Population fluctuations of Heterodera rostochiensis Woll. when susceptible potato varieties are grown continuously

Abstract

SUMMARYWhen seven potato varieties susceptible to Heterodera rostochiensis were grown every year on the same small plots for 6 years with precautions to minimize soil movements, yields were inversely proportional to the number of larvae in the soil before cropping in the first year only. With all the varieties, numbers of larvae after harvest fluctuated around mean values and the fluctuations became smaller in successive years. Although conditions from year to year were as uniform as possible, there were differences in weather, incidence of potato blight (Phytophthora infestans), seed size and quality, and in planting dates. Analysis of soil temperatures and of heat accumulation did not suggest that conditions after planting caused the fluctuations. The smaller fluctuations with repeated cropping probably reflect the influence of population density on the size of the root system and are what would be expected from the curve relating pre‐ to post‐cropping numbers, which reaches a maximum before the reproductive rate decreases to unity.A resistant potato hybrid ex andigena, also grown every year, usually outyielded the susceptible varieties and, by the sixth year, yielded at least twice as much as the mean of the six susceptible varieties.