On our previous episode, we excluded a number of the "usual suspects"
for the Illinois mass-murder of larches, and narrowed the investigation
down to two suspects isolated from the root systems of the newly
deceased tamaracks.

Episode 4: The first suspect was Fusarium. It causes root rot, but it
is a common member of the soil community, so it is almost always
possible to isolate it from a root system, whether it was causing
disease or not. I was not surprised to see it.

The second suspect was a fungus called Macrophomina phaseolina. It
causes a disease called "charcoal rot" on more than 300 species of
plants, including soybean, sunflower, and conifers. It also causes,
with the help of Fusarium, a black root rot of various plants. I
recognized it immediately because it forms little clumps of mycelium
called "microsclerotia" that can survive for years in soil. The roots
of the larch were covered with them (I'll post a picture shortly on the
IBC Gallery) and when I picked these propagules out of the root and
plated them on media, I got characteristic colonies of M. phaseolina.
What puzzled me, however, was that this is a common and serious disease
of *Southern* nurseries, but I couldn't really stretch the definition
of "southern" to fit Illinois, much less the Toronto area where John
Biel was reporting this problem. I also couldn't come up with a way
for the larches to have become infected: the first hypothesis, that the
trees had been infected when collected, was possible, but unlikely
(again, Minnesota isn't "the south"), and the second hypothesis, that
the trees had become infected since that time, was hard to make
plausible, since Robert had never used soil in his potting mix. How
would the microsclerotia have gotten to the roots?

So I read up on charcoal rot. I discovered that the fungus is present
in soil world-wide, but it doesn't cause disease except in hot, dry
climates. Hmmm- which, this year, included MOST OF THE US! I did an
internet search of disease clinic reports. Many plant disease clinics
publish their reports online, which is very helpful. I help edit a
journal called "New Disease Reports", and I'm always having to
fact-check manuscripts called "First Report of [disease] on [host] in
[geographic region]", and I have to confirm that it is, indeed, the
first report of whatever whereever. Anyway, I found a couple of
reports of charcoal rot on conifers in Wisconsin. Close enough.

I was almost convinced on the "who" and the "why", but I couldn't
figure out the "how". I sent Robert an email asking him aimless
questions about whether he set his pots on the soil surface, adding
"Here's an even stranger question: do you live near any soybean
fields?"

[we pause for commercials]


One sponsor is "Turface". A rugged football player on a
turface-enhanced playing field holds up a tiny potted pine and says
"Great for mame, too!"



Another sponsor is "Superthrive". A man in a bad toupee talks really
fast while a tomato plant next to him grows visibly while we watch.



Then there's a commercial for "Evergreen Gardenworks". Wow. Brent
looks like a lost member of "Z Z Top"!



[back to our program]

A few hours later, Robert wrote back saying "Last year there was a
soybean field located about 150' north of the larch. This year there
is a soybean field located about 100' south of the larch." Robert
owned the land and rented it to farmers.

I couldn't believe it! Here was "how"! I had read that in soilbean
fields, the microsclerotia persist in the debris from the last year's
soybean crop, and this debris, lighter than soil, can spread in the
wind. I wrote back asking Robert to ask the farmers if they had ever
seen charcoal rot in the field. He wrote back this morning saying that
the tenant had, indeed, seen charcoal rot.

Now, this is the part of the show where you arrest the snarling
perpetrator, the death of innocence is avenged, and future senseless
bloodshed is averted. Only in this case, how does one stop the
pathogen? There is no treatment for charcoal rot; everything I read
recommended starting over in fresh or fumigated soil. As usual with
plant pathologists, we think in terms of yield loss reduction, not in
terms of saving cherished trees.

I told Robert to put his remaining trees in a cool spot. Once autumn
comes, they will be able to fight off the pathogen, but there's nothing
else that can be done for them at the moment. But what about his other
conifers? Are they at risk? Is there anything that can be done to
protect them from blowing debris? As always, the best defense is a
well-fertilized, well-watered healthy tree. You can't stop the summer
sun from shining or the wind from blowing but you can make your trees
strong. I'd also recommend "Rootshield" and other biological-control
soil conditioners; they add mycoparasites (fungi that attack other
fungi) to your soil, and although they aren't totally effective, they
have been known to control diseases spread by microsclerotia. It's
worth a shot. These types of fungi thrive in freshly prepared
composted bark, so it would be worthwhile to repot bonsai into mixes
heavy with a pine bark component. Care should be taken when repotting
to injure the root system as little as possible- each fresh wound is an
entryway for soilborne pathogens.

And so we end this season of "CSI:Bonsai" with the cast standing in a
soybean field, hoping for cooler weather.....