Differences in susceptibility to attack by Brevicoryne brassicae (L.) on brussels sprouts

The attack by alate and other forms of Brevicoryne brassicae (L.) and the amount of leaf malformation they caused on each of eighteen varieties or plant breeders' lines of brussels sprout were assessed in both 1967 and 1968 over the first 6 weeks following planting out. The results of the two years agreed closely despite differences in the dates of planting relative to aphid immigration. As a group, the Dutch varieties Roodnerf, Gravendeel, Tribune, Hercules and Castricum Glory were preferred by the incoming alates but suffered less damage than the least preferred inbred line, 3BL, of Cambridge No. 3. Some evidence of antibiosis was obtained, especially from a later experiment with three Cambridge inbred lines. However, in 1969 when there was a high rate of aphid multiplication on six varieties representing sprout types from Holland, Cambridge, Bedford and Ireland, any expression of host non-preference or antibiosis was soon obscured by the development of large aphid infestations and much damage. Under these circumstances, considerable varietal differences in tolerance to attack were found. Irish Elegance sustained most damage yet its most heavily attacked plants supported little more than half of the aphid numbers of equivalent plants of the most tolerant variety Darkmar 21. The Dutch varieties Vremo Inter and Seven Hills were almost as tolerant as Darkmar 21 and recovered quickly from damage. Darkmar 21 also showed high tolerance to attack in 1970, as did the Dutch varieties Rollo, Roodnerf and Stiekema, when large aphid infestations developed on the eight varieties then grown. Lack of plant tolerance to damage appeared to be associated with incurling foliage, a characteristic of Irish Elegance, Cambridge No. 1 and No. 3.