The composition of ESMs varies widely, from simple mixtures of stones and native soil to patented commercial products. Highly porous ESM mixes provide ample infiltration and pore space for temporary storage of surface runoff. Also, they support tree growth by providing more water and aeration to roots than compacted native soil alone. ESMs can reduce conflicts between surface roots and sidewalks by promoting deeper rooting systems. In California alone, over $70 million is spent annually to remediate damage by shallow tree roots to sidewalks, curbs and gutters, and street pavement. In Davis, California, a bioswale installed next to a parking lot reduced runoff from the parking lot by 88.8% and the total pollutant loading by 95.4% during the nearly two year monitoring period. Furthermore, a bioswale installed next to a turf grass patch at the University of California-Davis campus eliminated dry weather runoff from an irrigated urban landscape. The ESM used in these studies offered several advantages over other ESMs because the main structural element was locally quarried and relatively inexpensive lava rock . This ESM had a high porosity, high infiltration rate, and a high water storage capacity . It effectively fostered the growth of biofilms that retain nutrients and degrade organic pollutants. Because vegetated bio-swale research is in its infancy, very few studies have monitored vegetation growth and its impacts on bioswale performance. Moreover, plant pot with drainage evaluation of system performance is generally conducted before vegetation is fully established.
In contrast, this study evaluated the effectiveness of two bioswales on surface runoff reduction, pollutant reduction, and tree growth eight years after construction. The control bioswale contained native soil and the treatment contained an ESM. At the time of this study, the trees in the control and treatment bioswales were fully established and approaching mature size. Measurements recorded the differences in surface runoff dynamics and pollutant reduction rates, as well as tree and shrub growth. This study provides new information on the long-term effectiveness of engineered bioswales in a region with a Mediterranean climate.The water collection system was installed in 2007 to collect composited samples from natural runoff . In this study, surface runoff samples from the control site were collected at a high frequency using grab samples to better observe pollutant concentration dynamics for each experiment throughout a storm hydrograph. A test run was conducted on 10 October 2013 to determine the optimal runoff sampling time intervals and the number of samples needed to capture the peak and total loadings. For the test run, the water soluble fertilizer was applied to both of the sites at a rate of 2.24 g m−2 . Grab samples were collected at a 10-min frequency over a two hour irrigation period from the control site. Water samples were collected immediately before the runoff was directed to the underground tank. The water samples were coarse filtered during sample collection with coffee filters to remove large tree leaves, grass clippings and large soil particles. Based on the results of the test run, the water sampling frequency for subsequent trials was extended to six hours with a variable sampling interval to better characterize the runoff pollutant pattern. The composite water sample from the treatment site was used for calculating the total loading of the treatment site where little surface runoff occurred in this study.
A 5.1 cm diameter and 0.9 m long PVC drainage pipe was vertically installed into the middle of the treatment bioswale to collect a representative water sample for monitoring pollutants concentration dynamics in the bioswale. The treatment site water samples were not affected by successive flow from the control site because the treatment site was located upslope of the control site. The water sample collected from the control site was surface runoff, which was not affected by subsurface flow of the treatment site because of the site’s relatively flat surface.This treatment bioswale had slightly higher pollutant reduction rates as compared to the bioswale with ESM installed adjacent to a parking lot in a previous study. The parking lot bioswale reduced the nutrients by 95.3% and organic carbon by 95.5%. The peak pollutant concentration reduction rates found in this study were a minimum of 53% higher than those reported in the parking lot study. One possible explanation for this difference is that the trees and shrubs in the bioswale were more extensive and older than the tree in the parking lot site. Tree and shrub roots can function as a biofilter, where pollutants are immobilized, transformed, or degraded. Although data were not available for below ground biomass, a more extensive rooting system and associated microorganisms in this study’s bioswale could be partially responsible for its improved performance. Another possible explanation is the difference in pollutant inflows. The primary pollutant source in the parking lot study was from atmospheric deposition, with lower concentrations when compared to the fertilizer rates applied in this study. Stormwater BMPs, such as bioswales, are reported to have higher removal rates when treating stormwater with high inflow concentrations. Concentration of Zn, Cd, Ni, Cu, and Pb are key water quality concern parameters.
They were excluded from this analysis because their concentrations were below detection levels in the irrigation source waters and these metals are not identified as impairments in the study area.The interpretation of results from this study is subject to some limitations. Pollutants can leave the system via infiltration deeper into the soil and potentially enter the groundwater. Deep leaching can be an important flow path affecting the fate of pollutants, and was not included in the scope of this study. Caution should be taken regarding the potential for groundwater contamination when considering the use of ESM in bioswale projects. In this experiment, the trees were eight years old and their root systems were well established. Trees received excess surface irrigation runoff during the hot/dry summer. Because the ESM in this study was 75% lava rock it may not retain enough moisture for tree roots during long dry periods. Trees in bioswales with ESM may require more irrigation than trees in native soils, especially for establishment. In this study, the pollutants were artificially added to the system by using dissolved fertilizer. Actual storm runoff includes pollutants from atmospheric deposition and has a more complex mixture of pollutants. These factors introduce uncertainty in extrapolating the pollutant reduction efficiency of the bioswale to other sites. It is unclear whether all of the pollutants retained by the bioswale were fully retained by the vegetation and soil, or if a portion of these pollutants were only temporarily immobilized in the system by the soil-tree root system. The bioswale system tested in this study was eight years old, relatively young when compared to its 20 to 30 year life expectancy. Long-term monitoring of system performance is needed to document bioswale performance over longer time periods typical of urban green infrastructure. Additional research is needed that follows the fate and transport of pollutants after infiltration. In particular, chemical analyses of soil and tree samples are needed to understand the fate and transport of the pollutants in the bioswale system.The oomycete Phytophthora cinnamomi Rands is the causal agent of Phytophthora root rot of avocado, the most destructive disease in the avocado industry worldwide . PRR affects approximately 75% of California avocado growers, historically causing losses of $40 million annually . The pathogen mostly infects the feeder roots, causing the roots to be blackened, brittle, or necrotic. Infected trees usually develop symptoms from pale green to yellow leaves, wilting, to heavy leaf fall and dieback, greatly reducing fruit yield. Trees eventually become leafless and die . P. cinnamomi is considered one of the most invasive pathogens in the world, because the pathogen has a wide host range, infecting over 3,000 hosts, including forest trees, such as eucalyptus, pine, and oak ; ornamental plants, such as camellia, Rhododendron, and Azalea; and fruit crops, such as pineapple, peach, and highbush blueberry . P. cinnamomi is a hemibiotrophic pathogen feeding initially from living host cells and then switching to necrotrophy by killing the host cells and feeding from the nutrients released by them . The entry into the plant is achieved by the adhesion of the motile zoospores to the host tissue, encystment, and germ tube formation. The germ tubes usually grow and penetrate the root surface via appressorium-like swelling structures and then plant tissue is rapidly colonized .
During its biotrophic stage, P.cinnamomi projects haustoria into the plant cells for the acquisition of nutrients and release of pathogen proteins to aid the infection process in the host . This is followed by a necrotrophic stage characterized by host cell death, hyphal proliferation, pots with drainage holes and production of numerous sporangia . P. cinnamomi is heterothallic and both pathogen mating types are pathogenic, however, the A2 mating type is more invasive and is generally recognized as being the more aggressive . P. cinnamomi isolated from infected avocado trees almost exclusively consist of an A2 clonal population with no sexual reproduction evident. Even when both mating types are present in the population, diversity develops asexually in the form of clonal populations. Only one A1 P. cinnamomi isolate has been discovered on avocado in California, which appears to infect mainly alternate hosts such as camellia . Once P. cinnamomi is introduced in a new avocado area, it cannot be eradicated, and for this reason, the ability to rapidly and accurately detect this pathogen, monitor its population, and differentiate their variants becomes more urgent to apply appropriate management strategies to reduce crop yield loss and pathogen spread. Despite the economic and ecological importance of P. cinnamomi worldwide, there are limited studies regarding: the genetic diversity of the pathogen population , the efficacy of novel Oomycota fungicides to manage avocado PRR, and the molecular and genetic basis of host-P. cinnamomi interactions .The first main objective of this dissertation was to assess the phenotype of avocado P. cinnamomi isolates representing the current clonal populations recovered in California. Recently, two different A2 clonal P. cinnamomi populations were found in California : the prevalent A2 clade I population and the more geographically specific A2 clade II population. The phenotypes of several avocado P. cinnamomi isolates corresponding to these A2 clades regarding in vitro mycelial growth rate, optimal growth temperature, sensitivity to registered Oomycota fungicides mefenoxam, potassium phosphite, and new compounds fluopicolide, and oxthiapiprolin, and virulence were evaluated. Finally, a detached leaf assay P. cinnamomi inoculation method using Nicotiana benthamiana was developed and validated to circumvent the difficulties associated with the avocado root inoculation method to assess the virulence of P. cinnamomi isolates. Control strategies of avocado PRR are limited, and include: the use of resistant rootstocks, cultural practices, and chemical treatments. Commercially available root rot resistant rootstocks include Dusaâ, Steddom, Thomas, Uzi, and Zentmyer . Among them, Dusaâ is the industry standard currently in California, which enables growers to cultivate avocado in P. cinnamomi infested soil and maintain avocado production. Cultural practices include using certified disease-free nursery stock, keeping well-drained soil, cleaning tools and equipment, and applications of gypsum and mulch . Resistant rootstocks and cultural practices are effective ways to manage PRR, however, losses of avocado production to PRR are still substantial. Resistant rootstocks may also be overcome by aggressive P. cinnamomi populations . Until rootstocks with strong levels of quantitative resistance are developed, there will be a need for chemical treatments to mitigate losses caused by PRR in avocado production. At present, the only fungicides available to control PRR of avocado are phosphonate fungicides, e.g. potassium phosphite, and phenylamide fungicides, e.g. mefenoxam. Potassium phosphite injection is the preferred treatment by avocado growers due to the lack of reported resistance as well as the ability to apply this chemical as a fertilizer. Mefenoxam is used to a much less extent because of resistance development and the relatively higher cost. Both chemicals have been used for decades against Phytophthora spp. including P. cinnamomi. Aluminum tris-O-ethyl phosphonate was first introduced by Rhone-Poulenc Agrochimie Laboratories in 1977 with the product name Fosetyl-Al . It was later discovered that potassium phosphite was the active ingredient and that alkyl phosphonates were degraded in the plant .