The assessment of populations of soil-inhabiting invertebrates

The nematode resistance locus Gpa2 was mapped on chromosome 12 of potato using information on the genomic positions of 733 known AFLP markers. The minimum number of AFLP primer combinations required to map Gpa2 was three. This demonstrates that a reference collection of potato AFLP markers may be a valuable tool for mapping studies in potato. By use of RFLP probes, Gpa2 was more precisely mapped at the distal end of chromosome 12. Gpa2 confers resistance to a distinct group of populations of the potato cyst nematode Globodera pallida and originates from the same potato accession as locus H1, conferring resistance to pathotype Ro1 of G. rostochiensis. This study shows that these two nematode resistance loci are unlinked and that Gpa2 is linked to the Rx1 locus conferring resistance to potato virus X. The efficiency of AFLPs for genetic mapping of a highly heterozygous crop like potato is discussed and compared with the RFLP technique.

Mapping of the cyst nematode resistance locus Gpa2 in potato using a strategy based on comigrating AFLP markers

Population dynamics have received a minimum of nematologists' and plant pathologists' attention from the time crop rotation was advised as a control measure against Heterodera schachtii in beets (2, 37) and H. ros­ tochiensis on potato (84). Al though population densities were not yet actually measured the advice was based on (i) the supposition that the damage to be expected is directly related to density of the nematode in the soil and (ii) the concept that mortality during a certain number of years in which no host is grown and therefore no food supplied balances population increase during a season in which a host is grown. If a is the rate of reproduction of the nema­ tode on a host, b the proportion of the population at the beginning of a year that survives through the year, and x the number of years in which no host plant must be grown to balance a, then ab" ::; 1 or x ;::: log a/log b 1.

Dynamics of Populations of Plant Parasitic Nematodes

PM 7/119 (1) Nematode extraction

Summary

Potato cyst nematodes (PCN) have been known to occur in the UK for nearly a hundred years. They are the most problematic pests of potatoes and can cause severe yield losses. Previous work has shown the two species, Globodera rostochiensis and G pallida, to be distributed throughout the UK. This paper reports the results of the first structured and statistically unbiased survey undertaken to assess their occurrence and distribution in the potato growing land of England and Wales. The survey showed that PCN were present in 64% of sites sampled. Of the populations found, 67% were G pallida, 8% were G rostochiensis and 25% contained both species. The results show an increase in the incidence of PCN since previous studies were completed and confirm the perceived shift towards G pallida as the predominant species. Of the infestations found, 62% had a population density of less than 10 eggs g−1 soil.

Potato cyst nematodes in England and Wales: occurrence and distribution

A method of recovering cysts from soils of not more than 10% water content is described. The method is continuous and the yield from it is only slightly lower than that from the dry flotation of soil. The yield was found to contain a slightly lower percentage of full cysts than that obtained by dry flotation but this drop was not considered sufficiently great to justify the drying of soil in large quantities. Quantities of soil in the neighbourhood of 1 cwt. can be handled continuously in a matter of two hours.

D. W. Fenwick

Papers

Methods for the Recovery and Counting of Cysts of Heterodera schachtii from Soil