Development of an in vivo bioassay to identify sugar beet resistance to the stem nematode Ditylenchus dipsaci

A bioassay was developed to evaluate sugar beet for resistance to the stem nematode Ditylenchus dipsaci. To produce large numbers of D. dipsaci for inoculation of sugar beet seedlings, a protocol for a monoxenic carrot disk culture was established. Ditylenchus dipsaci multiplication was greatest with 125 000 nematodes per carrot disk after 72 days at 20 degrees C with an initial inoculum density of 75 nematodes. Higher inoculum densities resulted in more rapid decay of carrot tissue and lower numbers of D. dipsaci. Inoculation of 200 D. dipsaci in leaf axils of sugar beet seedlings resulted in high penetration rates. The addition of 1% carboxymethyl cellulose (CMC) as a gelling agent resulted in a significantly increased penetration rate of 96% compared to tap water with 34%. Soil infestation was not suitable, with only 4 and 11% of the nematodes penetrating the sugar beet seedlings when inoculum was applied in water and CMC, respectively. The bioassay enables a rapid screening for resistance in sugar beet to D. dipsaci.