THE INFLUENCE OF CYST NEMATODES AND DROUGHT ON POTATO GROWTH .5. EFFECTS ON ROOT DISTRIBUTION AND NITROGEN DEPLETION IN THE SOIL-PROFILE

In two experiments in the Wageningen Rhizolab with potato cv. Mentor p lanted in soils with or without potato cyst nematodes (Globodera palli da, W) the number of roots per cm(2) was observed two weekly by video camera, in horizontally placed minirhizotrons at depths varying from 5 to 100 cm. In both experiments initial root growth was more rapid in the top soil of the uninfected soil. In the first experiment under opt imal water supply, root formation continued longer in the top 30 cm of infested soil leading to twice as many roots at the end of the growin g season as in uninfested soil. In the subsoil from 30 to 100 cm, howe ver, root formation was strongly reduced by cyst nematodes leading to an uneven distribution of roots throughout the profile. In the second experiment potato cyst nematodes only increased rooting in the top soi l with reduced irrigation. Potato cyst nematodes did not affect the wa ter use efficiency of the crop whereas reduced irrigation increased wa ter use efficiency by about 22%. Without potato cyst nematodes the soi l profile was depleted of mineral nitrogen until a depth of 1 m wherea s with high initial population densities no nitrogen was taken up in t he subsoil between 30 and 100 cm. The spational heterogenity of roots and nitrogen in the soil is an important mechanism of damage. This fin ding may lead to improved cultural practices and breeding for toleranc e.