Density dependent parasitism of Xiphinema diversicaudatum by Pasteuria penetrans in naturally infested soil.

Parasitism of Xiphinema diversicaudatum by the nematode pathogen Pasteuria penetrans was studied in a naturally infested peach grove in Italy. The nematode density and the numbers of specimens infected or encumbered with spores were determined at monthly intervals during 2 yr in six replicated plots. Nematode densities varied between 32.4 +/- 13.8 and 121.7 +/- 26.5 specimens per 100 cm3 of soil. The percentages of nematodes parasitized by P. penetrans were low, varying between 0.7 +/- 0.7 and 9.3 +/- 1.6. Both parameters remained relatively constant in time, fluctuating at 2- to 3 months delays with maximum 2.6- and 9.8-fold increases, respectively. Similar trends were observed for the percentage of nematodes with spores and the mean number of spores per nematode. Density-dependent trends for nematode abundance and rates of parasitism by P. penetrans were shown by Bulmer's and Pollard's tests. When the grove was sampled to determine the spatial distribution of X. diversicaudatum densities and parasitism, X. diversicaudatum were found in 87.5% of the samples. Parasitism by P. penetrans was observed in 80.3% of the samples containing X. diversicaudatum. A subset of 43 (76.8%) observations from the spatial sampling was described by the equation 0.026 ln(y) - 0.0038y = 0.000043x - 0.0052 ln(x) + 0.04, where x is the number of nematodes per 100 cm3 of soil and y is the percentage of parasitism by P. penetrans.