EFFECTS OF CROP-ROTATION AND REMOVAL OF CROP DEBRIS ON THE SOIL POPULATION OF 2 ISOLATES OF VERTICILLIUM-DAHLIAE

Microsclerotia of Verticillium dahliae are produced in large numbers o n senescing parts of host plants and remain viable in the soil for man y years, Changes in the population density, i.e. density of microscler otia, in the soil were measured in micro-plots using two isolates of V . dahliae, specific to either field bean or potato, several crop seque nces comprising potato, field beans and barley, and either the removal of aerial debris of the crops or incorporation into soil. Potato was more susceptible to the potato isolate and field bean more susceptible to the field bean isolate. Removal of debris of potato and field bean reduced numbers of microsclerotia in the soil in the subsequent years , but removal of barley straw had no effect. Initially non-infested co ntrol micro-plots became infested, probably by the growth of potato ro ots into the naturally infested subsoil. The rate of increase of the m icrosclerotial population in the non-infested control micro-plots was larger than in the initially infested treatments, because more coloniz ed debris was produced. It is concluded that removal of aerial debris of host crops is important to reduce the soil population of V. dahliae .