Jim Lewis wrote:
Actually, I learned somethin' here (which is always nice). I
thought Phytophthora was a fungus. So IS there a layman's
(common) term for Oomycetes (which actually sounds kinda neat).


The layman's term for Oomycete is "water mold". Pythium and
Phytophthora have motile spores (they look like sperm, only they have 2
tails) that require water to spread. One of my favorite things is to
induce zoospore formation and then watch thousands of them swimming
around. They're so cute!

Here's what makes Oomycetes so versatile, however: they have 4 kinds of
spores that are dispersed in different ways. Take the sudden oak death
organism: the sporangiospores are formed *on* the leaf and are spread
by water splash from tree to tree; these burst open and release the
zoospores, which either infect leaves directly, or are spread in rivers
hundreds of miles downstream. Meanwhile, thick-walled chlamydospores
form within the leaf, and when the leaf falls off, they are buried in
the soil, where they can persist for at least a year (by my studies!).
If male colonies meet female colonies, Oospores are formed. Oospores
are the sexual stage, and can therefore recombine characters to create
more virulent organisms; they are also thick-walled, and can persist in
soils for a very long time (in other phytophthoras, for at least 10
years). Luckily, with P. ramorum, we only have one mating type in the
US, so the sexual state can't form. Yet.

Anyway, these 4 spore types explain why phytophthoras have been the
most devastating plant pathogens in history. They caused the great
potato famine that changed the history of Ireland (and made the US a
more exciting and dynamic place, thanks to the Irish immigrants who
fled here).

To turn to bonsai: you can make life inhospitable to water molds by
using clean pots and potting mix, to avoid chlamydospores and Oospores.
You should irrigate so that water doesn't spread from pot to pot,
either by pooling or by splash. You should avoid heavy potting mixes
that get soggy. And you should be careful to root-prune in a clean
place so the fresh wounds don't get inoculated.