BIOLOGY OF THE HOP CYST-NEMATODE HETERODERA-HUMULI .2. HOST-PARASITE RELATIONSHIP OF THE NEMATODE AND ITS INTERACTION WITH VERTICILLIUM-ALBO-ATRUM

Heterodera humuli inhibited plant growth in pot experiments when the n ematode 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 n umbers of newly produced cysts on different cultivars but the sizes of the cysts differed significantly. The biggest cysts were found on a w ild 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 cv s Northdown or Target. The growth of nematode infested plants of a wil t susceptible hop cultivar was significantly reduced when the plant pa thogenic fungus Verticillium albo-atrum was also present. Verticillium did not significantly influence the reproduction of H, humuli under e ither laboratory or field conditions although hatching of juveniles in the field was delayed by two weeks and more juveniles were found in t he soil during late summer around the roots of fungus infected plants in the field.