Date of Award

12-2025

Degree Name

MS in Agriculture - Plant Protection Science

Department/Program

Horticulture and Crop Science

College

College of Agriculture, Food, and Environmental Sciences

Advisor

Shashika Hewavitharana

Advisor Department

Horticulture and Crop Science

Advisor College

College of Agriculture, Food, and Environmental Sciences

Abstract

Disease remains one of the major constraints to the productivity and economic success of the California strawberry industry. The most important soilborne pathogen is Macrophomina phaseolina, causing Macrophomina root rot (MRR). The most economically significant airborne pathogens include Botrytis cinerea, causing Botrytis fruit rot (BFR), and Podosphaera aphanis, causing powdery mildew (PM).

Disease severity of MRR is highly dependent on environmental factors. Therefore, understanding which abiotic stressors contribute most to disease development is crucial for effective management. A two-year field experiment at Cal Poly, SLO (2024 and 2025) evaluated the effects of drought stress, two elevated chloride stresses, and high ECW stress on MRR development in two cultivars, Fronteras and Sweet Ann, that were artificially inoculated with M. phaseolina. Across both years, there was no significant treatment ´ cultivar interaction on final plant mortality (P = 0.557 and P = 0.391 for 2024 and 2025, respectively). In 2024 and 2025, treatment had a significant effect on final mortality (P = 0.008 and P = 0.024, respectively). Final mortality, averaged across both cultivars, was significantly higher in the drought stress treatment in 2024 (60.9%) and the high ECW treatment in 2025 (47.3%), when compared to the standard treatment (37.2 and 19.9% in 2024 and 2025, respectively). Cultivar also had a significant effect on final plant mortality in both years (P < 0.0001 and P < 0.0001, respectively). ‘Sweet Ann’ consistently had higher final plant mortalities when compared to ‘Fronteras’ across both years. These results highlight the importance of utilizing cultural management tools, including maintaining optimal soil moisture using soil tensiometers, avoiding the use of poor-quality irrigation water when possible, and planting disease resistant cultivars, to minimize disease severity.

Citrus essential oils have potential as biological controls that could limit crop losses due to foliar pathogens, reduce the use of synthetic fungicides, and slow resistance to major fungicide classes. However, low solubility and high volatility present barriers to practicality. Nano-encapsulation of essential oils offers a solution by improving dispersibility and retention of volatile bioactive compounds. Using commercially available citrus oil products that were nano-encapsulated (NE), preliminary trials were conducted to evaluate their efficacy against B. cinerea and P. aphanis across in vitro, in planta, greenhouse, and field settings at Cal Poly SLO. Treatments were pure citrus oils with the addition of an emulsifier, NE citrus oils containing 10% active ingredient (ai), and commonly used organic and conventional fungicides. Efficacy results were inconsistent across experiments. For the BFR experiments, NE citrus oils applied at 3 and 4% ai (v/v) significantly inhibited B. cinerea in detached fruit assays, when compared to the non-treated control (3%: P = 0.045; 4%: P < 0.0001). However, no significant inhibition was observed in vitro or in the postharvest evaluations of the field experiment, which may be due to product degradation over time. In the detached leaflet assay for PM, treatment had a significant effect on disease incidence (P = 0.0178) and observed phytotoxicity (P < 0.0001), but not disease severity (P = 0.0552). These results reveal limitations to the practicality of using NE citrus oils in commercial settings, including concerns of phytotoxicity and reduced product stability over time. Future work will evaluate the efficacy of NE citrus oils produced from Cal Poly harvested citrus against BFR and PM.

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