MORPHOLOGY AND BIOLOGY OF HOP CYST EELWORM (HETERODERA HUMULI FILIPJEV 1934) BY A. DE GRISSE and A. GILLARD Rijkslandbouwhogeschool,Gent, Belgium Since 1957, all hop gardens in Belgium examined have been found to be infested by Heterodera humuli, with an average of 100 (5-390) cysts per 100 g of dry soil, each cyst containing about 16 (2-55) larvae, which is an average for the field population. Young mature females contain 229 (194-263) larvae and eggs. Many measurements gave good average dimensions of cysts (L: 480 µ X B: 371 µ), larvae (L = 425 µ), giant larvae (L = 560 u), eggs (L = 98 µ) and giant eggs (L = 139 µ). There was no correlation between the measurements of perineal patterns and those of the cor- responding cysts. In the field, only two host plants were found: Humulus lupulus and Urtica dioica. Cysts on Urtica dioica were smaller (L = 419 µ X B = 338 µ) than those on Humulus lupulus. In a hatching trial with root diffusates of H. lupulus, Cannabis sativa, U. dioica, U. urens and Ficus sp., using soil extract and water as controls, only diffusate of H. lupulus had more effect on the hatching of larvae than water. In 1957 Gillard & Van den Brande found hop cyst eelworms in several localities of the Asse-hop centre in mid-Belgium. This eelworm was found in all the hop gardens examined with no apparent connection between the health of the plants and the degree of infestation. Percival ( 1 895a, 1895b) and Theobald (1907-1910), thought that the hop cyst eelworm was the cause of "Nettlehead", but later, Ware (1939) and Keyworth (1946) proved that Nettlehead was a virus disease, transmitted by aphids. This led to later workers overlooking the eelworm as a disease agent, but recent workers, e.g., Sveshnikova (1956), Rademacher, Weil & Nuber (1958), Simon (1957) attempted to correlate presence of Heterodera humuli with some disease symptoms. Because no crop rotation is followed in hop gardens there is every likelihood that these eelworms influence hop production, but so far there is no experimental evidence to this effect. T'he distribution of Heterodera humuli in Belgium (Fig. I ) Thorough sampling led to the discovery of the hop cyst eelworm in the fol- lowing localities. 42 Brabant: Borcht-Lombeek, Liedekerke, Opwijk, Pamel, Wambeek. Henegouwen: Baudour, Flobecq. Oost-Vlaanderen: Baasrode, Eksaarde, Erembodegem, Gent (wild-hop). West-Vlaanderen' Elverdinge, Poperinge, Proven, Vlamertinge, Watou, West- Vleteren. Fig. 1. Distribution of Heterodera humulj in Belgium. Again all the fields examined were found to be infested. The samples from 26 fields gave an average degree of infestation of 100 ( 5-390) cysts per 100 g of dry soil. HostplantJ of Heterodera humuli In the literature the following host plants can be found: Humulus lupulus L. (Hop ) Urtica urera.r L. (Annual nettle ) Urtica dioica L. (Common nettle) CannabiJ sativa L. (Hemp ) Although Kirjanova ( 1 954 ) described the cyst eelworm found on Ficus sp. as a new species, other workers, e.g., Sher & Raski ( 1 956 ) and Nuber ( 1 958 ) consider Heterodera humuli and H. fici the same. Cooper (1955) could not differentiate between the two species using the perineal patterns. He also mentions Heterodera "urticae" on Urtica dioica roots. During our work on host plants of H. humuli in the field, H. humuli cysts were found only on hops, only a few cysts were found on U. dioica roots, whilst no cysts could be detected on U. ui-ens roots, even when they were growing between infested hops and U. dioica plants. In a glasshouse trial, hemp could not be infected with H. humuli cysts. .