Decision making for root disease control: a problem in reducing the nugget variance

Root disease has the potential to cause large economic losses of agricultural production. Techniques are available for assessing the amount of pathogen present in soil using DNA assays. There is spatial variability in pathogen levels across fields and spatial methods would appear an obvious tool to use to map the incidence and distribution and as a basis to plan cropping programs. The information required for agronomic decisions has to be obtained in sufficient time and at an acceptable price for this to be a viable technique. Two examples where this is being used are wheat in large (40 to 100 ha) fields and potatoes grown in centre pivots. The largest difficulty encountered is obtaining a sampling scheme that produces a small nugget variance. Alternative sampling strategies are considered.