ANALYSES OF THE EFFECTS OF POTATO CYST NEMATODES (GLOBODERA-PALLIDA) ON GROWTH, PHYSIOLOGY AND YIELD OF POTATO CULTIVARS IN-FIELD PLOTS AT 3 LEVELS OF SOIL COMPACTION

Field experiments were carried out in 1991 and 1992 on sandy soil high ly infested with the potato cyst nematode Globodera pallida. Half the trial area was fumigated with nematicide to establish two levels of ne matode density. Three levels of soil compaction were made by different combinations of artificial compaction and rotary cultivation. Two pot ato cultivars were used in 1991 and four in 1992. Both high nematode d ensity and soil compaction caused severe yield losses, of all cultivar s except cv. Elles which was tolerant of nematode attack. The effects of the two stress factors were generally additive. Analysis of the yie ld loss showed that nematodes mainly reduced cumulative interception o f light while compaction mainly reduced the efficiency with which inte rcepted light was used to produce biomass. This indicates that nematod es and compaction affect growth via different damage mechanisms. Nemat odes reduced light interception by accelerating leaf senescence, by de creasing the specific leaf area and indirectly by reducing overall cro p growth rate. Partitioning of biomass between leaves, stems and tuber s was not affected by nematode infestation but compaction decreased pa rtitioning to leaves early in the growing season while increasing it d uring later growth stages. The effects of nematodes and compaction on root length dynamics and nutrient uptake were also additive. This sugg ests that the commonly observed variation in yield loss caused by nema todes on different soil types is not related to differences in root sy stem expansion between soils of various strength. Cv. Elles, which sho wed tolerance of nematodes by relatively low yield losses in both expe riments, was characterised by high root length density and thick roots . These characteristics did not confer tolerance of soil compaction, s ince compaction affected root lengths and tuber yields equally in all cultivars. In the first experiment only, high nematode density led to decreased root lengths and lower plant nutrient concentrations. The yi eld loss which occurred in the second experiment was attributed to the effects of nematodes on other aspects of plant physiology.