I got an email from a Florida bonsai grower named Tom who grew lots of
figs and said:
"What happens is a swollen node develops on the trunk, usually toward
the base, although not always. From out of the burled growth myriad
buds burst, creating a mass of
bushy growth that never develops into healthy branches. " He said he
used to have only a few of these galls, but now he's seeing more and
more, and so are other Florida bonsai growers. He wondered what they
were.

I googled "fig gall" to see what possibilities there were, and saw
there were two: galling on fig was reported for two pathogens, a fungus
and the crown gall bacterium, agrobacterium tumefaciens. I suspected
crown gall, but not being a bacteriologist, I emailed Tom and suggested
he get a diagnosis from his county Cooperative Extension office. There
are kits made by AgDia where you can take a sample of plant juice, add
it to each well on a a plastic plate with about 90 wells on it, and see
which wells turn blue. You can use a computer program to copy the
pattern of blue wells, and you will get the percent match to any
bacterium that's entered in the computer program. Most extension
offices can send a sample to someone who has this program (on Long
Island, we had it). That would be the easy way; there are
old-fashioned ways to identify bacteria by growing them on 30-40
different kinds of media to see what substances they can or can't
digest, but few people are trained nowadays to do that.

Tom wrote back to say that Florida Cooperative Extension had a policy
of not doing diagnoses on bonsai, a response that shocked me.
Cooperative Extension exists to serve the customer! Undoubtedly they
were afraid of messing with plants that people give *names*. Normally
plant samples sent to a clinic are torn into pieces, but more care
would be needed with plants that have been trained for years. They
probably smelled "lawsuit". Anyway, that left me no choice but to tell
Tom to send a sample to me.

Stay tuned for the next episode, and in the meantime, go to the IBC
main gallery to see a picture of one of Tom's figs.

Nina, plant detective.

Part II: In our last episode, I was sent some fig trees from Florida
that had galls on them. In this episode, I examine them with the help
of an expert.

The samples were Fed-Ex'ed and arrived last Friday in beautiful shape.
This is especially important with bacteria; if the samples get old,
other bacteria begin to invade the tissue, and you can't tell which was
there to begin with.

I agreed to take the samples because my friend Fari is a bacteriologist
and I knew he'd help me. Fari was a professor in Iran and when the
Shah fell, he fled to the US to keep his son safe (otherwise he'd have
been drafted into the Iranian army). Fari had a hard time finding a
proper job, and for a while he worked on a turkey farm, and then for a
corporate plant-care company (the people who put the weeping figs in
the lobby, and the poinsettias by the elevator at Christmas).
Eventually he got a job at the USDA, where he brings great joy to
everyone who works here!

Anyway, he and I unpacked the box, and we were both sure the galls were
caused by the crown gall bacterium (look at the picture on the gallery-
those swollen warty nodules are characteristic). Fari said when he
worked for the plant-care company he saw hundreds of figs with crown
gall, and they looked just like this. Still, we had to be sure. The
first step was to try to isolate the bacterium. Although the galls
have quite a lot of volume, the bacterium is found just under the
surface, not inside it, so Fari used a sterile razor blade to pare off
the rind (which was not sterile), then sliced off a piece just under
the surface of the gall. He put the tissue in a sterile petri dish in
a few drops of sterile water and teased the tissue apart to release the
bacteria. After waiting for half an hour, he took some of the water
and streaked it on a plate of nutrient media.

On Monday I checked the plates and they were dotted with little white
convex colonies of bacteria. They looked like Agrobacterium, but were
they?

Stay tuned for our next episode!

Part III: What is crown gall, anyway?

It's a bacterium that's closely related to Rhizobium, which causes the
nitrogen-fixing nodules on legumes. It shares that organism's ability
to cause tumors, but it has no beneficial effects to the plants it
infects. And it infects a lot of plants: so many I couldn't find a
list of them, although apple, apricot, peach, rose, euonymus, cypress,
hibiscus, lilac, privet, viburnum, and willow are singled out as
suffering economically important damage. The galls don't kill their
hosts, but make them unsightly and poorer yielding. With bonsai, the
galls will weaken the branches above the gall, and will (at least with
fig) make the tree less appealing a subject.

The bacterium can live up to 10 years in soil, so once it appears in an
area, it stays there. All it needs is a fresh wound in order to infect
its host. That wound can be from pruning, rodent gnawing, lawnmower
damage, or even from wind whipping sand or debris against a trunk. For
bonsai, obviously, the major avenue of infection is through pruning.
The bacteria will stick to the tool and be spread from one plant to the
next.

Once the bacterium enters a wound, it does an amazing thing: it
releases a loop of DNA to be absorbed by plant cells and become
incorporated into the plant's chromosomes. The DNA disrupts the normal
hormone balance in the cellls, and they start to proliferate, creating
the gall. The altered cells also produce novel substances that leak
from the cells and can only be digested by the crown gall bacterium.
Nice set-up, isn't it?

As the gall grows, it eventually crushes the xylem in the plant's
vascular system, reducing the ability of the plant to use water,
stressing it, and making it more susceptible to cold damage and other
pathogens.

The cure? There is none. Small galls can be pruned off, but the
resulting scarred tree may be useless. If galls can't be removed but
the galled tree isn't unsightly, it can be kept, but there's always the
chance it will infect other trees, so generally such plants are
discarded. The soil should also be discarded along with it and the pot
sterilized before being used again.

Prevention is the key. Start with clean stock. Tools should be
cleaned after use on every plant: they can be sterilized in 70%
ethanol, 10% bleach, 0.5% potassium permanganate, or (according to Jim
Lewis) in Lysol. When pruning cuts are made, they can be treated with
a commercially available product that contains another bacterium,
agrobacterium radiobacter, which will colonize the wound and keep the
crown gall bacteria out, but which can't cause galls itself. This is
only practical on a site where crown gall is well-established, and
isn't necessary otherwise.

Stay tuned for the next episode, when we'll learn how to infect a
carrot with crown gall!

Part IV: Last episode, we learned more about crown gall; this episode
we learn a simple way to identify it.

Fari the bacteriologist had to go on vacation, but before he left, he
told me a simple way to identify crown gall using only a carrot. This
is something that any of you could do, or your children could do as
science projects.

First I cut some gall tissue as described before and teased it apart in
distilled water to release the bacteria. Then I surface-sterilized the
carrot in bleach, then used a sterile razor blade to cut it into disks.
I put the carrot pieces in Petri plates with moist filter paper on the
bottom and wet the top of each carrot slice with the
bacteria-containing water. I also took some of the colonies I isolated
last week and smeared them on top of some of the slices. I sealed the
Petri dishes to keep them moist and put them under fluorescent light.
Now we wait 2 or 3 weeks. If the bacterium is Agrobacterium, weird
tumors will arise from the carrot slices.

I inoculated the carrots on Monday; I'll be checking them every few
days to look for tumors. Wait patiently for the next episode.

Today when I looked at the carrot disks, I saw that the xylem
parenchyma had hypertrophied. That's how I talk at work: sorry. The
center of the carrot disk was becoming lumpy, and the lumpiness was
turning green, which is characteristic of crown gall on carrot. I've
posted a picture on the "General discussion" gallery on the IBC
webpage. This means that, without a doubt, the fig with galls was
suffering from crown gall.

Nina.

PS- My friend Fari wants us to inoculate a baby tomato plant: that's
another surefire method of identifying crown gall. Tomatoes are very
suceptible, and will form galls quickly where you prick them with a
needle coated in bacteria.