Available at: https://digitalcommons.calpoly.edu/theses/3028
Date of Award
6-2025
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
Baby kale (Brassica oleracea and other species) is a vital commodity as a salad green in spring mix and is widely produced on the Central Coast of California. Downy mildew, caused by Hyaloperonospora brassicae, is a devastating disease infecting economically important crops like cabbage, broccoli, cauliflower, horseradish, rapeseed, kohlrabi, Brussels sprouts, and kale. This foliar disease causes plant tissue to exhibit chlorotic and necrotic flecking and gray-white sporulation, affecting both the yield and quality of the crop and rendering the affected leaves unmarketable. The Central Coast’s cool climate and nighttime moisture, combined with dense plantings and sprinkler irrigation, make the region highly conducive for downy mildew infections. Downy mildew management primarily relies on chemical applications for disease prevention. However, the crop’s quick harvest time and the pathogen’s rapid polycyclic cycle and spore dissemination, paired with limited fungicide modes of action, present challenges for disease control.
Additionally, growers on the Central Coast face strict regulations limiting water and nitrogen inputs to protect natural resources and ecosystems while sustaining agricultural production for years to come. The Sustainable Groundwater Management Act was implemented to manage groundwater and prevent aquifer depletion caused by rising temperatures and intensified drought events in California. Ag Order 4.0 was put into effect, limiting nitrogen application amounts to address water quality and run-off issues. These limitations make it difficult for agriculture production to meet yield demands. However, plant breeding approaches can combat these obstacles by identifying and exploiting favorable genotypes to produce superior baby kale varieties with downy mildew resistance or high water and nitrogen use efficiency (WNUE).
This study focused on a collection of 212 accessions of baby kale. First, three rounds of baby kale host resistance screenings were conducted against eight downy mildew isolates collected from eight distinct locations across the Central Coast of California to identify sources of downy mildew resistance and facilitate resistance breeding in baby kale. The first screening test all 212 baby kale accessions against one downy mildew isolate (BKG22). The second screening tested 50 accessions against seven isolates. Finally, 25 accessions were tested against 4 downy mildew isolates across three replications. Artificial inoculation was performed by spraying a sporangia suspension onto baby kale plants, which were incubated in a humidity chamber. Disease severity was assessed by examining both surfaces of each leaf for chlorotic and necrotic symptoms and sporulation and then quantified using an established rating scale. Screening of all 212 accessions revealed an average disease severity of 28%, with severities ranging from 0 to 100%. The initial subset screening showed average disease severities ranging from 2.2 to 9.4%, depending on the isolate. The final subset screening demonstrated a range of 0.003 to 0.072% average disease severity among the four isolates, with 13 accessions exhibiting 100% estimated resistance probability, 11 accessions between 99.0 and 99.9%, and one accession below 99.0%. These results suggested that downy mildew could be effectively managed in baby kale through resistant varieties.
Following the downy mildew resistance screening, a subset of 46 baby kale accessions showing reduced disease susceptibility were assessed for water and nitrogen use efficiency. The study was conducted under greenhouse conditions, and all baby kale plant accessions were treated with two water and nitrogen treatment levels. Plants were harvested after approximately six weeks, and fresh weight was recorded. Accessions were also individually dried and encapsulated for isotope analysis. Screening of the 46 accessions revealed broad genetic variation among the three species, with species significantly affecting all measured variables under both water and nitrogen treatments. Among species under 50% inputs, B. napus maintained high performance, ranking first for yield, WUE, and PFP under 50% nitrogen and first for yield, WUE, and NUP under 50% water. Nitrogen treatment significantly affected all variables, with greater variation observed between the treatments. Notably, B. napus × B. oleracea accessions consistently ranked second across all measured variables under both nitrogen levels. These findings highlight the genetic capacity within Brassica spp., demonstrate the potential of interspecific crossbreeding, and suggest that breeding strategies can help growers remain productive while adapting to evolving regulations and constraints.
While collecting downy mildew isolates for disease resistance screening, a constantly high disease incidence was observed in nurseries. It was hypothesized that such management failure was caused by fungicide resistance as a result of frequent fungicide exposure. Therefore, a study was conducted to evaluate the impact of prior fungicide exposure on fungicide resistance in H. brassicae by comparing disease severity between high-exposure nursery isolates against presumed low-exposure isolates collected from a school garden and breeding plot. H. brassicae isolates were evaluated for their sensitivity to five fungicides through artificial inoculation of baby kale cotyledons treated with low and high concentrations of each fungicide. Results revealed distinct patterns in fungicide efficacy. Fluopicolide completely inhibited infection caused by the low-exposure isolates, while infection caused by high-exposure isolates was only partially inhibited. All isolates displayed varying levels of insensitivity to mefenoxam. Infections caused by each of the four isolates were completely inhibited by ametoctradin + dimethomorph, mancozeb, and fluoxapiprolin. Patterns observed with fluopicolide indicated that repeated exposure likely contributed to resistance development. The insensitivity to mefenoxam across all isolates indicated that resistance was likely to increase with prolonged use of chemicals. However, the lack of resistance to mancozeb, one of the multi-site modes of action fungicides used in the nurseries, suggested that factors other than fungicide resistance may significantly contribute to the ineffective control of downy mildew in the nursery settings.