An integrated breeding approach for improving levels of resistance for fusarium basal rot of onions

Onion is the second most important crop of New Mexico in terms of farm gate value, and is constantly challenged by several diseases and pests. Fusarium basal rot (FBR) of onions, caused by Fusarium oxysporum f. sp. cepae (FOC), is a major soil-borne disease, that accounts for enormous field, transit, and storage losses every year. Development of FBR-resistant cultivars has been difficult due to the variable nature of the host and the pathogen. The present study was designed to identify a way for increasing FBR resistance levels, while considering the variability of FOC in the breeding and the production fields of onions. The first chapter of this dissertation discusses the approaches used to determine a better procedure of screening to develop FBR resistance in diverse onion lines after a single cycle of selection. The seeds of a selected population were produced using three different screening methods, and were compared with their original population for two consecutive years to determine the effectiveness of screening methods. The field inoculation bulb approach was found to be unreliable, based on random chances of contact between the host and a virulent pathogen in the screening and the evaluation procedures. The seedling inoculation approach did increase FBR resistance in all seven cultivars at the seedling phase of plant growth, but was only partially successful to develop FBR resistance in mature bulbs. The artificial inoculation bulb approach seems promising based on the results of a single year of evaluation. A second year of evaluation is necessary to test the reliability of the approach before its incorporation into the breeding program. A comprehensive analysis, that includes the estimation of realized heritability, sensitivity ratios, and associated costs, revealed that a combination of the seedling inoculation and the artificial inoculation of bulbs would be more effective compared to a single approach. The second chapter discusses the methodology and findings of a study conducted to classify the available FOC isolates into sub-groups based on their virulence levels. Another objective of this study was to determine the future use of PCR-based protocol to classify FOC isolates into virulence based sub-groups, contingent on the correlation between the virulence levels and phylogenetic relationship of partial nucleotide sequences of amplified ribosomal DNA segments. A large number of bulbs exhibiting FBR symptoms were arbitrarily-collected from the onion breeding fields and several commercial onion producing fields during the summer of 2006. Multiple isolates of FOC were identified based on Koch's postulate, and were artificially-inoculated on four genetically-divergent onion cultivars. A significant interaction was observed between onion cultivars and FOC isolates, and would add another layer of complexity for selection of FOC isolate for artificial inoculation purposes. However, some FOC isolates were found to be severely virulent on all four onion cultivars, and could potentially be used for future artificial inoculation purposes. The future use of a PCR-based procedure might not be advantageous to categorize FOC isolates into virulence groups based on absence of correlation with virulence-based sub-groups.