Assessment of Verticillium biguttatum in agricultural soils

Two different methods were evaluated for the assessment of Verucillium biguttatum in soil: the pellet sampler (PS) and the modified Andersen air sampler (AS). Both techniques allowed the distribution of soil on selective Rhizoctonia plates (RP) as soil fine particles and cylindrical pellets, respectively. The white speculating colonies of V. biguttatum can easily be recognized on RP.

Compared with the PS, the AS method yielded more colonies of V. biguttatum over a wider range of artificially-produced densities, and with a lower detection threshold. It is assumed that for germination and subsequent development of colonies a more or less close contact is essential. Microscopical observations revealed that the soil particles are often not in immediate contact with hyphae of R. solani and then no outgrowth of V. biguttatum can occur.

Combined with the AS, sampling the soil volumetrically with PS can serve as a more efficient substitute for the time-consuming weighing procedure.

The observed differences for various soils suggest that besides the presence of R. solani, more factors affect the germination process. The value of the AS method is discussed in view of its applicability to studies on population dynamics of V. biguttatum in soil.