Light deprivation as a herbicide?


In the light of strategies to reduce pesticide usage for sustainable fruit production, the objective of the present work was to investigate mitigation strategies to reduce herbicide input with the expected glyphosate ban. Naturally reduced weed population/vegetation has been observed in the summer in the herbicide strip under fully-grown ‘Elstar’ apple trees cultivated organically or in IFP in a temperate climate zone. Hence, the objective of this field trial was to investigate the potential causes. Therefore, the vegetation in the ‘Elstar’ apple tree strips was compared with that under fully-grown ‘Golden Delicious’, ‘Gala’ and ‘Braeburn’ apple trees on the same M9 rootstock at the same location at Klein-Altendorf, University of Bonn, Germany (50°N) in 150 vegetation assessments. To determine the amount of light available to the vegetation in the soil of the tree strip, light interception of the tree canopy was measured. The 300 ceptometer and the 100 sunfleck measurements with the ‘point quadrat method’ (chessboard) in September, after herbicide application in May, showed a statistically significant 10% (ceptometer) and 35% (sunfleck) light reduction in the tree strip under ‘Elstar’ apple trees in August compared to ‘Golden Delicious’ in the adjacent tree row. The tree strip under fully-grown ‘Gala’ and ‘Braeburn’ apple trees was significantly more densely covered with weeds vegetation rated as 3.8-4.3 in comparison with that of ‘Elstar’ rated as 1.7-1.9 on a scale of 1 (0-20%) to 5 (80-100% soil surface covered); this finding of fewer weeds in ‘Elstar’ orchards, possibly due to light deprivation, applied to both chemical (IP) and mechanical (organic) weeding. Common groundsel (Senecio spp.) and daisy (Bellis perennis) were less abundant in the tree strips under fully-grown ‘Elstar’ apple trees than under fully-grown ‘Golden Delicious’ or ‘Gala’ apple trees of the adjacent row. The reported indirect weed species selection as a result of long-term glyphosate application was confirmed; this herbicide application allowed the subsequent germination of seeds of common chickweed (Stellaria media) and common groundsel (Senecio spp.). In the present experiment, cuckooflower (Cardamine hirsuta) and meadow grasses (Poa spp.) dominated in the tree strip. This weed suppression under fully-grown ‘Elstar’ appears related to shading by the canopy of vigorous fruit trees such as ‘Elstar’, irrespective of rootstock and cultivation (IFP with herbicide or organic production with mechanical weeding) and possibly influenced by further factors.