A Greener View by Jeff Rugg

Q: Is there a way to control nematodes that may be affecting my hosta plants? I have 128 different varieties and some of them seem to have some of the symptoms. I am not sure if that is the actual problem or not.

A: Nematodes are microscopic roundworms. There are many kinds. Some feed on fungi, some on the outside of plant roots, some are inside of plants and some are predators that feed on insects or on other nematodes. They are part of the invisible community that is part of every soil, from deserts to the tropics. They move around on the microscopic film of water that is stuck to soil particles even in soil too dry for plants to get any water. They are so numerous that it has been said that if everything but the nematodes was removed from an area, you would still be able to see the shape of the landscape. Most people have never heard of them, but farm soils that are treated for them will often yield much greater productivity.

In the past, many chemicals have been used to treat the soil-borne nematodes, but most have been eliminated or can only be applied by licensed applicators. Newer soil nematicides are safer for the whole community, but there are still none that work for foliar nematodes.

The nematodes that feed inside the leaves of plants seem to have become more common over the past decade or so. As they feed in a leaf, the area turns yellow and then brown as it dies. The damage is often spotty in a leaf.

The same or similar appearance can be caused by other problems including Anthracnose, sun burning and drought stress. Anthracnose causes brown spots on the leaves that can merge to form irregular dead areas and occurs more often on hostas in wet or over-watered locations. Sun burning often bleaches the leaves to cause white or light tan areas. Drought stress usually causes dead leaf edges that may become infected with fungi.

To be sure of your diagnosis, you need to check for the nematodes. You can send a sample of the leaf to a diagnostic laboratory. Your local Cooperative Extension office can probably tell you where to send the sample. Before spending money on treatments for nematodes, it is important to be sure you actually have them.

You can try to see them for yourself. Cut out some samples of the leaf tissue that is showing the yellowing symptoms. Place them in a clear dish in sufficient water to submerge the samples. After about a day, shine a bright light into the water. Look for very small snake-like threads moving about in the water. You may need a 10X magnifying glass. It is sometimes easier to hold the glass up to the light and look through the water.

Having nematodes is one thing, but there are nematodes that feed on dead leaf material and they are not the ones harming your plants, so knowing that you have nematodes is only half the answer. The lab will be able to tell you which ones you have and prescribe a treatment.

There are few products labeled for use against nematodes, but some insecticides work.

Once they get into a plant, it is very difficult to kill the whole population. The hosta leaves die in the fall and the nematodes spend the winter in the crown of the plant and soil surrounding the plant. In the soil, they feed on fungi to stay alive.

Removing all the leaves and shoots of the hosta in the fall before they naturally die back will help remove a lot of nematodes. The nematodes can move deep into the soil over the winter and move back up to re-infest the plant in the spring. Cold weather does not kill nematodes.

Insecticidal soap and systemic insecticides will help to reduce the population within the plants. Soil insecticides will help to reduce incidence in the soil.

If you want to try an unusual treatment that doesn't use chemicals, you can try the heat treatment. This method is labor intensive and can damage or kill the plant. The nematodes are killed in temperatures above 120 degrees Fahrenheit. You need a large bucket and a hose connected to a faucet with water running above 120 degrees. You may need to adjust your hot water heater.

Dig up the hosta with a large root ball of soil. Drop it in the hot water bucket for 10 minutes. Take it out and put it in a bucket of regular tap water to cool it off. At the same time, pour a bucket or two of boiling water around the soil in the original hole to kill the nematodes left in the soil around the hosta. It is best to do this when the plant is dormant. If the leaves are out or have started to sprout in the spring, they will probably die. The crown may be damaged and it may take a year to get it back into full health. People doing the heat treatment report losing some plants and having the surviving plants remain nematode free for several years.

The heat treatment works best on potted plants bought early in the spring before they begin to grow. If you are adding hostas to your collection, it is wise to treat them all with suspicion. A quarantine system is a good idea. If it is potted, leave it in its pot, or re-pot it into a larger one for the first year. Watch it for symptoms. If it doesn't have any, you can plant it into the garden or possibly a holding area. If you get freshly dug hostas, they should go into the holding area. Pick an out-of-the-way area and plant all-new hostas. If they develop symptoms, you can decide to treat them or throw them out. Dig up the surrounding soil and treat it or destroy it, too. This method gets hostas into the collection a little slower, but also a little wiser. They can't infect the collection and cause more headaches if they are diagnosed early.

In an effort to slow the spread, try these tips. The nematodes can move on the leaf if it is wet, so when possible, water the plants without getting the leaves wet. Remove all dead leaves because the nematodes can go dormant for over a year in dead leaves. When the dead leaves are wet, the nematodes can revive and move into the soil to get back into the plant. Use mulch around the plants, as it stays drier on top and, drying out, kills nematodes. In the fall, clean up all stems and leaves. Using insecticides will kill adults but not eggs, which can survive for several years. Even if you treat a plant, you will need to watch it for several years for new symptoms.

E-mail questions to Jeff Rugg, Kendall County unit educator, University of Illinois Extension at jrugg@uiuc.edu. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

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