If a plant is infected, can the cycle be broken?
Billy on the Florida Space Coast


It's easy to break the cycle with cedar-apple rust, because the spores
on juniper can ONLY infect apples, and the spores on apples can ONLY
infect junipers. Keeping a reasonable distance between apples and
junipers will break the cycle (as long as you don't live near an apple
orchard or a juniper waste area). I've told this story dozens of
times, but once mo I had a yard with a big juniper in it. I had
two apple trees: one was a yard away from the juniper, the other was 3
yards away. The close one was being killed by constant exposure to
rust spores, but the one 2 yards farther away was barely infected.
This is because spore deposition from a point source follows an
exponential decay pattern: a lot of spores are splashed near the tree,
but very few get even a few yards away.

The decay curve is steep for water-dispersed spores (the spores on
juniper), because water droplets are large and don't splash very far.
In addition, the infectious period is only a few weeks in spring. So it
is easier to keep roseaceous plants disease-free than junipers. You
could bring them into the garage during rainstorms, for instance. It
is not as easy to keep junipers disease-free. The air-dispersed spores
coming off of apple leaves travel much farther in the wind; there is
still an exponential decline in spore concentration, but it is much
shallower; a juniper a mile from an apple could still get infected.
However, the galls on juniper can be pruned off easily with little harm
to the design of the bonsai except in extreme cases (someone once gave
me a tiny juniper bonsai with quince-apple rust on the main trunk: it
was cute, but doomed).

Nina, proud owner of red cedar bonsai as well as crabapples.