The natural decline of potato cyst nematodes in the absence of a host crop and their movement by cultivation operations
This thesis has investigated the variation in PCN population density declines in the absence
of a host crop and, the movement of PCN by cultivation operations in a potato rotation.
The use of crop rotation as a management strategy for the control of potato cyst nematodes
(PCN) is dependent on the natural decline rate of PCN. Any substantial variation among
PCN populations will influence the adequacy and duration of crop rotation. Two
experiments were undertaken to assess the extent of variation in PCN decline. Experiment
1 consisted of the annual sampling of semi-permanent field test stations to monitor the
population decline in five infested fields, over four years. Large variations within the PCN
population densities were found over the duration of the experiment, these included
apparent increases in population density. These increases were not found for the controls in
the field boundaries. The observed variations were probably due to the net movement of
cysts in and around the stations as a result of soil movement by cultivation, as identified in
further studies. Experiment 2 consisted of 45 plunge pits with soil from different PCN
infested fields. The plunge pits were sampled at four monthly intervals, for twenty months.
The range of decline was 11 to 69 % per annum. The variation in decline rates between the
populations could not be accounted for by differences between G. pallida and G.
rostochiensis populations or completely by soil type. This suggested that decline
information was required on a site-specific basis and that sampling within a field is not
reliable if cultivations are carried out between sampling dates.
Further studies determined that PCN cysts are moved by cultivation operations. With the
exception of the bed-former, all cultivations investigated were found to result in the
movement of PCN. The range of movement by a cultivation operation was from within 1
m up to 5 m. The cultivation operation employed and its purpose determine the direction
and extent of cyst movement