INVASION OF BASSENDEAN DUNE BANKSIA WOODLAND BY PHYTOPHTHORA-CINNAMOMI

The rate and mechanism of the spread of Phytophthora cinnamomi in Bank sia woodland of the Bassendean Dune system north of Perth, Western Aus tralia was studied. Aerial photographs were used to measure the spread of disease in fronts longer than 5 km over 35 years. Fronts in upper slope positions moved downhill and uphill at 1.01 and 1.13 m year(-1), respectively, while fronts in low-lying positions moved downgrade and upgrade (with and against the direction of water table flow) at 1.30 and 1.20 m year(-1), respectively. Fronts in low-lying areas spread si gnificantly faster than those on upper slopes. Excavations to 2 m dept h of the root systems of 21 dying plants of Banksia attenuata R.Br., a n overstorey co-dominant, revealed that 28% (3.9 roots/tree) of all fi rst-order roots were infected. Assessment of the proportion of roots i nfected revealed a significant (P < 0.01) departure from uniform distr ibution of disease down the profile in both upper slope and drained fl at sites. In 14 trees on dune slopes, 6-40 m above the aquifer, infect ion rate was higher than expected in roots lying at 21-40 cm depth, bu t lower than expected in roots below 1 m. Seventy three percent of inf ected roots lay in the top 40 cm of soil. In seven trees on a drained flat, 2-3 m above the aquifer, infection rate was high in the 21-60 cm horizon and also in roots below 1 m. Even though Phytophthora cinnamo mi was active in the vicinity of the water table in trees on drained f lats, the slow, steady spread of the disease in all landscape position s suggested that the primary mode of fungal invasion was through roots of susceptible vegetation. No evidence was found of accelerated sprea d of disease caused by the dispersal of zoospores.