One of the peculiarities of the eelworm Heterodera schachtii which makes it a very difficult parasite to control is the length of time it can survive in a dormant condition in the soil. In this state the embryonated eggs are protected by the resistant cyst wall, and, in the case of the potato strain of the eelworm, the larvae only hatch in large numbers in the presence of the host plant. On account of this property, land infected with the parasite remains infected even though the host plant is not grown. It is however well known that if “potato sick” land is rested from potatoes for a few years it is then sometimes possible to grow a potato crop giving a fairly good yield and showing little or no sign of sickness in the overground parts. In these cases, however, it is often found that the roots of the plants are heavily infested with eelworm. These and other observations have led to suggestions that other factors may be associated with H. schachtii in giving rise to the condition known as “potato-sickness.” Such factors may be due wholly or in part to soil conditions, climate, fungal attack or the time of attack by the eelworm. In some cases more than one factor may operate in addition to the nematode, and in different cases it is probable that different factors come into play.