Evaluating resistance to Alternaria dauci and related traits among diverse germplasm of Daucus carota

Alternaria leaf blight (ALB) is among the most devastating and economically significant disease of carrots worldwide. Partial genetic resistance has been reported, but carrot germplasm has not been extensively evaluated. Therefore, the first objective of this thesis is to evaluate the USDA carrot germplasm collection to identify new sources of ALB resistance. The second objective is to examine the variation of epicuticular leaf waxes in D. carota ssp., since glossy leaf surfaces are a relatively rare trait that occurs in some wild carrots, and have been hypothesized as a possible mechanism to curtail ALB infection and possibly aid in resistance to abiotic (heat, drought, and salinity stress, etc.) and biotic (pathogens, pests, etc.) stresses. Finally, in some new wild carrot sources of ALB resistance, intercrosses with cultivated carrot presented several reproductive abnormalities that may be characterized as “hybrid breakdown” so those abnormalities are also evaluated as part of this thesis. In this thesis, 993 Plant Introduction accessions (PIs) were evaluated for ALB disease resistance across years and replications. Complete resistance was not observed, however 33 PIs exhibited partial resistance. A comparison of rank scores for 64 accessions grown previously in FL with scores for the same accessions in WI revealed no significant location effects between scores (p-value=0.128). An evaluation of the association between ALB iiseverity and domestication status, flowering habit, leaf glossiness, storage root color, and geographic origin revealed that PIs with purple or white root colors had slightly more resistance than those with orange, yellow, or red color, but no other traits were correlated with ALB resistance. Further, in examining the quantity, structure, and chemical composition of epicuticular leaf waxes, leaves that exhibited the glossy phenotype had more total wax load than non-glossy, and had globular wax granules on the leaf surface. Triterpenoids constituted 35-65% of all identified leaf wax compounds followed by alkanes (comprising 10-45%), carboxylic acids, and butane-1,1-dicarbonitrile. Five triterpenoid precursors were identified. The most abundant was α-amyrin, followed by oleanitrile, lupeol, β-amyrin, and betulin. However, more wax load did not correlate with higher triterpenoid leaf cuticle content. ALB resistance was not found to correlate with leaf glossiness. Lastly, in an effort to introgress resistance to Alternaria leaf blight (ALB) from an inter-subspecific cross between D. carota ssp. sativus and D. carotassp. commutatusRILs were advanced, yet only 23 lines produced adequate seed to the F6 generation. The incidence of male sterility increased from 18% to 38%, and the yield of viable seed produced decreased from 55% to 17% during RIL development. Forms of male sterility identified resembled several described before and a novel source was also discovered. The average RIL height of flowering plants was reduced and 75% exhibited an abnormal umbel morphology, like the wild parent, but 68% of the roots of the F5-F6 RILs were smooth with no lateral branching, a trait in common with the cultivated parent. Though there were two generations of selection for ALB resistance, a significant improvement of resistance was observed (p-value <0.001) in the mean resistance scores of RILs selected for high and low levels of ALB resistance.