Effects of mechanical thinning on botrytis bunch rot on Sauvignon blanc wine grapes

Mechanical fruit thinning could be a practical and cost-effective alternative to hand thinning of Sauvignon blanc grapes to increase quality by reducing yield. Botrytis bunch rot, caused by the fungus Botrytis cinerea, is the main seasonal disease risk for grapes grown in New Zealand but it is unknown if this disease is exacerbated by mechanical rather than manual thinning of the vines. It was hypothesised that the damage caused by mechanical thinning would result in more disease or increase disease pressure than hand thinning or no thinning. Botrytis bunch rot was determined in the field at harvest following mechanical thinning in the 2009, 2010 and 2011 seasons compared with an un-thinned control. In the 2011 season, possible mechanisms that may have influenced disease severity were investigated. The parameters investigated were: bunch openness; berry susceptibility to infection; and percentage of bunch debris infected with Botrytis cinerea. Mechanical thinning resulted in the same or lower observed disease severity compared with the un-thinned control in the 2009, 2010 and 2011 seasons while reducing yield as desired. In all seasons, both heavy and light machine thinning treatments reduced incidence of botrytis compared to the un-thinned control and the heavy machine treatment always reduced disease severity compared to the un-thinned control. Berry susceptibility to Botrytis cinerea was a complex interaction between various factors. Heavy machine thinned berries without wounding and inoculation were significantly less susceptible than the un-thinned control. Further investigation will be required to determine if the significant differences observed in berry susceptibility to infection and total infected bunch debris per bunch can be correlated with observed field disease levels.