The influence of plant density and position in field trials designed to evaluate the resistance of carrots to carrot fly (Psila rosae) attack

The results of field trials designed to evaluate the resistance of carrots to carrot fly (Psila rosae) attack were influenced by plant density and position within trials. Five trials are described, their results analysed and implications for future work discussed. Density effects were of major importance when the range of densities within a trial was greater than about three-fold. Density and damage were associated, carrot cultivars and families with the most roots having the least damage; plant densities achieved should therefore be as close as possible to the targets set. Positional effects were often very important, so trials should ideally have no more than about 10 plots in a block, possibly by using an incomplete block design. Conventional analyses of variance removing block effects may be sufficient but it is desirable, especially with large blocks, to use some form of nearest neighbour analysis for which the various possible techniques gave similar results.