Available at: https://digitalcommons.calpoly.edu/theses/2971
Date of Award
3-2025
Degree Name
MS in Agriculture - Plant Protection Science
Department/Program
Horticulture and Crop Science
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Shunping Ding
Advisor Department
Horticulture and Crop Science
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
Grapevine powdery mildew caused by the fungal pathogen Erysiphe necator is a persistent threat to wine grape production in California. While synthetic fungicides remain the primary management strategy, increasing concerns over pesticide resistance, environmental impact, and regulatory restrictions have driven interest in biofungicides as a sustainable alternative. However, their efficacy is inconsistent, requiring research into optimized application strategies, integration with synthetic fungicides, and their effects on disease suppression, grape yield, and fruit quality.
This two-year study (2023–2024) was conducted at two vineyard sites, Trestle Vineyard in the San Luis Obispo Coast AVA and a commercial vineyard in the Arroyo Grande Valley AVA. In each experimental trial, 14 treatments were tested, which included three biofungicides applied alone at three different intervals, weekly, biweekly, and based on powdery mildew risk assessment index (RAI), and in rotation with synthetic fungicides, a grower standard, and a non-treated control. The treatments were assessed by disease incidence and severity, grape yield, juice chemistry (Brix, pH, titratable acidity), and economic profit.
Field trials were conducted using a randomized complete block design, and the tested biofungicides contained the active ingredients extract of Reynoutria sachalinensis, Streptomyces lydicus Strain WYEC 108, and Bacillus subtilis Strain QST 713. Disease pressure was monitored using RAI, and an economic analysis was conducted to assess treatment viability based on input costs, application expenses, and revenue differences.
Results demonstrated that treatments with biofungicide-synthetic rotations provided disease suppression comparable to synthetic-only programs, particularly when applied at optimized intervals. Standalone biofungicide treatments exhibited variable efficacy, with greater disease suppression under moderate disease pressure but reduced effectiveness under high-pressure conditions. At Trestle Vineyard, refinements in application timing and a typical disease progression led to improved control in 2024 compared to 2023. In Arroyo Grande Valley, biofungicides performed better under moderate disease pressure but declined in effectiveness when disease pressure intensified, emphasizing the role of environmental conditions in treatment success.
Beyond disease suppression, biofungicide applications indirectly influenced grape chemistry, as improved disease control correlated with higher yield and balanced juice composition. Diseased clusters exhibited elevated °Brix and pH, suggesting stress-induced ripening, whereas healthier clusters maintained a more desirable sugar-acid balance, which is crucial for wine quality.
Economic analysis confirmed that integrated biofungicide-synthetic programs consistently provided strong disease control and financial returns, reinforcing their potential as sustainable tools for powdery mildew management. This study highlighted the importance of strategic application timing, environmental conditions, and disease pressure in optimizing biofungicide efficacy. By demonstrating that biofungicides can effectively contribute to disease management when integrated with synthetic programs, these findings support efforts to reduce synthetic fungicide reliance while maintaining vineyard productivity and fruit quality.
