Effects of water potential on the infection of potato tubers by Streptomyces scabies in soil

When soil was maintained at a mean water potential (Slatyer & Taylor, 1960) of the order of -80 J kg^-1 at 25 cm depth throughout the growing period, much infection of potato tubers by Streptomyces scabies occurred, but when soil was irrigated to maintain it at potentials greater than - 13 J kg^-1 at 10 cm depth infection was negligible. Until about 5 weeks after initiation, tubers were very susceptible to infection; irrigation during this period reduced scab considerably, but subsequent irrigation reduced it only slightly. At the low water potential, actinomycete populations on lenticels were high and bacterial populations low; these effects were reversed at the high water potential. Since also actinomycetes were more frequently isolated in the absence than in the presence of bacteria it was deduced that there was an interaction between these two groups. It is suggested that irrigation may decrease the population of S. scabies in tuber lenticels by increasing populations of bacteria antagonistic to it.