Nematodes, or microscopic eelworms, are rapidly assuming the role of Public Enemy Number One among the organisms that cause plant diseases.
Last year, they seriously damaged one out of every ten acres of crops grown in the United States. California farmers lose between 200 and 300 million dollars annually because of nematodes.
But farmers are not the only victims of damage by these pests. They attack all kinds of plants, including ornamental and shade trees, annual and perennial flowers—even lawn grasses.
So how do they launch their attacks?
Nematode Damage
Nematodes have piercing mouthparts inserted into plant tissues to extract the juices, causing the plant cells to be invaded by fungi and bacteria.
As a result, they may soon die. However, in some cases, the pierced cells may not die but may be stimulated to grow abnormally.
In either situation, the plant’s natural development is upset, and stunted growth, leaf blight, reduced size of fruit and flower—occasionally even death of the whole plant—occur.
Hard To Diagnose Nematodes
Such nematode damage, however, is not easily recognized by most gardeners. As a result, many cases of injury are incorrectly diagnosed as winter injury, unfavorable soil conditions, or even lack of plant food.
It is because of their small size that they are not more generally recognized. Although nematodes are thousands of times larger than bacteria, they are just too small to be seen without a hand lens.
The plant-parasitic kinds range from less than 1/8 inch in length at maturity down to 1/125 of an inch. Thus, if a person sees little white worms crawling over the soil or on plants, he may not see nematodes!
A second reason they are hard to diagnose is that many species of nematodes attack below-ground parts, particularly the roots, and their presence is not discernible unless the plants are lifted, the soil shaken or washed away, and the roots carefully examined.
Of course, where large plants or trees are suspected of harboring nematodes, only a portion of a root can be dug up for examination.
Diagnosing Root-Knot Nematodes
Perhaps the easiest type of injury to diagnose is produced by the so-called root-knot nematodes belonging to the Genus Meloidogyne. They produce knots or galls on the roots, varying from a slight swelling up to an inch or more in diameter.
If such galls are pulled apart with two needles, pearly-white, pear-shaped nematodes will be just barely visible without a hand lens.
The shape and size of this nematode vary greatly from most other species, which are much tinier and have long, slender, eel-like bodies.
Where no galls are present on the roots, and the gardener still suspects nematodes because of the generally unhealthy condition of the plants, the wise procedure is to submit adequate root specimens to the state pathologist.
The so-called meadow nematodes, for example, do not produce galls on the roots but cause lesions on some roots and decay in others. Boxwood, iris, and pin oaks are some of the plants subject to meadow nematodes.
Some nematodes, such as the root-knot nematode, can attack 1700 different kinds of plants; others attack only one or two.
One species infests the bulbs of narcissi and onions; another causes a shortening and twisting of the leaves and stems of perennial phlox, sweet Williams, and hydrangeas; and still, another blights the leaves of ferns, begonias, and gloxinias.
Life Cycle of Nematodes
The life cycle of nematodes is not a complex one. Eggs are deposited in the soil or the plant the female feeds on.
The immature larvae from these eggs emerge, which feed on the roots, stems, or leaves, depending on the species involved.
After several molts, the larva matures and is capable of reproducing. The entire cycle from egg to egg-laying female is completed in several days to several weeks, depending on the kind of nematode and the surrounding conditions.
Some species can withstand long, dry spells. They remain alive but dormant for several years and then, once they are moistened, become active in a short time.
Nematodes found in the colder parts of our country can survive winters, even in frozen soil.
The gardener, receiving little help from nature in controlling some species, is forced to use special measures.
The measures to adopt depend on the kind of nematode involved, the crop affected, and the planting size.
Though some reduction in injury is possible by crop rotation, this is not practical in small gardens as the entire garden is usually infested. Then, too, rotation is possible only where annuals are being grown.
Use of Resistant Varieties
The use of resistant varieties also offers a partial solution because nematodes, like other parasites, have plant preferences.
Among the ornamentals resistant to root nematodes are African marigolds, gaillardias, narcissi, tulips, and zinnias.
Resistant vegetables include corn and soybeans; resistant fruits are currants, gooseberries, and certain varieties of grapes.
Lilacs, peonies, snapdragons, and violets are extremely susceptible to root-knot nematodes, as are such vegetables as carrots, celery, lettuce, peppers, and tomatoes. Among the very susceptible vine fruits are muskmelons and some watermelons.
Fertilizing plants infested by root nematodes sometimes helps to overcome harmful effects caused by these same nematodes.
Controlling Leaf-Infesting Nematode Species
For leaf-infesting species of nematodes, control measures may vary from keeping water off the leaves (where plants are grown indoors) to dipping pot-grown plants such as begonias and African violets in hot water or applying a highly poisonous chemical to the soil.
The leaf-blighting nematode of the chrysanthemum can be controlled by applying a dilute solution of sodium selenate to the soil.
A two-percent sodium selenate powder, sold as P-40, can be substituted for the liquid. However, sodium selenate is a highly poisonous chemical. Therefore, it must be handled with care and cannot be used on soils used to grow food crops.
Soil-infesting nematodes can be controlled in several ways. Small lots of soil can be heated to 180° for a half hour as a control measure.
Larger amounts can be treated outdoors by injecting special fumigants into the soil, where their volatile gasses kill the pests.
Nematode Killers
The most commonly available fumigants usually contain dichloro propene or ethylene dibromide as the active ingredient.
The former is sold under trade names such as D-D and Nemafume; the latter as Garden Dowfume, Soilfume 60-40, Bromex, and Bromofume.
Ethylene dibromide can also be obtained in capsule form under the name Fumi-Soil. These capsules are merely placed in the infested soil as directed on the container.
One of the newest nematode killers to appear on the market, Nemagon, is capable of controlling nematodes in certain crops without killing the roots of nearby plants. This is a characteristic not shared by most older fumigants.
Pest Control Chemical For Nematodes
The ultimate in pest-control materials is a chemical that will control not only nematodes but also soil-infesting organisms such as parasitic fungi as well as weeds.
Such a product is Crag Mylone, which at this writing, is available in commercial quantities for use in Florida in the control of dry rot and nematodes of gladiolus. If present tests hold up, this product will eventually be widely available.
Vyapam, another of the newer chemicals, can control certain fungi, bacteria, soil insects, weeds, and nematodes.
44659 by P. P. Pirone