An ecological study of some soil nematodes associated with apple trees in a New Zealand grassed orchard

Of the 35 nematode genera found in the orchard soil 12 were in the order Tylenchida. These 12 genera, represented by about 34 different species, constituted about 50%–60% of the total nematode populations. Although no obvious differences exist between the species composition of the tylenchid populations of the unvegetated 0m and the grass‐covered 1m† sites, distinct quantitative differences were found where Pratylenchus and Helicotylenchus species, only barely represented at 0m, were predominant at 1m. Tylenchus species occurred in both areas, but were far more abundant at 0m. Total nematode figures for both areas had no marked differences. Nematodes were most abundant between 0 cm and 10 cm, and the density decreased with depth. No differences were observed between the vertical zonation patterns of total nematodes at 0m and 1m. The populations of Helicotylenchus and Alaimus , the rhabditids and the dorylaimids, fell sharply below 10 cm. The vertical gradients of Tylenchus and Pratylenchus species were more gradual. The vertical distribution of the Tylenchus spp., the total nematodes, Pratylenchus and Helicotylenchus spp., were influenced by soil moisture levels, although root distribution was more important in influencing the distribution of the plant feeders. The effect of the seasons on the vertical zonation patterns of the total nematodes is illustrated and discussed. The mean values of the total nematodes recovered had a spring depression and a peak which seemed to rise through summer to autumn. The seasonal pattern at 0m with autumn and spring peaks differed from that at 1m, which had a summer to autumn maximum. There was a winter depression in both, but the significant spring fall at 1m corresponded to the high summer reduction at Om. The seasonal distribution of the Tylenchus species was very close to that for Om totals, while Pratylenchus and Helicotylenchus species followed a pattern similar to that recorded for 1m totals. Alaimus species had maximal populations in autumn, with a gradual decrease to summer minima. The seasonal population fluctuations of Tylenchus and Hemicycliophora species coincided with the period of lower soil temperatures (58°F), but vegetation cover constituted the most important ecological factor in this orchard.