Effects of inoculum depth on the activity of Rhizoctonia solani and on bean root rot development in sterile soil

The effects of different inoculum depths on the activity of Rhizoctonia solani and the development of root rot on beans were studied under greenhouse conditions. Inoculum was grown on rice grains and buried at depths of 0-5 cm, 510 cm, 10-15 cm and 15-20 cm. Experiments were carried out in PVC cylinders with removable rings, where the sowing was performed directly in infested soil, and in pots, where seeds were sown in the top layer (3 cm deep). Sowing was done immediately after soil infestation, as well as at 15, 30, 60 and 90 days after the soil infestation. In the cylinders, high disease severities were observed at all evaluation dates; emergence and plant weight were reduced by R. solani independently of the depth in which the inoculum was originally buried. The pathogen activity was not consistently reduced after 90 days in absence of host tissue in both experiments, although effects of R. solani on emergence and plant growth decreased over time. Moreover, the pathogen could very hardly be recovered from soil samples collected above the layer originally infested in the cylinders when it was deeply buried. In the pots, disease incidence and severity were high, and emergence and weight of plants were consistently reduced at all evaluation dates when the inoculum was confined in the upper 10 cm of soil. Damaging effects of R. solani on emergence and plant growth were reduced with deeper placement of inoculum. Emergence and plant weight were not affected at any evaluation date when the inoculum was buried deeper than 15 cm, although infected plants were frequently observed. As an epidemiological consequence, the primary inoculum for the next crops would be increased. The results indicate that the recommendation of deep burial of R. solani inoculum is not always valid, especially not in tropical lands, where it is common to prepare the soil twice or three times a year, considering that soil movements can deposit inoculum close to the soil surface.