SummaryThe effects of chloropicrin fumigation of field soils before replanting apple are frequently predicted from bio‐assays using pot‐grown apple seedlings. The results of 506 such bio‐assays carried out during 12 years are discussed with reference to modifications which were made to the test method. The interpretation of the most recent and valid bio‐assay results is considered in relation to the different purposes for which such tests may be conducted.When all test soils were amended with phosphorus (to compensate for the eradication of vesicular‐arbuscular mycorrhizal fungi by fumigation) the proportion of old orchard soils yielding economically significant growth responses increased from 39% to 68%. When, additionally, plant growth was assessed by shoot fresh weight (rather than shoot length) the proportion further increased to 82%.The traditional method of expressing growth responses to soil sterilisation, as proportionate growth increases, was found to be subject to skewing effects of non‐biological fertility factors; this feature was avoided when results were given as actual growth increases. The latter value therefore is considered preferable when assessing the relative disease status of soils for etiological purposes (as opposed to predicting the cost‐benefits of field soil treatment).The data indicated that disease severity was on average diminished in the most acidic soils (pH 4.8–6.1), but did not support the generalisation that disease severity increases progressive