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  • Author or Editor: Elzbieta Z. Krzesinska x
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Au excised twig assay was developed to evaluate cherry rootstocks (hybrids of Prunus avium L.; P. canescens Bois.; P. cerasus L.; P. fruticosa Pall.; P. mahaleb L.; P. pseudocerasus Lindl.) for their tolerance to Pseudomonas syingae pv. syringae van Hall. Twigs of `Napoleon', `Corum', and F12/1 in 1988 and 10 more rootstocks in 1989 were inoculated with water, one avirulent (K4), or one of three virulent strains (W4N54, AP1, and B-15) of bacteria at 105, 106, and 107 colony forming units (cfu)/ml in 1988 and with 107 cfu/ml in 1989. Evaluation of browning and gummosis at the inoculation site after incubation for 4 weeks at 15C and high relative humidity revealed no gummosis or browning on twigs inoculated with water or the avirulent strain. The amount of browning and gummosis induced by concentrations of 106 and 107 cfu/ml of the virulent strains was, in general, not different within genotypes. `Napoleon' and `Corum' had significantly higher browning and gummosis ratings in 1989 than F12/1 and the 10 rootstock selections, most of which did not differ from F12/1. Rootstocks Gisela (Gi.) 172-9 and Gi. 169-15 had higher incision browning than F12/1 in some instances.

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The syrB gene required by Pseudomonas syringae pv. syringae van Hall to produce the phytotoxin syringomycin is activated by plant signal molecules. Extracts from twigs of 12 cherry (Prunus) genotypes were tested for their ability to induce syrB::lacZ fusion in P. syringae pv. syringae strain B3AR132 to determine whether signal activity is correlated with susceptibility to bacterial canker. One-year-old twigs of `Napoleon', `Corum', and 12 cherry rootstocks (F12/1, `Colt', M×M2, M×M39, M×M60, Gi 148-1, Gi 148-9, Gi 154-2, Gi 154-5, Gi 169-15, Gi 172-9, and Gi 173-9) were tested at concentrations of 0.2, 1.0, and 2.0 mg twig dry weight/ml solution for their ability to induce syrB::lacZ fusion, as measured by β -galactosidase activity. Extracts from all cherry genotypes induced syrB::lacZ fusion, but to varying degrees. The highest β -galactosidase activity was observed in `Napoleon' and `Corum'-the most susceptible genotypes; activities were two to four times higher than that of F12/1, a disease-resistant genotype. Activities higher than that of F12/1 were induced at the lowest extract concentration by rootstocks M×M60, Gi 148-1, Gi 148-9, and Gi 154-5, whereas rootstocks M×M2, M×M39, Gi 154-2, Gi 172-9, Gi 173-9, and `Colt' were not significantly different from F12/1. At the two highest extract concentrations tested, only `Napoleon' and `Corum' consistently had higher induct&m activity than F12/1. At high extract concentrations, interfering substances seemed to suppress or antagonize the induction of syrB::lacZ fusion. These results suggest that susceptible genotypes contain higher signal activities than resistant genotypes.

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