Synchronous Conidiation of Piricularia oryzae by the Replacement Culture

Abstract

For the purpose of studying the mechanism of conidiogenesis in Piricularia oryzae, methods were developed to separate the phase of mycelial growth from that of conidiation and also to evaluate them quantitatively. When P. oryzae was grown on media with low carbon: nitrogen ratio and high nitrogen concentration, conidiation did not take place, in spite of its vegetative growth. Conidiation occurred in a very short period of time when the above mycelia were replaced on nutritionally poor media. Cellophane membrane was overlaid on solid medium and conidia were spread uniformly. Evenly grown mycelial mat which could be easily transferred onto the post-culture medium was obtained. As the preculture medium, MSA medium with carbon: nitrogen ratio of 6.3 and nitrogen concentration of 1.5g/liter was used. The evenly grown mycelial mat was cut into small square mats of 1.44cm2 each and the small square mycelial mats were transferred onto the post-culture medium together with the cellophane membrane. The conidiation took place in the post-culture and the vegetative growth in the preculture and the conidiation in the post-culture could be observed separately and quantitatively. Conidiation did not occur at all in the preculture and the degree of conidiation which took place in the post-culture varied according to the precultural conditions. This means that it is a certain state of physiological condition in the preculture which determines the degree of conidiation in the post-culture. The anthers designated this state of physiological condition as the “latent activity of conidiation” (LAC). For the purpose of the quantitative estimation of this activity, we expressed LAC in terms of the degree of conidiation in the post-culture under a defined cultural condition. The LAC was subject to change very easily, declined rapidly and disappeared upon prolonged preculture. Only young mycelia showed to have this activity. The influences of the precultural condition on the development of the LAC and vegetative growth were generally parallel. However, the LAC was generally more sensitive to the environmental condition than the vegetative growth, especially to the temperature change. The conidia formed were uniform in size and had high rate of germination. Several strains of P. oryzae tested showed very similar behavior.