Symptomless infection with Clavibacter michiganensis subspecies sepedonicus during tissue culture propagation of potato

Abstract

Nodal cuttings of tissue cultured potato (Solarium tuberosum L. ’Sangre’) were inoculated with the bacterial ring rot (BRR) pathogen, Clavibacter michiganensis subsp. sepedonicus (Cms). Under commercial tissue culturing conditions, the pathogen persisted through as many as three micropropagation cycles, each consisting of production of a plantlet from an apical cutting. Infected plantlets did not develop BRR symptoms of leaf wilt or chlorosis. Signs of bacterial presence, cloudiness of tissue culturing medium or foliar lesions, were not present when the inoculum level was low, 5.0 × 102 cfu mL−1. A higher inoculum concentration (5.0 × 103 cfu mL−1) reduced plantlet height, but the lower concentration had no effect on height. Growth and viability of tissue from Cms-inoculated plantlets grown on Murashige and Skoog’s medium without sucrose, a source of carbon, were reduced compared to those grown with sucrose, but BRR symptoms were not induced at these inoculum levels. When transplanted to soilless potting medium, a basal portion of cycle 1 plantlets either died or produced a symptomatic miniplant, indicating that Cms survived during the growth of a plantlet into a miniplant. Nonsymptomatic miniplants were produced by portions of some cycle 2 and 3 Cms-infected plantlets, indicating that successive nodal propagation decreased the number of symptomatic miniplants. Bioassays of these nonsymptomatic miniplants were Cms-negative. Incubating plantlet tissue in Richardson broth for 14 d and visually observing for turbidity was not effective in detecting Cms in cultured potato tissue. Because phytopathogenic bacteria may persist in tissue cultures without causing symptoms, pathogen-specific testing of source plants and plant cultures is necessary to prevent transmission of bacteria to cultured stock and field-grown progeny.Key words: Potato shoot cultures, Solanum tuberosum L., Clavibacter michiganensis subsp. sepedonicus, medium with no carbon, bacterial persistence and detection, symptomless infection, Solanum melongena L.