S15 - Session P2 - Light deprivation as a herbicide?

Authors: Antonia Engel, Michael Blanke *

In a changing climate, the objective of the present work was to investigate mitigation strategies to reduce herbicide input with the expected glyphosate ban in 2023/4. Naturally reduced weed vegetation has been observed in the summer under fully-grown cv. ´Elstar` apple trees. To investigate the potential causes, the vegetation under cv. 'Elstar' apple tree was compared with that under fully-grown cv. ´Golden Delicious`, ´Gala` and ´Braeburn` apple trees on the same M9 rootstock at the same location at Klein-Altendorf, University of Bonn, Germany (50º N) in 160 vegetation assessments with the following results: (1) Light interception of the tree canopy, as a potential cause, was examined by 300 ceptometer and 100 sunfleck measurements with the ´point quadrat method` (chessboard) in September after herbicide application in May and showed a y significant 10 % (ceptometer) and 35% (sunfleck) light reduction under ´Elstar` apple trees relative to ´Golden Delicious`. (2) The tree strip under fully-grown cv. ´Gala` and ´Braeburn` apple trees was more densely covered with weeds rated as 3.8 n 4.3 in comparison with cv. ´Elstar` rated as 1.7 n 1.9 on a scale of 1 (0-20%) to 5 (80-100% surface covered); this finding of fewer weeds in cv. 'Elstar' orchards, possibly due to light deprivation, applied to both chemical (IP) and mechanical (organic) weeding. (3) Common groundsel ( Senecio spp .) and daisy ( Bellis perennis ) were less abundant under 'Elstar` apple trees than under 'Golden Delicious`. The long-term glyphosate application had selected certain weeds and enabled the germination of seeds of common chickweed ( Stellaria media ) and common groundsel ( Seneccio spp .). In the present experiment, cuckooflower ( Cardamine hirsuta ) and meadow grasses ( Poa spp .) dominated. (4) The weed suppression in the tree strip by fully-grown ´Elstar Michielsen` on M9 after the May herbicide application appears not only cultivar dependent, but may be related to shading by the canopy of vigorous fruit trees such as cv. 'Elstar', irrespective of cultivation (IFP with herbicide or organic production with mechanical weeding) and possibly influenced by further factors.