Poor drainage is not exactly what it sounds like - the plants aren't
sitting in a puddle - as long as there is water flowing,
the zoospores are disseminated and infect feeder roots. This year I
lost a grevillea growing on a sandy bank, and I have read of avocado
orchards in Australia and also, I think, Israel, where there were heavy
losses due to a rainstorm where there was water in the soil for just
a few hours.