ASSESSMENT OF SUITABLE DESIGNS FOR FIELD EXPERIMENTS INVOLVING AIRBORNE DISEASES

The suitability was assessed of various designs for held experiments i nvestigating plant diseases caused by airborne pathogens that can be s ubject to interplot interference. Use of a model to describe such inte rference showed that the treatments with the most dissimilar effects o n controlling the disease should be allocated to experimental plots fu rthest apart in each block, in order to minimize the interplot interfe rence within a block. When using large square plots, rectangular block s were more efficient than square blocks in minimizing treatment-compa rison biases due to interference between neighbours. For rectangular b locks with the square plots side by side, less biased treatment compar isons were obtained from designs with complete blocks than from design s with incomplete blocks, especially when larger numbers of treatments were included in the experiment. However, when interplot variance is taken into account, incomplete blocks may give better treatment compar isons. Similarly, unbalanced designs composed only of incomplete block s that yield less biased treatment comparisons may be better than bala nced incomplete block designs when interplot variance is low. For high levels of variation, balanced incomplete block designs may be more ap propriate, as increasing the precision of the treatment comparisons be comes more important than reducing the bias.