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Saturday, June 13, 2009

Iris Borer and Bacterial Soft Rot: Perils of Poor Horticuture

I have been working like a crazy woman in the garden....trying to catch up on tasks I should have started in April (plant division and moving plants to better locations) and weeding.

I have also been in a panic as my irises need major attention. Because of poor horticultural habits (I let the weeds and other plants crowd the iris and also didn't divide last year) and my heavy soil, I was beginning to lose many of my clumps of German or bearded iris.

At left you can see a clump which is suffering. When you get up close to it and poke around a bit, you can easily see the problem.

One of the biggest pests for German iris is the iris borer. The borer is the larval stage of a moth. The moth lays the eggs in or around the iris and when they hatch, they start eating.

Here is the tell-tale sign of the iris borer. See the eaten edge of the leaf? The discoloration? The healthy leaf is a really cool shade of green, many are bright green but some have a blue-green cast to them. See that area which looks like it is wet down near the base of the leaf? That's the sure sign. The small circular brown spots are from a fungus which attacked in the cool spring we had, and aren't related to this problem.

This next shot, you can easily see the holes the borer chewed into the leaves and when pulled the leaf away from the fan, the borer was right there. Look closely and you can see this ugly beast. He is sort of yellow/tan with a reddish maroon head, right above my thumb in the eaten area of the leaf.

Most garden manuals say that you can press the leaf and kill the borer, but in several of the leaves which I have been working with, the borer is so small that I wouldn't have squished him. The borer will eat it's way all the way down through the leaf and tunnel through the rhizome (the root of the iris which some people incorrectly refer to as the "bulb").

One of the other problems with the borer is that they carry a bacteria on their body which enters into the rhizome from the damaged areas. I didn't have much of a problem with this in Connecticut, so I can only think that the heavier soil I have here makes this a greater problem.

Here, you can see where the soft rot has gotten into a part of the rhizome. The central yellowish blob was once a stalk and now has completely rotted away. There is slime in the soil nearby and the whole thing smells rotten. We're lucky this time that there isn't a smell-o-cam as this is pretty nasty.

You can see a little bit better an area of soft rot at the tip of my knife.

Conventional wisdom says to destroy all of the clumps infected. I am afraid that I can't do this and I've had pretty good luck with doing the following.

I cut away all the offending parts down to the good tissue. I then dunk the rhizome and what is left of the leaves (which have usually also been cut back because of the borer in them) into a 10% bleach solution.

I then leave the rhizomes out to dry for a couple of days, or at least overnight. I then dust them with a fungicide and replant them, usually amending the soil with a lot of compost and some sand as well. They may not bloom well the next year as I have been pretty ruthless in removing leaves (the leaves are where the plant gets the energy for more flower production next year), but the following year, they are usually fine.

Any particularly nasty rhizomes, I do throw out. Usually the bigger rhizomes throw new growth as the plant is thinking it is going to die and it is going to do everything in it's power to live. Thank goodness!

Here's a better shot at what the borer looks like.

Good gardening practices, such as cutting away and destroying the leaves after a good, hard frost, keeping the plants in open area for good air circulation and amending the soil to provide better drainage, or making raised beds are good methods to avoid the mess I have this year.

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