Nematodes are generally tiny, microscopic animals that can contain as few as 1,000 cells. Some, however, are large enough to be seen with the minor magnification of a 10X hand lens. The don't have a skeleton and appear similar to worms in body type.

Plant nematodes may infest roots, stems, foliage or flower parts. They have piercing mouth parts and suck sap from the plant.

Currently, there are two types of nematodes that have been found to attack hostas - Foliar Nematodes and Root Knot Nematodes. The foliar type is by far the most common type seen in hostas.

Foliar Nematodes

Before about 1990, foliar nematodes were not considered a problem of hostas. Then, over the next decade or so, people began to notice the browning of the space between the leaf veins as the growing season progressed. Upon closer examination in the lab, it was discovered that a small roundworm (i.e. a nematode) was causing this discoloration.

One of the problems in identifying this pest is that it often looks similar to normal browning of leaves that occurs in the fall. Unless a person was very familiar with the problem they might easily overlook it.

Foliar nematodes are rather rare. Most types of this plant pest infect roots and are known as root knot nematodes. The foliar types go up into the leaves to feed and then move back down into the crown of the plant to overwinter. The veins of the leave form a type of barrier so that the activity may be between one set of veins and might not show in the next interveinal area.

What do these critter do to the hosta? Well it appears that they do two things. First, they make the plants less thrifty and cause them to loose vigor. The other impact is that they make the foliage look bad starting in about August.

Control of the nematodes once they are in your garden is difficult. They do not seem to infect all types of hostas and some cultivars are much more susceptible than others. For specimen plants, some people have suggested the use of a hot water bath that would kill the nematodes but not damage the plants crown. Chemicals called nematicides may work but they must be applied by a licensed pesticide applicator and are generally not available to the home gardener.

For now, exclusion seems to be the most effective approach. Purchase plants from only reputable nurseries who are aware of the problem and who take measures to exclude this pest from their stock. Be careful when "swapping" plants with other hosta collectors. Finally, if you find nematodes (and are absolutely sure by a lab test) eliminate the infested plants from your garden. Be sure to dispose of them away from the garden and not in the compost pile.

The American Hosta Society is continuing to investigate this problem in cooperation with university researchers.

Root Knot Nematodes

Although not as common as foliar nematodes, hostas are also susceptible to microscopic root knot nematodes. These critters infest several types of landscape plants and are identified by the small knots or galls that they cause to form on the roots of plants. These growths will interrupt the root's ability to take up water and nutrients resulting in a stunted plant of poor vigor. The plant will show signs of wilting during hot, dry weather or may show nutrient deficiencies in the leaves.


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