Graduate Thesis Or Dissertation
 

Development of an in vitro and modification of an in vivo bioassay to screen cherry genotypes for response to inoculation with Pseudomonas syringae pv. syringae

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  • The bacterium Pseudomonas syringae affects different crops worldwide. In the Willamette Valley of Oregon, P. syringae causes bacterial canker in sweet cherry, severely limiting its production. High grafting of susceptible sweet cherry cultivars to resistant rootstocks is practiced in the Willamette Valley to reduce incidence of this disease. The research objective was to screen for potential resistance to bacterial canker using a modified in vivo and an in vitro bioassay. The severity of infection of sweet cherry genotypes inoculated with Pseudomonas syringae pv. syringae was rated, using both an in vitro excised leaf bioassay and an in vivo twig bioassay. The response of in vitro excised leaves after inoculation with a mixture of four highly virulent P. syringae pv. syringae strains (SD443, SD447, W4N54, and W4N108), at two concentrations (106 and 108 cfluJml) was evaluated. The response to inoculation with water and a P. syringae pv. syringae strain (JL2000) with low virulence was also rated using the in vitro bioassay. Necrosis of leaves was recorded on a scale from 0 to 4 with a score of 4 indicating complete leaf necrosis. The in vivo bioassay used twigs as the plant material. A browning response to inoculation with water and a mixture of four pathogenic strains (SD443, SD447, W4N54, and W4N 108) at 108 cfulml was rated on a scale from 1 to 4. A score of 1 indicated yellow pith while a 4 indicated completely brown pith. Gummosis and callus were also noted in the in vivo bioassay. Results from the in vivo and in vitro bioassays showed Mazzard x Mahaleb (MxM) genotypes had the smallest response to inoculation with the mixed pathogenic treatments. The in vitro bioassay further indicated that Pi-Ku genotypes 4-11, 4-22, 4-20 and 4-17; and Giessen clones 148-1 (GiSelA 6), 192-2 (GiSeIA 12), 148-9 (GiSe1A 8) 148-8 (GiSelA 7), 196-4, 3 18-17,195-20, 497-8 and 473-10 (G1Se1A 4) might be regarded as potential resistant rootstocks. Weiroot genotypes, tested only in the in vitro bioassay, showed a large necrotic response. In the in vivo bioassay Giessen clone 169-15 showed a low browning response and a high gummosis response. This investigation identified genotypes possessing potential resistance to bacterial canker. These genotypes should be evaluated under field conditions to determine disease resistance in the orchard environment. Results from field trials will help growers determine the optimal rootstock for their production system. Ultimately, these assays may be useful in screening sweet cherry scion genotypes for resistance to bacterial canker.
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