Pathways of infection of Phytophthora ramorum in rhododendron Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/t435gh182

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  • Phytophthora ramorum, a plant pathogen, is the cause of sudden oak death and ramorum blight and shoot die-back. It has a wide host range including many native forest species and common nursery plants. The lack of knowledge regarding infection biology of P. ramorum limits our understanding of its ecology and epidemiology. Pathways of infection were investigated in Rhododendron 'Nova Zembla' using tissue culture plantlets and greenhouse-grown container plants planted in artificially-infested potting medium, or inoculated with a zoospore suspension or mycelial plugs. The presence of the pathogen in plant tissue was determined by isolation onto selective medium and PCR analysis. Histological examinations of tissue samples were performed with fluorescence, scanning electron and scanning laser confocal microscopy. Inoculated roots, stems and leaves were examined to identify pathways by which P. ramorum infects and colonizes rhododendrons. Results indicate that roots can be infected by P. ramorum without causing root rot. Below-ground infections arising from artificially infested potting medium resulted in infection of above-ground stems and leaf petioles. P. ramorum was found in the primary xylem of below-ground and above-ground stem tissue. Inoculation of roots with zoospore inoculum resulted in inter- and intracellular penetration of root tissue. Cysts appeared to aggregate at wound sites and near root primoridia. Examination of inoculated leaves revealed that P. ramorum does not require stomata for leaf infection. P. ramorum spread from inoculated leaves into petioles and stems via the vascular tissue (primary xylem). These results indicate that P. ramorum may be present but not cause obvious symptoms in certain plant tissues. This may contribute to difficulties in detection of infected plants, a requirement for limiting the long-distance spread of the disease with infested nursery stock.
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