Interaction between phyllosphere yeasts, aphid honeydew and fungicide effectiveness in wheat under field conditions

Aphid honeydew can reduce fungicide effectiveness against necrotrophic pathogens of wheat under controlled conditions. In 1988 and 1989, field experiments were carried out at different locations in The Netherlands, to assess the interference of honeydew with fungicide effectiveness under field conditions, and to establish whether honeydew consumption by naturally occurring saprophytes could prevent this effect. Different combinations of chemicals were used to create differences in saprophytic population densities and aphid populations, which were expected to result in differences in the amount of honeydew and other nutrients on the leaves. In two of the experiments, Septoria nodorum was the only necrotrophic pathogen present; in the other two the only necrotroph was S. tritici . On several sampling dates, the effect of the fungicides was significantly enhanced by addition of insecticide, most often when the saprophytes were reduced by maneb/anilazine. This effect of insecticide is ascribed to prevention of interference of honeydew with fungicide activity, and not to a direct effect of insecticide, as insecticide alone had no effect on necrosis. On the basis of the fact that interference of honeydew with fungicide activity was most apparent when saprophytic population densities were reduced, it is concluded that the naturally occurring saprophytes can generally prevent the effect of honeydew. The same interactions are likely to occur between other nutrients and fungicides, and it is therefore argued that the naturally occurring saprophytes should be protected by elimination of harmful chemicals.