THE RELATIONSHIP BETWEEN ANTHRACNOSE SEVERITY AND POPULATIONS OF BACTERIA ON THE PHYLLOPLANE OF THE TROPICAL PASTURE LEGUME STYLOSANTHES SCABRA

Populations of gram-negative bacteria, spore-forming bacilli, pseudomo nads, and total bacteria were monitored on leaves of the tropical past ure legume Stylosanthes scabra cv. Fitzroy showing different severity levels of anthracnose caused by Colletotrichum gloeosporioides. Leaves were either disease-free or were grouped on the basis of percentage o f leaf area visibly diseased into low-, moderate-, and high-severity c lasses. Data on bacterial populations, collected on nine different occ asions during a growing season, were analyzed with the population esti mates as dependent variables and anthracnose severity class as the tre atment. Meteorological variables were summarized using the maximum lik elihood factor analysis and used as covariates. Of the three bacterial groups, only the spore-forming bacilli showed a statistically signifi cant negative relationship between their population density and anthra cnose severity. Total bacterial populations were assessed independentl y and also tended to rise with increasing disease severity. This sugge sted a possible antagonism of spore forming bacilli toward the anthrac nose pathogen. A simple in vitro method for differentiating modes of a ntagonism was developed. Twenty-four bacterial isolates collected from the leaf surface were tested using this method; of these, 19 showed a degree of antagonism. The method proved incapable of differentiating between modes of antagonism. Further work is necessary to establish th e nature and effectiveness of this putative antagonism. (C) 1995 Acade mic Press, Inc.