Understanding the Causes of Flower Necrosis in Grapevines

A series of experiments was carried out in ‘Pinot noir’ grapevines to better understand why flower necrosis occurs. Previous research investigating late bunch stem necrosis indicated that an abnormal accumulation of the polyamine, putrescine, was the causal agent leading to necrosis. We tested if putrescine was also responsible for causing flower necrosis by feeding single flower node cuttings various metabolites, or by applying metabolites via a needle-delivery method to developing clusters in the field. Both approaches showed that high levels of putrescine in the rachis can cause flower necrosis in ‘Pinot noir’ and induce pedicel abscission in the field. The concentration of putrescine that induced flower necrosis was similar to the concentration previously shown to induce late bunch stem necrosis. However, further work comparing healthy and necrotic clusters from ‘Pinot noir’ grapevines grown in sand-culture (with a history of flower necrosis) showed that flower necrosis was not due to the accumulation of putrescine. These findings combined with other observations led to the hypothesis that flower necrosis may also be caused by an imbalance in the root to shoot ratio of vines, presumably related to whole vine carbohydrate stores and the number of competing sinks at flowering. We tested this hypothesis by manipulating shoot number in the sand culture vines and other vines with no prior history of necrosis. In both cases, flower necrosis was reduced by increasing shoot number per vine while vine nitrogen status was not altered. These results show that putrescine can cause flower necrosis in some cases, but at least one other mechanism (presently unknown) also causes flower necrosis in grapevines that appears to be related to above and below ground vine balance. Vineyard blocks with a history of flower necrosis could be managed by increasing shoot number per vine.