Going back to considering the Melampsora rust fungus in the leaves of my poplar trees (Nov 28).
My rough calculations suggested that the tree had about 100,000 leaves.
I estimated that on average, each leaf had about 100 small areas of encysted teliospores
(though some have over 1,000, often contiguous and covering fairly large areas of leaf).
Now I have looked at sections through some of those small areas,
and suggest that each of them contains about 500 (resting) telial cells. Now for some real guesswork. I think that about 25% of the teleial cells may be eaten
by invertebrates during the winter, though they are heavily impregnated with a melanic substance, and may not be at all palatable. Each teliospore should give rise to a basidium, producing and discharging 4 basidiospores. So we can put together the following sequence.
100,000 (leaves) x 100 (telial areas) x 500 (telial cells) x 0.75 (some eaten) x 4 (basidiospores per cell) = 15,000,000,000 or 15 billion basidiospores. My calculations should not be off by more than one order of magnitude, so the number of basidiospores floating in the air next spring will be enough to ensure that the new leaves are well and truly infected. That's how the fungi work...