SummaryPotato tubers (cvs Cara and Bintje) were grown in compost in a glasshouse and immature tubers harvested 57, 68 and 78 days after planting. Two moisture levels were imposed after the first harvest by disconnecting the water supply to one of the treatments and allowing the soil in that treatment to dry naturally. Tubers from wetter compost (59.4% moisture holding capacity) were more resistant toPhytophthora infestansthan those from drier compost (14.7% moisture holding capacity) 78 days after planting. The potential causes of this difference were investigated. Aqueous extracts of wet compost did not inhibit the growth ofP. infestans.The susceptibility of the internal tuber tissue, from which the periderm had been removed, was different to whole tuber susceptibility. The internal tissue of tubers from wet compost was more susceptible (cv. Cara), or as susceptible (cv. Bintje) as that of tubers from dry compost 78 days after planting. Fungi were isolated from the surface of whole tubers and there were no differences between the populations of potentially antagonistic fungal genera on tubers from wet and dry compost. As the experiment progressed, the number of bacteria per gram fresh weight on tubers grown in wet compost increased, whereas that on tubers from drier compost decreased (cv. Bintje) or remained similar (cv. Cara). There were significantly (P= 0.008) more bacteria on the surface of tubers from wet compost 78 days after planting. WhenP. infestanswas co‐cultured in Petri dishes with randomly selected tuber surface bacteria, some isolates (≤ 16.7%) inhibited the growth of the fungus. The percentage of the total bacterial population that was antagonistic toP. infestanswas not significantly affected by soil moisture level (P= 0.368). The greater numbers of bacteria, of which a proportion are antagonistic toP. infestans, on the surface of tubers grown in wet compost may account for the greater resistance to tuber blight in that insta