Available at: https://digitalcommons.calpoly.edu/theses/2901
Date of Award
6-2024
Degree Name
MS in Agriculture - Environmental Horticultural 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
Strawberry is an economically important crop in California, with an estimated value of $2.68 billion in 2023. In California strawberry production, mitigation of low plant health and yield often focuses on major soilborne pathogens, while the contribution of minor pathogens and abiotic disorders to production shortcomings are often overlooked. The objectives of the first project in this thesis are to determine the pathogenicity of multiple minor pathogens and quantify other biotic and abiotic factors that can reduce plant health such as viruses and soil salinity. Two pathogens of the black root rot complex, Pythium spp. and Rhizoctonia spp. as well as Neopestalotiopsis rosae are included in this study. The pathogens were identified using ITS DNA sequencing and evaluated for optimal colony growth temperatures. Over the course of two trials, Koch's postulates of P. ultimum and P. irregulare isolates were confirmed for pathogenicity of strawberry roots and crowns, Rhizoctonia spp. isolates were confirmed over 2 trials for pathogenicity of strawberry roots and crowns, and Neopestalotiopsis rosae isolates were confirmed for pathogenicity of strawberry crowns, leaves, and fruit. In 2022 and 2023, 60 root zone soil samples of symptomatic plants that tested negative for major soilborne pathogens were evaluated for electrical conductivity (ECe). Average soil ECe was 1.17 dS/m ranging between 0.18 and 2.45 dS/m, categorizing all samples as non- or slightly-saline except for two which were moderately-saline. Virus testing between 2022 and 2024 diagnosed eight positive samples out of 38 total samples: three in 2022 for Beet pseudo-yellows, two samples in 2023 for Strawberry mild yellow edge and Strawberry polerovirus 1 and one for Strawberry polerovirus 1, and two samples in 2024 for Beet pseudo-yellows. The results from this study suggest that while the tested minor pathogens can infect and reduce strawberry plant health, it is unlikely they are the sole cause of the observed plant mortality from recent diagnostic samples. Additionally, the low to moderate soil salinity levels and infrequent positive virus diagnostics are also not likely the sole cause of observed plant mortality. Future research into these topics could focus on the combination of major and minor pathogens as well as abiotic disorders to decipher how each factor affects plant health.
Additionally, Macrophomina phaseolina (MP), an important soilborne pathogen in California strawberry production, was observed to cause 29.7%-52.0% of late-season strawberry mortality in major strawberry growing districts in recent surveys. The objective of the study of this thesis is to assess the efficacy of crop termination and cover cropping on MP suppression to reduce disease incidence of Macrophomina root rot and improve strawberry yield and soil health. Studies were conducted using conventional field soil as a pot trial and a field trial, as well as an organic field trial. The greenhouse pot trial utilized strawberry cultivars Albion and Royal Royce planted in soil collected from a conventional grower field in the Santa Maria district in a randomized complete block design. Treatments included untreated control soil (C), untreated control soil planted with wheat (W) 'Summit 515’ (C+W), soil fumigated with metam potassium (crop termination) planted with wheat (CT+W), and soil fumigated with metam potassium (crop termination) and chloropicrin (flat fumigation) (CT+FF). Plant infection and soil pathogen levels were assessed via plating on semi-selective media and using a pour plate method, respectively. Two repetitions of the trial were conducted. There was no significant soil treatment × cultivar interaction or cultivar effects in trial 1 and 2 for the MP CFU/g soil, but there was a significant soil treatment effect in the MP CFU/g soil of both trials (P = 0.0001). The trial 1 CT+W treatment had the highest-level MP CFU/g soil, which descended in significance to C+W, then C, and then finally CT+FF. In trial 2 CT+W had a significantly higher MP CFU/g soil value than the other treatments, while C and C+W were comparable to each other and both higher than CT+FF. Chemical soil evaluations for mineralizable carbon (MinC) and permanganate oxidizable carbon (POXC) were also performed. Trial 1 C+W had a significantly higher MinC value than CT+FF, while C and CT+W were comparable with both treatments. In the trial 2 MinC soil test the soil treatment × cultivar interaction and the main effects were not statistically significant. In trial 1 and trial 2 POXC soil tests the soil treatment × cultivar interaction and the main effects were not statistically significant. The second year of this study took place in the field and soil samples were collected pre- and post-soil treatments of CT, CT+W, CT+Triticale ‘Pacheco’ (T), CT+W+FF, and CT+T+FF to be evaluated for MP CFU/g soil. T was added as a cover crop treatment to compare a triticale variety to wheat as well as its prevalence as a cover crop in California. There was a significant soil treatment effect (P = 0.02) with post-cover crop wheat treatment having a significantly higher MP CFU/g soil than post-cover crop triticale. The strawberry cultivar Portola was planted after fumigation across all cover crop blocks. Additionally, an organic strawberry field trial compared wheat and triticale cover crops for MP suppression with strawberry cultivars Valiant and Monterey. Soil and plants were tested in the same manner as the pot trial. MP CFU/g soil, MinC, and POXC were not significantly different between soil treatments. Preliminary results suggest single season cover cropping cannot manage high pathogen levels, while crop termination can reduce the pathogen inoculum if the application is timed correctly. The completion of this project will include plant mortality evaluations and microbiome analyses from the field trials. This research aims to help the California strawberry industry by enhancing disease management and reducing fumigant use.
Award received:
CAFES Spring Research Symposium, 1st place, CAFES Outstanding Graduate Student Award 2024