars have wider and light brown. The lesions coalesce each other giving dried and burning appearance to field [78].
