Photosynthetic inhibition of 'seyval blanc' grapevines with terbacil

'Seyval blanc' (Vitis vinifera L.) is an important French-American hybrid white wine cultivar grown in the Great Lakes region because of its disease and phylloxera resistance and cold hardiness. It does, however, have some limitations. It forms large, compact clusters susceptible to Botrytis bunch rot under Michigan's cool, wet climate and post-rainfall veraison conditions. The large clusters can also result in overcropping and delayed fruit maturity. Fruit thinning of most horticultural crops (peaches, apples, pears and grapes) is essential for achieving adequate fruit size and quality. Hand-thinning continues to be widely practiced, but increasing costs and decreasing availability of labor accentuate the appeal for chemical thinning agents. Terbacil (a Photosystem II inhibitor herbicide), when sprayed to thin apple fruits, caused no leaf injury, pigmy fruit, fruit malformation, or detectable residue levels in the fruit. Thus an application of Terbacil at flowering may decrease cluster size and compactness, alleviating disease pressure, overcropping and pesticide use. A wholeshoot open gas exchange system was constructed and used to measure carbon exchange in the field. Terbacil was applied 24 hours after the first ΔCO₂ measurements at 40 ppm + Herbimax surfactant (10 ml L ^-1) on two shoots, 50 ppm + surfactant (10 ml L^-1) on another two shoots. The control consisted of a distilled H₂O + surfactant (10 ml L^-1) spray which was applied on four shoots. Terbacil inhibited photosynthesis by 80% initially and had an effect for three days. Thus, this preliminary study resulted in a decreased photosynthate production on a whole-shoot scale. Further studies are necessary to assess the true effects of Terbacil and the surfactant (crop oil) on grapevine photosynthesis, source-sink relation at bloom and potential for fruit thinning.