The rate of population increase of Pratylenchus penetrans is greatest at moderate soil moisture tensions (pF 2-3) and is least at very low or very high tensions. Soil type influences population growth: the higher the silt and clay content of a soil the greater is the moisture tension necessary for satisfactory population growth. In general, the number of nematodes surviving in soil decreases with increasing moisture and temperature above freezing. No nematodes survive 15 days at -15° C or at 37° C. Soil type influences survival: at tensions of pF 3.7-4.2 survival is much greater in clay than in sandy soil. Nematodes can remain viable at tensions as high as pF 5.0, but death is rapid at pF 5.06. The results show that the half-life concept is of limited value for measurement of nematode viability. In sandy soil some P. penetrans enter alfalfa roots under saturated conditions (pF 0) and under dry conditions (pF 3.0) but entry is greatest under moist conditions (pF 1.8-2.5). The results suggest that only fourth stage larvae and adults of P. penetrans can penetrate into roots. The experimental results indicate that the widespread occurrence of high Pratylenchus spp. populations in sandy soils can be explained in terms of the interaction of soil moisture with soil type. © 1968 BRILL.
