Influence of the number of CPPU applications on growth, mineral composition and Bunch Stem Necrosis incidence in table grape clusters

The forchlorfenuron (CPPU) application is recommended in table-grape after fruit-set to boost berry sizing, albeit growers also apply CPPU during pre-flowering with controversial advantages. We examined the effect of single (BBCH 15) and double (BBCH 15 and 57) CPPU applications (2.25 mg/L a.s.) in a commercial vineyard. At each time, 75-100 bunches belonging to 6-9 vines were sprayed, and compared with unsprayed (CTRL). Leaf stomatal conductance (g_s), cluster stem diameter and length were measured. At harvest, 25 berries/repetition were sampled for chemical composition, BSN incidence was counted (N° necrotic laterals/10 cm of stem) in 40 bunches/repetition. To test the role of air VPD on mineral composition, at BBCH 77, 50 CTRL clusters were bagged to induce a low VPD.

Preliminary results showed a significant effect of CPPU on stem diameter when compared to that in untreated being 5.49 ±0.22 SE, 6.05 ±0.20 and 6.17 ±0.24 mm in CTRL, single and double CPPU applications, respectively. Cluster length and g_s remained comparable across treatments. The number of CPPU applications did not affect berry Ca content (0.84 ±0.08 –single- and 0.85 ±0.03 mg berry^-1 -double), whereas BSN incidence was significantly higher (2.63 ±0.33a) in the double CPPU applications than single (1.29 ±0.18b) and comparable to CTRL (1.75 ±0.24ab), leaving some open questions.

Based on the significant effect of VPD on berry Ca content (0.39 ±0.04 –bagged-, 1.81 ±0.84 mg berry^-1 -CTRL), the use of management options (i.e. training systems, plant distances, covering, canopy manipulation) to increase Ca accumulation is discussed as alternative to chemical spray.