Recovery of resting spores of Polymyxa betae from soil and the influence of duration of the bioassay on the detection level of beet necrotic yellow vein virus in soil

Abstract

In a bioassay, the recovery of resting spore clusters ofPolymyxa betae from artificially infested sand was 1.7%. A recovery between 1 and 2% implies that one infective unit ofP. betae estimated with the most probable number method represented 50–100 resting spore clusters.

Neither prolongation of the duration of the bioassay from 6 to 18 weeks, nor a second and third 6-week replant resulted in a lower dilution endpoint of BNYVV in naturally infested soil. Estimates of inoculum potential of BNYVV were not significantly increased when incubation was extended over 6 weeks. In the 18-week bioassay, tap root weights showed a decrease with increasing inoculum levels. The presence of BNYVV in the tap root, as detected by ELISA, was associated with the presence of leaf symptoms and root browning.

Using the most probable number (MPN) method, assessment of vector and virus after 6 weeks yielded 3- and 16-fold higher MPNs, respectively, than when determined after 3 weeks. Prolongation of the bioassay to 12 weeks did not result in a higher MPN of BNYVV. Total and tap root fresh weight showed a decrease with increasing inoculum levels, but lateral root weight was hardly affected, after 3, 6 and 12 weeks of incubation.