Antagonistic interactions between Colletotrichum graminicola and yeasts on maize leaves

During the last two decades it has been demonstrated many times that populations of leaf surface micro-organisms are capable of reducing the levels of disease caused by various fungal pathogens. The various mechanisms by which this may be achieved have been studied and reported. In many cases, the disease depressing interactions are effective in depressing germination of spores of pathogens, in causing lysis of germtubes or in hyperparasitising the pathogen. This work is reviewed. The objective of the present study was to determine how the leaf-colonising yeasts, Cryptococcus laurentii var. flavescens and Sporobolomyces roseus reduced the infection of maize by the anthracnose fungus, Colletotrichum graminicola. Initial experiments did not support the hypothesis that the yeasts suppressed germination, nor did they demonstrate an effect of the yeasts on appressorium production. Experiments on attached leaves, however, provided data which showed that maize leaves inoculated with equal inocula in the presence and absence of the yeasts produced many more infection courts than with yeasts. This suggested, since there were equal numbers of appressorium on each set of leaves, that appressorium behaviour was different in the presence of yeasts from in their absence. In further experiments, using different techniques, it was found that in the presence of the yeasts, the proportion of appressoria which germinated was reduced. It is suggested that the effect of the yeasts on appressorium behaviour is similar to fungistasis. It was also found that in the presence of yeasts fewer acervuli developed, and individual acervuli produced fewer setae and conidia. The manner in which the cohabiting yeasts influence the behaviour of C. graminicola during its post-penetration phase was not investigated here, but offers another challenging research topic. The implications of the present findings are discussed in relation to the natural and manipulated control of plant pathogens by biological agents