Effect of Flooding on Development of Phytophthora Root Rot in Fraser Fir Seedlings

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Abstract
The effect of flooding on root rot caused by Phytophthora cinnamomi in Fraser fir (A. fraseri) seedlings was assessed. Soil infested with propagules of P. cinnamomi but without seedlings, and infested and uninfested soil into which 2-yr old Fraser fir seedlings were transplanted, was continuously flooded for 0, 24 and 48 h. Inoculum density of P. cinnamomi was quantified at 2, 9 and 23 days and seedling mortality was assessed 33 days after flooding. In treatments with infested soil and seedlings, inoculum density was greater (P = 0.01) with flooding than without flooding at days 9 and 23. Final seedling mortality and infection were also higher (P = 0.01) with the 2 flooding treatments (24 or 48 h) compared to no flooding. Inoculum density declined similarly among the treatments without the host regardless of the flooding period. A separate series of experiments demonstrated that significant increases in the production of secondary inoculum occurred within 3 days of flooding infested soil containing the host. The greatest rate of increase in secondary inoculum was recorded between days 5 and 7 following flooding. In experiments with zoospore suspensions used as inoculum, no difference in seedling infection was found between flooded or unflooded treatments at any inoculum level tested. Flooding (24 or 48 h) apparently increased seedling infection and mortality by promoting productin and dispersal of inoculum rather than predisposing the host.