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The way it works is like this. When the plant is short of water the tips and edges of the leaf go brown. This is logical as these areas are where there is most air flow (remember that things dry out quickest where these is a bit of breeze) and is furthest from the water supply system within the plants, so this is likely to dry out first. In this case the browning is reported as being within the leaf and not at the edges. Therefore the damage is seemingly inconsistent with a drought situation. Experienced gardener often find that plants with water-logged roots develop browning within the leaf and not at the margins or tips. The mechanism for this is less clear-cut. One assumes that the physiology of the leaf is adversely affected by the reduced root function brought about by lack of oxygen following excessive water filling air spaces in the soil. Inexperienced gardeners frequently underestimate the importance of the root zone to the overall health of the aerial part of the plant.
It may be that the trees in question are being grown in a slow draining soil and are being watered lavishly - this might explain the reported symptoms. The trees in question are clearly large and one would assume have high water demand in the antipodian spring but the evidence does not appear to to support that this is involved here. Such large trees will probably have been bought from a nursery specialising in such trees and such an outlet should be able to supply irrigation advice relevant to the soils and climate of the region. Their advice should be sought. My second cousin lectures on horticulture at a college in Sydney and she reports that British gardening experience is of limited value in the Australian climate so I might be quite wrong!
Boggy
Beware the bat-eared bogweevil
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