The effect of waterlogging on infection of Eucalyptus marginata seedlings by Phytophthora cinnamomi

Three‐ to four‐month‐old Eucalyptus marginata Donn ex Sm. (jarrah) were grown in the glasshouse in peat/sand at 20° C root temperature, infected with zoospores of Phytophthora cinnamomi Rands, added either 10 cm below the soil surface or flooded onto the soil, and various waterlogging regimes were imposed. When plants were infected but not waterlogged, the proportion of infected root length adjacent to the point where zoospores were added was low. More lesions were formed per metre root length on tap roots than on lateral roots, but lesions on lateral roots were longer than on tap roots. When plants were waterlogged for a 4 d period before the addition of zoospores, there was no increase in root infection. When plants were waterlogged at the same time or after the pots were inoculated, there was a significant increase in the proportion of root length infected. This was due to an increase in the number of lesions and not, in four of the five harvests, to an increase in lesion length. When inoculation and waterlogging were imposed at the same time, the increased number of infections was probably due both to the increased attraction of zoospores to anaerobically respiring roots and to increased zoospore motility in flooded soil. When waterlogging was imposed several days or weeks after the plants had been infected, the increased number of lesions was unlikely to have been due to residual zoospores in the soil, because their numbers declined exponentially. It was also unlikely to have been due to sporangia developing de novo on the roots after waterlogging had been imposed, because the O2 concentration of the soil solution was inadequate. It is probable, therefore, that the new infections originated from zoospores liberated from sporangia already formed on lesions before waterlogging was imposed. When plants were infected but not waterlogged, there was no effect on the leaf diffusive resistance, leaf relative water content and shoot xylem pressure potential between 1 and 28 d. When plants were flooded, the daily water loss decreased more rapidly and plants died more rapidly in the inoculated and waterlogged treatments than in the treatments which were only waterlogged. Infection by P. cinnamomi did not affect the proportion of xylem vessels occluded with tyloses in seven of the eight unflooded, and seven of the eight flooded, comparisons