Understorey management increases grape quality, yield and resistance to Botrytis cinerea

A bare ground control was compared with four mulch types (anaerobically and aerobically fermented marc (grape pressings), inter-row grass clippings and shredded office paper) which were applied in winter under 10-year-old Riesling vines in a 10-replicate randomized block design in New Zealand, over 2 consecutive years. Functional soil biological activity, as measured by Biolog Ecoplates and bait lamina probes, was increased 2–4 times in the two marc and paper treatments, compared with the control, an effect relating to the elevated soil moisture and reduced temperature fluctuations under these mulches. Nutrient levels and the C:N ratio were also affected in these treatments. The mulched paper lowered vine canopy density by up to 1.4 times that of the other treatments, an effect which probably led to elevated light penetration into the canopy and consequently increased canopy temperature and photosynthesis and lowered canopy humidity. These changes to soil and vine characteristics increased grape skin strength by up to 10% in the paper treatment and sugar concentrations by 1.2–1.4 °Brix in the two marc and paper treatments. The severity of Botrytis cinerea infections in the anaerobic marc, aerobic marc and paper treatments were reduced to 12, 3 and 2.2% of the control, respectively, in field assessments averaged over two consecutive harvests.