This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
The Potato Eelworm (a thread worm, Hetero-dera Radicola) is about one twenty-fifth of an inch long, and works in the mature tuber. It has been found in Nevada potatoes shipped to California. The following is from Nevada Experiment Station "Bulletin 76".
The accompanying photograph shows the external appearance of badly diseased potatoes. The surface of the potato is more or less wrinkled, and dotted with circular or oval pimples somewhat smaller than a pinhead, or with more irregular and larger nodules. The nodules are of grayish or brownish color, more or less depressed in the centre and sometimes surrounded by a slight furrow. In early stages the potato may be full and firm and the pimples so inconspicuous that they may easily be overlooked. When the disease is more advanced the nodules are more prominent, the specimen more or less shriveled and of softer consistency than normal. The easiest way to determine whether a suspected tuber is diseased or not is to cut off slices. If diseased, the cut surface will show several dry, brownish spots somewhat smaller than the head of a pin and extending from a sixteenth to a quarter of an inch into the flesh. They are usually circular or oblong in shape and consist of a brownish ring enclosing a central, whitish, pulpy core. Beneath the pimples there is a similar brown dry rot-like area which may or may not connect with the interior spots or worm burrows. Sometimes the burrows are so numerous and close together as to form an irregular continuous mass like a number of small shot close pressed together. More rarely the burrows may extend deeper into the flesh.
Badly diseased potatoes may shrivel up to one half the natural size, are softer and less nutritious than normal and of course are not desirable for human food. The burrows afford entrance for the bacteria of decay, so that infested potatoes will not keep as well as healthy ones.
If a portion of the pulpy centre of one of the burrows is scraped out and examined with the microscope, it will be found to contain numerous eggs, larvae, young and adult worms like those figured in the illustration. The potato cells have broken walls and the starch grains are fewer in number than in healthy tissue and those present are of smaller size.
From Bulletin 76 of the Agricultural Experiment Station of the University of Nevada.
Iron Age Sprayer in operation.
The disease is spread by planting infested seed potatoes. We do not know to what extent the worms may live and multiply in the soil itself or how long a soil may remain infected. This important point can be settled only by careful observation and experiment.
How to free infected soil from the parasite is a question which the knowledge at present at our command will not permit us to answer satisfactorily. Sterilization or disinfection on so large a scale is not practicable. Possibly deep plowing, letting the ground lie fallow for a year, or, where feasible, covering the fields with water during the winter months, may prove to be effective. The best advice for the present, it seems to us, is to plant infected fields with some other kind of crop, preferably grain or alfalfa rather than a root crop, such as sugar-beets, which might be attacked by the same pest."