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, pseudomonads, and total bacteria were monitored on leaves of the tropical pasture 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 of leaf area visibly diseased into low-, moderate-, and high-severity classes. Data on bacterial populations, collected on nine different occasions during a growing season, were analyzed with the population estimates as dependent variables and anthracnose severity class as the treatment. Meteorological variables were summarized using the maximum likelihood factor analysis and used as covariates. Of the three bacterial groups, only the spore-forming bacilli showed a statistically significant negative relationship between their population density and anthracnose severity. Total bacterial populations were assessed independently and also tended to rise with increasing disease severity. This suggested a possible antagonism of spore forming bacilli toward the anthracnose pathogen. A simple in vitro method for differentiating modes of antagonism 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 the nature and effectiveness of this putative antagonism.