The suitability was assessed of various designs for field experiments investigating plant diseases caused by airborne pathogens that can be subject to interplot interference. Use of a model to describe such interference showed that the treatments with the most dissimilar effects on controlling the disease should be allocated to experimental plots furthest apart in each block, in order to minimize the interplot interference within a block. When using large square plots, rectangular blocks were more efficient than square blocks in minimizing treatment-comparison biases due to interference between neighbours. For rectangular blocks with the square plots side by side, less biased treatment comparisons were obtained from designs with complete blocks than from designs 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 comparisons. Similarly, unbalanced designs composed only of incomplete blocks that yield less biased treatment comparisons may be better than balanced incomplete block designs when interplot variance is low. For high levels of variation, balanced incomplete block designs may be more appropriate, as increasing the precision of the treatment comparisons becomes more important than reducing the bias.