Past Articles Library | Plant Diseases and Pests of the Iris
The iris is a beautiful bulb that flowers in many areas after many other spring bulbs have bloomed. While this bulb is typically viewed as pest and disease-free, this is not necessarily true and depends on the area of the country you may live in. But one thing for sure, if you keep your garden space clean you will less likely have to deal with the diseases and pests described below.
To begin this analysis of diseases, let’s start from the bottom up.
Bacterial Soft Rot
Bacterial soft rot is a bacterial disease that is caused by Erwinia carotovora. This disease is brought on when you have wet springs that are warm. If you notice that your leaf fans are wilting and eventually die before their time then you probably have bacterial soft rot. Another clear cut way of finding out is to dig up the rhizome. If you do this and you are greeted with a foul smell, chances are you have bacterial soft rot.
While most people would just throw out the rhizome, you can save the rhizome but first remove the diseased tissue. The easiest way to do this is with a spoon. You will need to just scoop out the dead tissue and throw away. Once the rhizome is completely cleared of any diseased tissue, dip the rhizome into Dial antibacterial soap. This soap will kill any bacterium left.
Once this is done, you can replant the rhizome. At this point, there really is nothing you can do to prevent this bacterium since it is environmentally linked.
Fungal Crown Rot
Fungal crown rot is caused by Sclerotium rolfsii, which thrives during warm and wet days. This plant disease appears as a gray to white cottony mass that can cover the base of leaves, rhizomes, and soil around the iris. The affected leaf will start off with a yellow tip, which will travel down the leaf until the leaf falls over and dies.
Prevention is the first step one should take before this disease takes hold. This means picking up spent leaves and throwing them away. Also, exposing the rhizomes to sunlight prior to planting will sterilize the surface of the rhizome. As a secondary preventive measure, dip the rhizomes in 10% bleach water before planting. Both of these techniques will kill any remaining fungal spores.
Fungal Leaf Spot
The fungal leaf spot can be seen as circular to oval yellowish spots on the leaves. These spots then turn brown and finally end up with a red ring around the spot. The fungus that causes this problem is the Didymellina macrospora.
This disease is caused by wet weather. The best approach to dealing with this is to cut off the affected leaves and destroy the leaves. While you can apply a fungicide, once the weather dries up some of this issue will disappear.
Bacterial Leaf Blight
If you see watery spots along the leaf margin, which turn brown and finally grayish-white then you probably have bacterial leaf blight. Once this starts the blight spots will “grow” together on the leaves and follow the leaf vein. Environmental conditions cause this condition along with the bacterium Xanthoman tardicresens. This bacterium loves foggy and rain days. Since there is not much you can do about the weather, the best approach is to keep the garden clean. Once the leaves are finished in the fall, remove and destroy them. Also, make sure that your garden tools are clean and sterilized before heading out to the garden.
While bacterial leaf blight does not affect the rhizome, one technique that one may utilize if you find this bacterium in your garden is to divide the irises. This should be done starting in July and finishing up in late September.
Pests are another problem that iris growers can be faced with but do not be discouraged. There are a few techniques one can use to control these little creatures.
There are two common creatures that everyone who plants bulbs or rhizomes complains about and that are moles and voles. I am sure that almost everyone has heard of a mole. They are small rodents that are blind. They create hills in the turf and are very seldom seen above ground, unless you have a dog or cat.
Voles, on the other hand, look like a mouse. They will eat anything in their path and as far as a path they many times use those created by moles. While you can trap them, a good approach is to make it so that they cannot get to the rhizome. How do you do this? The answer is simple. You put something distasteful or discomforting in the hole. As an example, you can put holly leaves in the hole with the rhizome. The spines of the leaves will poke the creature and in doing so will discourage feeding.
Aphids are another pest that is commonly found on irises. They suck on the leaves and in doing so produce a sweet flavored substance called honeydew. This honeydew is a favorite food of ants. How do you know if you have aphids? Well, one sign is the appearance of ants. Another sign is the aphids themselves, which are small green or gray creatures that can be seen gathering on the leaves. If you see these creatures, do not immediately reach for the insecticide. They are easily picked off by hand and placed in a bucket of soapy water. If you do not want to use this approach, consider spraying your plant material with a combination of water and dish soap. The soap itself will suffocate the aphids, which also takes care of the ants in an environmentally friendly way.
The last pest that is commonly found in irises is the iris bore. Prior to knowing how to control this pest, you must understand its lifecycle. This pest starts out as a white caterpillar and can be found early on the young growth. As the season continues, this caterpillar will eat and make its way into the rhizome. Once the season begins to cool, the caterpillar will turn into a pupa where it will overwinter. Once spring arrives, the pupa will hatch into a moth, mate, and lay its eggs in garden debris.
To break this lifecycle, one must be diligent as far as keeping the garden clean. This means removing any dead or diseased plant material from the area. Do not compost.
As you can see, there seems to be a theme when it comes to prevention of any garden pests. This means, one needs to keep the garden as clean as possible. While removing dead and diseased plant material from the garden space is very important so is keeping your tools and equipment clean. The cleanest garden space can quickly become infected by the simple use of a “dirt” shovel. Lastly, while I know you love your irises do not think twice about disposing of diseased rhizomes. This is a better approach then spreading disease throughout your garden due to one diseased rhizome you tried to save.