Episodic Abiotic Stress as a Potential Contributing Factor to Onset and Severity of Disease Caused by Phytophthora ramorum in Rhododendron and Viburnum.

Phytophthora ramorum attacks many forest and nursery species, primarily causing trunk or stem cankers, foliar blight, and dieback, and in some species root infection has been demonstrated. However, the abiotic and edaphic factors that influence infection and disease development are unresolved. Root infection by P. ramorum and the potential for mild abiotic stress in disease predisposition was examined with Rhododendron hybrid Cunningham's White and Viburnum tinus cv. Spring Bouquet. To impose water stress in a uniform and synchronous manner, osmotic stress induced with 0.2 M NaCl was selected. Roots were exposed to NaCl for 16 to 24 h in modified hydroponic culture or standard potting soil, removed from the NaCl, and then inoculated with zoospores. In the hydroponic regime, disease symptoms developed in Rhododendron and V. tinus plants within 1 week after inoculation of salt-stressed roots, whereas symptom development was delayed in nonstressed, inoculated plants. Microscopic examination of roots from both species revealed that their apices were covered with sporangia of P. ramorum. On potted Rhododendron plants inoculated by applying zoospores directly to the soil, stem lesions developed rapidly in salt-stressed plants, with death of the plant occurring within 4 weeks after inoculation. Nonstressed plants survived for 6 to 8 weeks before succumbing to disease, and symptom development in these plants was delayed by 1 to 2 weeks relative to the inoculated, salt-stressed plants. A postinfection episode of salt stress to inoculated roots in the hydroponic regime resulted in significantly faster development of stem lesions in Rhododendron relative to nonstressed, inoculated plants.