In a more years-lasting field trial the influence of a second crop cultivation of rye and rape in winter time on the incidence of Rhizoctonia solani Kühn on potato or mustard as indicator plants was studied. The second crops were ploughed in during spring time. On part of the trial was artificially infested by this causal agent. Three crop rotations with different proportions of host plants (potato) were compared. The results obtained demonstrate the following: 1. On the part of experimental area arficially infested the incidence rate was higher for 3 till 4 years than on the control plots, but later it became equally to the control one, which also was very high (approximately 50). 2. In the last very dry year (1969) the disease developed only a little. 3. The second crop cultivation decreased the incidence of R. solani in 7 (rye) or 8 (rape) of 10 cases, but mostly the rate of decreasing was low. 4. A high weed density induced by a diminished mechanical cultivation increased the incidence of R. solani at the end of the experiment (1969), specially in the rotation with 80% host plants. 5. The number of soil bacteria, the respiration rate, and partly also the activity of dehydrogenase and the soil acidity were increased by green manuring of the second crop, however, the number of the actinomycetes, and the cellulolytic activity didn't be influenced remarkably. 6. The second crop cultivation ist nounced as a measure stimulating the soil fertiligy and discussed in connection with the microbial processes taking place in soil.