The effect of a sequence of host and non-host crops on the population dynamics of Pratylenchus penetrans was monitored for 2 years in a field. The sequence was pea (Pisum sativum, 25% cv. Colombia and 75% cv. Coca), French marigolds (Tagetes patula nana cv. Sparky); fallow; carrot (Daucus carota, cv. Amsterdamse bak); bean (Phaseolus vulgaris, cv. Avignon) and again fallow. Samples were taken every 21 days during the marigold crop and following fallow, and at planting and harvest for the other crops. From five 2 × 5 m 2 plots, 15 cores were taken per plot to a depth of 70 cm and split into seven segments of 10 cm each. Within the plots, segments from corresponding depths were pooled and nematodes were estimated from a 200 g subsample. Densities of P. penetrans increased 2.2-fold under pea, but decreased by 90% after marigolds had been grown for 105 days. The P. penetrans population was reduced by about 90% in the soil layer above 40 cm while the reduction was about 80% below 50 cm after 105 days of marigold culture. Densities of P. penetrans declined exponentially in time under marigold and fallow. The effect of marigold was persistent as no increase in P. penetrans densities over the whole soil profile was noticed after two crop cycles of host plants.

The influence of temperature and root diffusates from different plants on the hatching behaviour of Pratylenchus penetrans was studied. Diffusates were obtained from maize (cv. Husar), carrot (cv. Masei), black salsify (cv. Omega), pea (cv. Alouette), bean (cv. Polder) and marigold (cv. Single Gold) at different plant ages. Soil leachate was used as a control treatment. Hatching was also examined in 10% dilutions of the root diffusates and the soil leachate. Hatching was monitored at 5-day intervals until 60 days at most. Test solutions were refreshed at the same intervals. Pratylenchus penetrans hatched more at 20°C than at 10, 15 or 25°C. Hatching was stimulated by all host diffusates, as well as marigold; it was highest in maize and bean diffusates (ca 50%). Except for carrot diffusate and soil leachate, hatching activity decreased with increasing plant age. It also decreased in 10% diluted root diffusates, except for pea, black salsify and soil leachate. There was no evidence for the presence of hatching inhibitors in the diffusate of any host.

The vertical distribution of Pratylenchus penetrans was monitored in four fields cropped with maize, black salsify, carrot, or potato. Soil samples were collected at 21-day intervals from May 2002 until April 2003 from five plots (2 x 5 m(2)) per field. Per plot, 15 cores were taken to a depth of 70 cm and split into seven segments of 10 cm each. Within the plots, segments from corresponding depths were pooled. After mixing, 200-g subsamples were taken and nematodes were extracted by zonal centrifugation from the root fraction and the mineral soil fraction separately. In most crops, the root fraction contained more than 50% of the total number of P. penetrans. Because the ratio between the numbers of nematodes in the root fraction and mineral soil fraction changes during the growing season, numbers of P. penetrans found in the mineral soil fraction cannot be used to estimate the total number in the soil. Therefore, both fractions have to be processed to obtain a reliable estimate of the density. No nematodes were recovered below 50 cm soil depth, except in the maize field where nematodes were found at 70 cm. The optimum sampling depth for maize, black salsify, carrot, and potato was 45, 25, 25, and 35 cm, respectively. The percentage of nematodes per soil layer was independent of the sampling date, indicating that a defined optimum sampling depth will be applicable throughout all seasons. The cumulative vertical distribution, modeled with a logistic equation, can be used to estimate the sampling error when samples are collected at different depths.

The influence of both host and temperature on the vertical migration of Pratylenchus penetrans was examined in a series of experiments using maize, carrot, black salsify, pea, bean and potato. Plants were grown in a pot placed on top of a plastic pipe of variable length in a growth chamber. This pipe was planted into a second pot to maintain the humidity in the pipe. Nematodes were inoculated at the bottom of the pipe and were allowed to migrate upwards for 14 days after which the pipe was cut into 5 cm sectors from which the nematodes were extracted and counted. More nematodes migrated at 21 • C than at 16 or 11 • C. Migration of P. penetrans towards a host decreased with increasing distance between the nematodes and all hosts, with maize and bean eliciting the most attraction. They were the only hosts able to stimulate nematode migration up to the distal end of 31 cm pipes. Potato and carrot were the least influential hosts. Hosts differently influenced the migration only of second-stage juveniles (J2), with a greater percentage of J2 moving under maize and bean than under carrot. Under all hosts, adults migrated in greater proportions with increasing distance than did other stages. The attractiveness of hosts was age dependent. Generally, infectivity decreased with distance travelled by the nematode.