Alaphilippe, Aude; Derridj, Sylvie; Gessler, Cesare

Research on biocontrol agents (BCAs) such as yeasts, bacteria and fungi against plant
pathogens is developing fast, but the effects of the introduction of these microorganisms on non target
organisms and on the plant host physiology are poorly known. We study the effects of the spray of an
epiphytic yeast (Y16), a potential biocontrol agent against the powdery mildew, on scab, the codling
moth and the phylloplane chemical composition. A change in the codling moth egg quantity laid on
the apple tree treated with the yeast suspension was strong. In the second season of experiment, during
the second flight period, we observed an increase of the egg laid on the treated tree, especially on tree
site close to the apple fruits. These results are however, in contradiction with the preliminary
experiment. A third season will be conducted this summer in order to clarify the effect of the yeast on
the quantity of egg laid. Several yeast and bacterium isolates were tested for their ability to control
apple powdery mildew in the frame of a screening program. The yeast Y16 showed a good potential as
BCA of this disease. Three weeks from the inoculation, disease severity (percentage of leaf area
covered by symptoms) in the water control was 37.4%. The severity of the yeast treated plants was
reduced to 7.6%. We are currently analysing the effect of the yeast suspension on the germination of
V. inaequalis conidia. Primary metabolites (sugars) of the phylloplane stimulate the egg laying of the
codling moth, C. pomonella. Modifications of ratios and quantities of these metabolites can change the
number of eggs laid on the plants and could modify the development of pathogens. We analyse the
soluble carbohydrate, sugar alcohol and free amino-acid composition of leaf surface water washing on
the different sites by gas chromatography. On the first set of analysis, we observed on the upper side
of the distal leaf surfaces one day after spraying that the treatment increased the quantity of saccharose
(from 180 µg/cm² on the untreated trees up to 243 µg/cm²) and sorbitol (from 89 up to 135 µg/cm²)
compared to the non treated trees. Analysis of free amino acids did not show any significant difference
on the distal leaves on both leaf sides. We are currently doing a second set of analysis using liquid
chromatography.