Since nematodes are so widely distributed throughout the United States, this might seem like locking the barn door after the horse is gone. However, the introduction of new and more virulent species is always a possibility. I feel certain that the use of southern-grown tomato and cabbage plants in the North has been a means of annually reinfesting many growing fields with nematode species which do not ordinarily survive northern winters.
A case in point regarding infestation of soil by introducing plants is that of Radopholus similis, the burrowing nematode which causes
the spreading decline of citrus. This nema does not survive in the North, but is known to infest more than 100 ornamentals, many of them house plants which are shipped to northern markets from Florida. This pest is showing up in greenhouses in the North, apparently brought in on Florida foliage plant shipments.
Formerly we were limited to controls which could be used only when the soil was not occupied by living plants. These included applications of chloropicrin, dichloropropene, ethylene dibromide and similar volatile chemicals. Newer chemicals of this type include Dow Telone, Shell D-D, Vapam and Mylone. These are employed on greenhouse and coldframe soil used for propagating purposes (where the chemicals can kill 100 per cent of all nematodes) and for treating rows in the nursery (where complete control is not achieved, but nematodes are set back sufficiently so plants can make near-normal growth).
There is another chemical, scientifically designated as 1,2-di-bromo-3-chloropropane, and sold under the trade names Nemagon and Fumazone, which can be used around living plants without injuring them. Also, certain phosphate insecticides are available that can be applied to the soil for absorption by the plant to kill internal nematodes. To date, however, chemical control is not 100 per cent effective.
Several years ago, I was at the U.S.D.A. station at Beltsville, calling on Dr. Charles Wechsler, a root-rot specialist. He had been studying nematodes as carriers of disease organisms, and at the moment happened to have a culture which contained both nematodes and a ring fungus. He asked me to look in the microscope and observe the natural death of a nematode. As the nema poked its way into the odd ring-shaped organ on the fungus, the ring closed, trapping its victim so it could not escape. This was my first introduction to the fungus enemies of nematodes, which I discovered were fairly numerous. Some, like the specimen I saw, actually close on
the body of the nematode. Others work like an eel-pot; these have a ring that allows the victim to enter with its head, and then trap it so it cannot move either forward or backward.