Effect of Cluster Tightness on Botrytis Bunch Rot in Six Chardonnay Clones.

Cluster tightness was measured and Botrytis bunch rot was evaluated for six Chardonnay clones in each of two trials, one cane-pruned and the other spur-pruned. In both locations, University of California (UC) Foundation Plant Materials Service (FPMS) clone 4 was most compact and FPMS 15 was the least compact (P < 0.01). Cluster tightness among the six clones, measured by the UC Firmness Tester Method, ranged from 0.0028 newtons (N) for clone 15 to 0.0112 N for clone 4. Cluster tightness was positively correlated with cluster weight. Botrytis bunch rot development was greater in the cane-pruned trial than in the spur-pruned trial. In the cane-pruned trial, disease incidence was greatest in FPMS 16 (81.5%) and least in clone 15 (44.5%; P< 0.025), while disease severity did not differ significantly. Neither incidence nor severity were significant in the spur-pruned trial. Clone 15, the clone with the least compact clusters, had the lowest disease severity levels.