On our previous episode, Dr. Shishkoff determined that the corpses died
of a root or crown problem. She set up moist chambers containing plant
tissue, and read up on larch diseases.

A diagnosis of root problem limits the field of possible murderers. It
also brings up the key question that has to be answered: how did the
pathogen get in? Root rot pathogens spread very slowly, unlike foliar
pathogens that are spread by wind or rain. Robert's trees were
collected in Minnesota and raised in Illinois, so one possibility is
that the trees had the problem (on their roots or in the soil) when
collected, and it took a trigger (the hottest July in recorded history)
to elicit disease. The other possibility is that the disease came in
later and somehow got on more than one of the trees.

One obvious possibility is Phytophthora, common root killer. Although
found on roots, it can be spread by rain or irrigation. This hypothesis
was easily ruled out when none was isolated from tissue.

Another is verticillium wilt, which is ubiquitous in soil and will
often kill trees suddenly with the onset of a hot dry spell. I didn't
see verticillium in my plates, but it is often hard to isolate it from
dead tissue. On the other hand, most references call larch "resistant"
to vert. So I rule it out.

Two other possibilities were ceratocystis wilt and pine wilt nematode.
However, both of these are vectored (transported) by bark beetles, and
I saw no evidence of beetle damage. Nor did I isolate ceratocystis or
pine nematodes. It's too bad, because beetles would be a good way to
answer the question of how the pathogen got there.

So what did I isolate? Nothing much from the stem, but two things from
the roots. We'll discuss these suspects in our next episode of CRIME
SCENE INVESTIGATION: BONSAI!