Effect of CPPU on Fruit Development of Selected Seedless and Seeded Grape Cultivars

The effects of CPPU [forchlorfenuron, N-(2-chloro-4-pyridinyl)-N-phenylurea] on berry development of Vitis labrusca and V. labrusca × V. vinifera cultivars was evaluated under field conditions. A concentration response was initially established by spraying clusters of `Himrod' at a mean berry diameter of about 5 mm with 0, 5, 10, or 15 mg·L –1 CPPU. Berry enlargement was monitored (16, 30, 44, and 59 days after treatment) during development. Cluster mass, number of berries per cluster, berry mass and firmness, and °Brix were determined at harvest. Berry mass was dramatically increased (2.3 versus about 3.6 g/berry) at harvest by all concentrations of CPPU. Cluster mass and compactness were also increased and berry firmness was linearly related to CPPU concentration ( r 2 = 0.997). There was no significant effect on number of berries per cluster (79 to 86). °Brix, rachis necrosis at harvest, and berry abscission after 30 days of refrigerated storage (1 °C) were significantly reduced. Effect of time of CPPU application (0, 5, and 10 mg·L –1 ) was established by treatment of clusters at mean berry diameters of about 4, 5, 7, and 9 mm. Response was indexed by following berry enlargement at 14, 28, 42, and 56 (maturity) days after treatment. Maximum berry size for both 5 and 10 mg·L –1 was obtained from applications at 4 to 7 mm berry diameter. Relative response of seedless and seeded cultivars was compared by application of CPPU at 0, 5, 10, or 15 mg·L –1 to clusters (4 to 6 mm berry diameter) of seedless `Vanessa' and `Lakemont' and seeded `Concord' and `Niagara'. Bioresponse was determined by a time course of berry enlargement and berry and cluster mass, number of berries per cluster, and rating cluster compactness at maturity. Except for `Lakemont' at the 5 mg·L –1 concentration, CPPU at all concentrations increased seedless berry diameter significantly from the first measurement at 14 through 56 days after application. Berry and cluster mass and cluster compactness were significantly increased in `Vanessa'. In contrast, the only effect of CPPU on the two seeded cultivars was an increase in berry size in `Concord' and an initial increase in berry size 14 days after application in `Niagara'.