Cellulose-decomposing ability of some cereal foot-rot fungi in relation to their saprophytic survival

Abstract

The cellulolysis adequacy index has been employed as a measure of the extent to which rate of cellulose decomposition by a fungus is adequate to supply its needs for saprophytic survival in buried wheat straw tissue in the absence of supplementary nitrogen from the surrounding soil. The index has been calculated for five fungi causing cereal foot- and root-rot diseases by determining: (1) percent-, age loss in dry weight of inoculated filter-paper after incubation for 7 weeks at 22·5°C; (2) linear growth rate of the fungus expressed in mm./24 hr. at 22·5° C., and then dividing dry-weight loss by mycelial growth rate. An index of less than unity indicates a need for supplementary nitrogen if maximum longevity of saprophytic survival is to be obtained. An index of more than unity indicates that a fungus will achieve maximum longevity of survival without supplementary nitrogen. Amongst the five fungal pathogens tested, Helminthosporium sativum was characterized by the highest index (3·5), and its survival was shortened by supplementary nitrogen. This effect has been explained by postulating that nitrogen promotes a rate of mycelial development of H. sativum within the straw tissue that is in excess of the minimum rate required for survival of the fungal colony, so that substrate reserves become exhausted sooner than in the absence of nitrogen. This hypothesis is supported by the results of an experiment in which nitrogen was supplied to the straw tissue in two alternative ways: (1) before colonization of the straw by H. sativum in the pure-culture flasks; (2) after burial of the colonized straw in the soil.