GardenBanter.co.uk - CSI:Bonsai

On the previous episode of CSI:Bonsai, Robert had a group of larch that
were dying one by one. He suspected a mysterious wilt that had been
reported by other larch growers, but whose cause had never been
determined. Robert decided to send a couple of dead trees to Dr.
Shishkoff, forensic botanist.

Today's episode: Robert packed the trees better than 90% of people
sending specimens to plant diagnostic clinics: he cut each tree into
several pieces for easy packing, then placed the pieces in a ziplock
bag along with a moist paper towel, then shipped the specimens by
priority mail. They arrived today in good shape.

I immediately unpacked them and examined the symptoms. One tree was
extremely dead: the inner bark was a uniform brown from root to shoot
tip. The other tree, however, showed a gradation from extremely brown
bark on the roots and crown, going to healthy bark at the tip. This is
extremely important, and explains why I insisted on getting very recent
corpses. First, the disease obviously started at the bottom and moved
up: this tells us we're dealing with a root or crown pathogen, not a
stem blight pathogen. Second, the most likely place to isolate the
pathogen is at the margin between diseased tissue and healthy tissue.
In a plant that's been dead for a while, saprophytes and secondary
pathogens swoop in like vultures and make it hard to find the causal
agent.

I took samples and plated some on regular media (V-8 agar, made from
V-8 juice and a solidifying agent)and a selective medium specific for
water molds (PARP, with sterols and fungicides added). I also put
pieces of plant tissue on moist filter paper in a moist chamber, to
encourage the pathogen to fruit (easier to identify that way). I'll
check on these next week and see what shows up.

Nina