Most home gardeners think of nematodes as just another burden to be borne by the farmers. The tiny pests infest tobacco, cotton, potatoes, and many other agricultural plants, but most home gardeners think they don’t invade the garden.
Recent scientific studies present a different picture. Endless types of nematodes may be found in countless garden plants indoors and out.
Nematodes are responsible for hundreds of millions of dollars of loss to home gardeners and farmers. Unfortunately, nematodes can be anybody’s nemesis.
Webster describes nematodes as “any of a class (Nematoda) of worms, including the pinworm, trichina, and Guinea worm.”
Nematodes are a fascinating subject. There are thousands of species, and new ones are identified each year.
Harmful Nematodes
Most nematodes are destructive pests, but a few aids the gardener. The harmful group includes the root-knot nematode, which is endoparasitic—that is, it enters the root tissue or permanently attaches itself to the plant.
These root-knot nematodes have 1,700 known host plants, including the iris. The meadow nematode is a far more serious and common nematode.
It does not form knots or galls as the root-knot nematode does but causes black lesions on the roots and eventually kills their roots.
Control measures are the same for both the root-knot and meadow nematodes. Nematodes are microscopic, much too small for the naked eye to see. But don’t worry; the damage they do is ample evidence of their presence.
A Problem For Iris Growers
In recent years, nematodes have become a major problem for an increasing number of iris growers. In the 1956 report of the scientific committee of the American Iris Society (published in the January 1956 AIS Bulletin), Dr. L. F. Randolph, head of the committee, wrote:
“The discovery of the root-knot nematode in the fibrous roots of irises imported from Europe and gardens of northern Texas has complicated the problem of disease and pest control.” He was so right! And now he can add that nematodes have been found throughout the South and almost every other state.
Irisarians first detected nematodes when their plants, for no apparent reason, became weak and malnourished.
In this condition, the iris were extremely susceptible to almost every disease common to iris—leaf spot and rhizome rot, particularly. The real devotees of iris naturally began to search for the causes of the trouble. They soon discovered the nematodes.
If your iris has any symptoms mentioned, it is time to start your search. Dig up a sickly plant, shake off the loose dirt, and take a good look. You may be confronted with swollen roots with knots or lumps on them.
These roots may be much shorter than usual, with the ends dry and withered. What has happened?
A nasty little nematode has burrowed into the root and excreted an irritant that caused nearby root cells to grow wildly. The resulting lump or knot of tissue on the root blocked the flow of nutrients.
It is generally conceded that products that effectively destroy most pests do not destroy the root-knot nematode of the iris. However, special preparations, called nematicides, are being developed.
Using Hot Water Treatment
Dr. L. F. Randolph used the hot water treatment prescribed for peonies infested with nematodes to eliminate the nematode infection in his iris collection from Europe.
He immersed the iris for 15 to 20 minutes in water heated to a temperature of 100° degrees Fahrenheit, and then transferred the iris to 119° to 120° degrees Fahrenheit water for 30 minutes.
Dr. Randolph reported that this treatment injured the shoot tissue and killed or severely retarded the growth of the rhizomes, which were dormant when treated.
However, other iris growers have used the hot water treatment at an even higher temperature (125° degrees Fahrenheit) and found that it did the job without ill effects.
Dr. Randolph next tried the drastic method of cutting off with a sharp knife all of the fibrous roots close to the rhizome, then thoroughly scrubbing the rhizome under running water to get rid of any adhering eggs, cysts, or adult nematodes.
He reported that this cleaned up most of the diseased plants. He concluded that the root-knot nematode infected only the fibrous roots and rarely entered the rhizome.
However, Dr. Randolph was convinced that the nematode survives shipment for long distances, as was proven by those who arrived on iris imported from Europe. It is generally agreed that nematodes can be spread from garden to garden and state to state.
Treating The Infested
If there is any doubt about the possible infestation of new rhizomes, it is better to treat them before planting. The same goes for the soil where the rhizomes are to be planted.
One grower who has had several severe infestations is now convinced that his nematodes entered his garden in topsoil brought in to make new beds.
How is the soil sterilized? Many soil-sterilization products are available. Be sure to follow directions carefully.
Some of these products will kill all plant life; therefore, it is best to remove all plants from the area which is to be sterilized.
Soil Fumigants
Soil fumigants for agricultural purposes began to sell in 1945, and sales have increased ever since.
Among these are the following:
- Ethylene dibromide
- Dichloro propane-dichloro propene
- Methyl bromide
- Chloropicrin
Two soil fumigants used to combat nematodes in the iris are Dowfume MC-2 and Dowfume W-40.
Gas Treatment Method
The gas treatment method of killing nematodes with Dowfume MC-2 is very expensive because it requires that a plastic covering be spread over the entire area.
Dowfume W-40 is a liquid; it is less expensive and much easier to apply. After treatment, the soil must rest for two weeks before planting.
D-D-Soil Fumigant is rated high by those who have used it. It has been used commercially for years and is now available to home gardeners.
D-D-Soil Fumigant must be applied to empty beds to kill all plant life. A two-week period must elapse before planting. Fumi-Soil, a product of Sherman Laboratories, is another excellent soil fumigant.
Another relatively new product is Nemagon. a soil fumigant that may be applied in granular form or as an emulsifiable concentrate.
The latter is the type generally used on lawns and nursery stock. It is still in the experimental stage as regards the control of nematodes in the iris.
VC-13 is a nematicide that can be applied with a drench to garden soils before planting, to house plant soils, or to some ornamental plants already growing in the garden, including iris. VC-13 should not be used on edible plants yet, as toxicological tests are still being made.
If you cannot, or do not wish to, sterilize your soil immediately, your iris can be given one of the two treatments outlined by Dr. Randolph or a third one suggested by Geddes Douglas, a well-known iris authority:
- Cut back the foliage severely.
- Trim off all of the feeder roots flush with the rhizome.
- Wash the rhizome in a solution of Clorox and water (4 tablespoons of Clorox to each gallon of water).
After being scrubbed in the solution, the rhizome should be dried in the air for several hours before planting in a new location.
44659 by Frances G. Satterfield