Available at: https://digitalcommons.calpoly.edu/theses/3411
Date of Award
7-2026
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 diseases, such as powdery mildew and Botrytis bunch rot, require repeated fungicide applications to maintain fruit quality and reduce yield losses. However, fungicide performance can vary among products, application programs, disease pressure, and pathogen sensitivity. This thesis evaluated fungicide programs for control of powdery mildew in ‘Chardonnay’ grapes and assessed fungicide sensitivity of Botrytis cinerea isolates collected from California vineyards.
Powdery mildew, caused by the fungal pathogen Erysiphe necator, is a major threat to the production of Vitis vinifera grapes worldwide. Because powdery mildew management relies heavily on fungicide applications, field evaluations are needed to compare the efficacy of different products and spray programs under field conditions. To evaluate fungicide efficacy, field trials were conducted in 2024 and 2025 on ‘Chardonnay’ grapes at Trestle Vineyard, in San Luis Obispo, California. These trials compared organic, conventional and experimental or unregistered products for controlling powdery mildew. Evaluations were conducted by rating powdery mildew severity and incidence on 25 clusters per treatment. In the 2024 trial, all treated plots were significantly different from the untreated control, but not from each other, indicating that all treatments reduced disease severity under the conditions of the trial. Treatment costs for the growing season ranged from $276.41 to $816.56 per acre, with Treatment 14 having the highest cost. Treatment 14 was a fully organic program that included nine applications, compared to six applications on all other treatments. The 2025 trial resulted in more differentiation among treated plots, with several organic, conventional and unregistered products maintaining low disease severity. Of the treatments with the highest disease suppression, Treatment 19, a ProBlad Verde-only treatment, had the highest estimated cost at $1,333.19 per acre. In contrast, the positive control Treatment 6, which was a growers’ standard consisting of a rotation of conventional products, had an estimated cost of $424.30 per acre. This difference was due to Treatment 19 receiving 12 applications of ProBlad Verde at 7-day intervals, while the positive control received 6 applications at 14-day intervals. Overall, these results suggested that powdery mildew can be managed with a range of organic and conventional fungicide programs, but product selection should consider mode of action, application frequency, and cost.
Botrytis bunch rot, caused by the fungal pathogen Botrytis cinerea, is another major threat to grape production worldwide. This pathogen has a wide host range, can affect more than 200 plant species, and is primarily managed through fungicide applications. Due to high exposure to fungicides, Botrytis cinerea is considered a high-risk pathogen for fungicide resistance. In this study, 72 Botrytis cinerea isolates were collected from California vineyards in 2024 and 2025, and their sensitivity was assessed to six registered fungicide active ingredients: fludioxonil, fenhexamid, iprodione, boscalid, trifloxystrobin, and cyprodinil. After exposure to two discriminatory concentrations, mycelial growth data were collected and evaluated using both categorical and statistical approaches. For the categorical approach, differences in relative mycelial growth were used to classify isolates as highly sensitive, moderately sensitive, moderately resistant, or highly resistant based on criteria established by previous research. Relative mycelial growth was also analyzed using a mixed-model analysis of variance, followed by Tukey’s HSD test for mean separation. Results indicated that fludioxonil provided the highest level of inhibition in both years, with 100% of isolates categorized as highly sensitive. In contrast, trifloxystrobin had the highest frequency of highly resistant isolates, with more than 20% of isolates categorized as highly resistant in both years. These results indicate that continued fungicide sensitivity monitoring is needed to detect shifts in pathogen response and support resistance management through careful fungicide rotation.