Emergence of juvenile potato cyst-nematodes Globodera rostochiensis and G. pallida and the control of G. pallida

Speed of emergence of juveniles from cysts in potato root diffusate (PRD) in vitro differed between Globodera rostochiensis and G. pallida and between populations within each species. Early emergence in vitro was slower in most populations of G. pallida than in most populations of G. rostochiensis. Fewer G. rostochiensis juveniles emerged from 4 or 6 month old than from 4 yr old cysts. More G. rostochiensis emerged in solutions of sodium metavanadate at concentrations of 10(-2) and 10(-3) m than in PRD and as many G. pallida emerged in the same solutions as in PRD. In plots of bare fallowed sandy loam, emergence of G. pallida was stimulated by 10(-3) m sodium metavanadate. The emergence of two populations of G. pallida in PRD was stimulated by the addition of benomyl at 0. 1 ppm (3.4 X 10(-7) m). In microplots, cv. Cara potatoes grown for 8 wk decreased four populations of G. pallida by up to 93%. During a 4 wk period in PRD, more than 20 juveniles per gravid female emerged from five of 25 populations of G. pallida. In root observation boxes in which cv. Desiree was grown, oxamyl applied to the top 15 cm of a peaty loam soil greatly increased G. pallida in soil 15-45 cm deep. In another peaty loam, but not in a sandy loam, the same treatment appeared to increase the nematode in soil 15-30 cm deep. Oxamyl incorporated in the uninfested top 15 cm of all three soils largely prevented nematode increase from juveniles migrating upwards from untreated heavily infested soil 15-30 cm deep. These experiments suggest that inadequate control of G. pallida increase on susceptible potatoes by an oximecarbamate nematicide of short persistence, such as oxamyl, is primarily due to the slow rate of juvenile emergence in most populations of G. pallida, with a second generation and the upward migration of juveniles from deeper untreated soil later in the growing season as potential contributory factors.