Benefit transfer approaches, SFA, and DEA yield results that can evaluate how well an NTS site is performing and how to improve their functioning, but have associated advantages and disadvantages . Benefit transfer approaches can provide an estimate for ecosystem services value, which can be used in cost-benefit analyses to determine net changes to well-being. In addition, benefit transfer can identify drivers of change through the application of regression models. Regardless of net costs or benefits, there exists a maximum level of ecosystem services provision that can be provided with given inputs. SFA and DEA can help determine whether systems are performing at this maximum level and where improvements can be made. Both methods apply somewhat similar regression models to benefit transfer approaches, yielding quantitative relationships among variables. Other options include a fixed effects model to estimate the best-practice frontier , and the multi-product distance function which allows for a stochastic error term and multiple outputs including negative externalities . In prioritizing ecosystem services,vertical hydroponic farming geographic location can dictate physical and social factors that can influence their value. For example, water issues have long been important in southern California and they are predicted to become more contentious due to climate change.
Further, new policies in California put an emphasis on climate-regulating services. The region is also a biodiversity hot spot at risk of alteration . Urbanization has reduced the amount of suitable habitat for organisms as natural ecosystems are replaced by human structures. Urban green spaces such as NTS can be important patches of habitat and act as corridors for organisms seeking refuge . One of the biggest challenges in characterizing and valuing ecosystem services associated with NTS is the lack of accessible data. There is no standard monitoring program for NTS, which makes it difficult to compare across time and across sites, and there is no monitoring of ecosystem services associated with NTS. Monitoring could help identify effective management strategies, e.g. timing of maintenance, and improve the above quantitative models for more accurate estimates of value and efficiency. In situ, mesocosm studies are also needed to evaluate NTS performance under actual environmental conditions . Other questions that still need to be addressed include how networks of NTS compare to single systems regarding both targeted and non-targeted ecosystem services, and how NTS operate over time. Prior to the widespread use of soil fumigants, Verticillium dahliae Kleb. was considered among the most important pathogens affecting commercial strawberry production in California . Plantations established in infested soils often suffered 50% or greater mortality , and wilt from this pathogen continues to be a major concern where strawberries are managed in perennial planting systems .
Breeding for resistance has been an important strategy for minimizing damage caused by V. dahliae in strawberry for several decades, and genetic variation for resistance to this pathogen has been demonstrated in many breeding populations . The University of California strawberry breeding program was initiated in 1930, and its goal since initiation has been to release cultivars specifically adapted to California production environments. In part due to concerns about the continued availability of effective soil fumigants, researchers in the UC strawberry breeding program began to develop a field-based resistance screening method in 1992 , and an ongoing resistance evaluation program has been conducted for this disease since 1994. Preliminary experimental results demonstrated substantial genetic variation for an overall low level of resistance to V. dahliae in the UC breeding population . Researchers also verified that resistance to the pathogen in this population has polygenic inheritance, is conditioned by both additive and dominance genetic effects, and that genotypes with high resistance were rare in the germplasm at that time. These results suggested that the infusion of non-California germplasm was not needed to obtain genetic progress, but that substantial time and effort would be needed to change the genetic composition of the breeding population and generate Verticilliumresistant strawberry cultivars with adequate frequency.Two broad strategies have been used to improve genetic resistance to soil disease in crop plants. One common approach is to first identify resistant germplasm and generate highly resistant lines, then back cross the resistance into elite cultivars.
This method works well when the inheritance of resistance is conditioned by one or a few genes and the crop-specific breeding system permits back crossing and self-fertilization for rapid genetic transfer and stabilization. Intensive selection for resistance within the UC germplasm base has resulted in genotypes that exhibit few symptoms following inoculation with V. dahliae , and these genotypes have been useful in evaluating selection limits and the mechanisms of resistance within this genetic base . However, even asymptomatic plants suffer some yield loss under conditions of high disease pressure . Furthermore, due to the intensified focus on resistance in generating these genotypes, they all express substantial deficiencies for horticultural or productivity traits, and as a consequence this strategy has not been successful in generating cultivars that meet the commercial standards required of modern strawberry cultivars in California.An alternative breeding strategy is to obtain incremental increases in resistance through population improvement, by inclusion of V. dahliae–screening results in a multiple-trait selection system to choose the parents of elite breeding populations. This strategy is generally a superior alternative for developing resistance in cross-fertilized crops such as strawberry. This system is especially useful when inheritance of the resistance is polygenic, or determined by many genes, and back crossing systems are consequently less effective. Furthermore, with multiple-trait selection, Verticillium resistance is considered as one among the many characteristics required to render a cultivar useful to strawberry growers in commercial production. This selection system permits flexibility in the relative weighting of selection intensity for resistance and horticultural traits, and can result in cultivars of balanced commercial utility. Back crossing methods for resistance can be combined with multiple-trait selection for horticultural traits, but this strategy is generally most effective for inbred crops. UC has conducted strawberry breeding continuously since 1930 , and the improvement of traits important for commercial productivity has been substantial . The choice of parents for the population-improvement component of this program has depended in part on resistance to important pathogens,vertical gardening systems and the longest continuous screening effort has been dedicated to Verticillium dahliae Kleb. resistance. This study reports on progress in developing resistance to V. dahliae within the UC strawberry breeding program obtained through parent selection and population improvement since 1994.Between 1994 and 2008, 481 genotypes from the UC strawberry breeding program were screened for resistance to V. dahliae. This sample included 461 advanced selections, genotypes identified with superior characteristics and which might either serve as parents for future generations or eventually be released to growers as cultivars. These selections were obtained from controlled crosses conducted over 18 years , with 11 to 49 genotypes from each cross year. Twenty genotypes were tested from the original germplasm base present prior to 1988. Strawberry breeding proceeds with overlapping generations, but the yearly changes in resistance for the genotypes used as parents from 1988 to 2005 provided a reliable empirical index of genetic progress for this population. The advanced selections tested for resistance in each trial year included those genotypes considered most promising based on their performance in fruiting trials conducted at either the UC South Coast Research and Extension Center near Irvine, Calif. , or the Watsonville Strawberry Research Facility . The genotypes included from any cross year had not been evaluated for resistance prior to their selection for horticultural and productivity traits, and they provide a representative sample of the variation for resistance in that cohort. Changes in resistance parameters over time reflect the outcome of including resistance in the multitrait selection strategy for prior parent choice.
The selection intensities — which reflect the relative importance of the selected trait — applied to the improvement of Verticillium wilt resistance during the study period are impossible to quantify with precision, but the genetic progress reportedhere results from two sources. first, the most susceptible genotypes usually were eliminated entirely, and the participation of moderately susceptible genotypes as parents was limited to a few crosses. Moderately susceptible genotypes were included if they expressed outstanding horticultural characteristics, but they usually served as parents in crosses for just one year. Second, individual genotypic selections were made with knowledge of parental resistance scores, and fewer genotypes were retained from crosses expected to contain high frequencies of susceptible individuals.Runner plants from each tested genotype were immersed in an aqueous suspension of 1.0 x 106 V. dahliae spores per milliliter. All evaluations were conducted at the Wolf skill Experimental Orchard near Winters, Calif. . The inoculum included spores from one to three isolates originally obtained from symptomatic plants found in commercial production fields in California . Each genotype was represented by two plots of five inoculated runner plants per trial year; two noninoculated plants of the test genotype were placed adjacent to each plot, and disease ratings were made relative to these control plants. Plots were distributed between two replicates, and each year’s trial comprised a randomized complete block design. Inoculated plants and controls were established in the field from Oct. 5 to 18 each year. Many of the individual genotypes, 204 of the 481, were tested in more than one year, and the average number of test years per genotype was 1.7. In general, genotypes under consideration for release as commercial cultivars were tested with greater replication over years, and these superior genotypes were also heavily represented in crosses conducted to facilitate population improvement. Genotypes were evaluated in the spring following inoculation, by rating plants for symptoms of Verticillium wilt on a scale of 1 to 5, where 1 =severely diseased and 5 = no symptoms of disease . Individual plots were evaluated four to seven times in each trial year at approximately 3-week intervals beginning with the first symptoms on susceptible genotypes , and a combined score was obtained as the arithmetic mean of scores for all dates . An average resistance score was calculated for each genotype and year combination as the mean for the two replicates.Variation in resistance scores over trial years due to environmental factors — such as differences in ambient temperature, isolate source or inoculum quality — was confounded with genetic differences generated due to selection over years. Several of the genotypes were tested in most of the evaluation years, and the examination of these samples demonstrated relatively small yearly fluctuations in genotypic score. For example, on a resistance scale of 1 to 5, the relatively susceptible cultivar Camarosa had an average score of 2.52 and a standard deviation of 0.46 over all years, and the moderately resistant cultivar Camino Real had an average score of 4.25 and a standard deviation of 0.41 for trials conducted in 11 of the 15 years. This suggests that more than two-thirds of the samples will vary by no more than about 0.435 scoring units from year to year. The effect of trial year was not considered further, but is reflected as part of the error variation in our analyses. Changes in the Verticillium resistance scores due to selection were evaluated by first calculating a composite genotypic score as the average for each genotype over all trial years in which it was tested, then plotting and regressing these composite scores by cross year. The 20 genotypes from the original germplasm sample were considered representative of a base population present in 1987, and are included as such in plots and regression analyses. The average resistance scores were further resolved by calculating the percentage of genotypes in each cross-year population with a composite score greater than 3.0, and thus exhibiting at least moderate resistance. The percentages calculated for each cross year were plotted and treated by regression analysis as described for the resistance scores.The absence of reliable estimates for selection intensities and the presence of overlapping generations precludes precise determinations of the genetic response; however, the general trends for inclusion of Verticillium resistance in the population improvement program are evident from the steady increase in resistance scores over cross years . Regression of resistance scores for the 481 genotypes on their cross year demonstrated a highly significant increase over time, with b = 0.061 ± 0.007 . The predicted change in resistance score from this regression analysis over 18 cross years affected by selection is 1.10 resistance-score units, or a 46.3% improvement over the average resistance score for the original germplasm .