Biology of the hop cyst nematode Heterodera humuli

SUMMARY Heterodera humuli inhibited plant growth in pot experiments when the nematode population exceeded 500 eggs g‐1 of soil; below this density there was evidence of stimulated growth. The development of H. humuli was tested on seven hop cultivars. No differences were found in the numbers of newly produced cysts on different cultivars but the sizes of the cysts differed significantly. The biggest cysts were found on a wild hop and the smallest on the cultivar ‘OR/55’. The nematode and the fungus, alone or in combination, did not affect the bine length of cvs Northdown or Target. The growth of nematode infested plants of a wilt susceptible hop cultivar was significantly reduced when the plant pathogenic fungus Verticillium albo‐atrum was also present. Verticillium did not significantly influence the reproduction of H. humuli under either laboratory or field conditions although hatching of juveniles in the field was delayed by two weeks and more juveniles were found in the soil during late summer around the roots of fungus infected plants in the field.