> >I think that the > seasonal nature of moisture is what makes these bulbs adapted to heavy > soils. I agree partially. I suspect the seasonal nature of the moisture is what excluded most grasses and other forbs, allowing bulbs the opportunity to compete, and evolution provided the tolerance for dense soils. But it is true that phytophthora usually requires moist soil for zoospores to function so a dry period would tend to limit their ability to spread and re-infect. I have seen a dramatic increase in tolerance for clays among tree species due to steep slope which prevents waterlogging. This means that the small, polar pores that would normally be saturated with water would be free to accept air. Tim Eck