Brown spot, caused by Bipolaris oryzae (Breda de Haan)
Shoemaker, is the most important disease on rice world-
wide (Ou 1985). Distinctive symptoms of this disease
include light reddish-brown or lesion with a gray center
surrounded by a dark to reddish-brown margin with a bright
yellow halo (Ou 1985). The invading mycelium may enter
the leaf through the stomata or penetrate directly into the
epidermal cells (Ou 1985). Lesions appear around 18 h
after pathogen inoculation on plant (Dallagnol et al. 2009).
As lesion develops, the cells next to the invaded cell
containing fungal mycelium became brownish in color and
collapse (Tullis 1935). As reported by Dallagnol et al.
(2009), the intense lesions coalescence and the large
chlorotic halo around them completely destroy the rice leaf
blade. Toxins produced by B. oryzae quickly causes
browning and death of the parenchyma cells (Tullis
1935). The non-host selective toxins ophiobolin A and
ophiobolin B were detectable in fluids of germinated
conidia of B. oryzae as well as in infected rice leaves and
caused leaf chlorosis on non-host plant species of this
pathogen (Xiao et al. 1991). The ophiobolin A stimulated
electrolyte leakage of maize roots cells (Tipton et al. 1977),
promoted depolarization of the trans-membrane electrical
potential, and increased both leakage and efflux of