CONSUMPTION OF APHID HONEYDEW, A WHEAT YIELD REDUCTION FACTOR, BY PHYLLOSPHERE YEASTS UNDER FIELD CONDITIONS

Aphid honeydew on wheat leaves can affect host physiology directly and can stimulate infection by necrotrophic pathogens. The role of naturally occurring saprophytes on wheat flag leaves in removing aphid honeydew was examined in five field experiments at different locations in the Netherlands in 1987 and 1988. Aphid populations, saprophyte populations and aphid honeydew were measured weekly. Diseases were monitored at 1-4 week intervals. In the control treatment, sprayed with water, the naturally occurring saprophytes consumed the honeydew almost completely, except in one experiment in which the aphid population increased very rapidly. In the treatments in which the saprophytes were reduced by the broad-spectrum fungicide maneb in 1987 and with maneb alternated with anilazin in 1988, honeydew accumulated. The observed honeydew consumption by the naturally occurring saprophytic population is ascribed mainly to pink and white yeasts (Sporobolomyces spp. and Cryptococcus spp., respectively). Additional treatments showed, that in the presence of the naturally occurring saprophytes the yield loss per aphid-infestation-day was lower than when the saprophytes were inhibited by fungicides, showing that yeasts can reduce the detrimental effect of aphid honeydew in wheat.