The results of a research into the extent of biological degradation of a typical cernozem, into the state of its soil microorganism complex, and into the phytosanitary state of agrosystems in cases of root rots are presented. The increased numbers of micromycetes in general and pathogens of genus Fusarium in particular, and the higher frequency of root rots on winter wheat and sugar beet was observed in field conditions. More virulent forms of Fusarium pathogens were found to be selectively accumulated in soil under field crops.