Introduction Botrytis bunch rot, caused by Botrytis cinerea, is a significant disease affecting grape production. The 2006 grape bunch rot trial, conducted by W.D. Gubler and his team at UC Davis, aimed to evaluate the effectiveness of various fungicides in controlling this disease. This blog explores the trial’s methodology, findings, and implications for grape growers.

Trial Overview The trial was conducted at a vineyard in St. Helena, Napa County, California. The Johannesburg Riesling grape variety was used, and the vineyard was established in 1991 with a density of 908 vines per acre. The trial utilized a randomized complete block design with multiple treatments and unsprayed control plots.

Materials and Methods

  • Objective: Evaluate the efficacy of fungicides for controlling Botrytis bunch rot.
  • Experimental Design: Randomized complete block design with unsprayed vines used on all sides of the experimental area.
  • Application Method: Backpack sprayers.
  • Vine Spacing: 6 ft row spacing and 10 ft vine spacing.
  • Replicate Unit: 3 vines per treatment, covering an area of 720 ft² per treatment.
  • Applications: Conducted in late May, June, and August 2006.
  • Evaluation Stage: Disease severity and incidence were evaluated 1.5 weeks following harvest in late October and early November 2006.

Experimental Treatments Various fungicides were tested, including chemical and biological control agents. The treatments included products such as Vangard, Endura, BioNatrol-M, Micro 108, and several experimental compounds. Each treatment was applied at specific rates and intervals throughout the growing season.


  • Disease Severity: Fungicide treatments significantly affected disease severity (p=0.04). The lowest disease severity was observed in plots treated with BioNatrol-M (1841 ml/acre) and Vangard (10 oz/acre).
  • Disease Incidence: No significant effect on disease incidence was observed across treatments.
  • Berry Sugar Content: The increase in berry sugar content was monitored throughout the trial, showing variations among treatments.

Key Findings

  • Treatments had a mild influence on disease severity but no significant effect on disease incidence.
  • BioNatrol-M and Vangard treatments showed the lowest disease severity.
  • An additional pre-harvest application of fungicides might have improved the efficacy of both biological and chemical controls.

Conclusion The 2006 UC Davis trial highlighted the varying effectiveness of different fungicides in controlling Botrytis bunch rot. While some treatments showed promise, the overall influence on disease incidence was limited. These findings underscore the importance of integrated disease management strategies, combining chemical and biological controls for optimal results.