Effect of contrasting irrigation regimes on populations of Streptomyces and potential antagonists and the control of common scab

Streptomyces species cause common scab of potato. Diagnostic techniques were used to monitor changes in their populations under contrasting irrigation regimes. Samples from the trials were also used to identify populations of potentially antagonistic microorganisms present in soils where scab levels were lower. 

Field experiments were conducted in 2009, using the varieties Maris Piper, Hermes and Vales Sovereign. Five irrigation regimes were used, either irrigation at 20 mm SMD throughout the experiment, 20 mm SMD throughout except for between four and six weeks after tuber initiation, over-irrigation for three weeks post tuber initiation, over-irrigation between 10 and 13 weeks post initiation, and unirrigated. Tubers were sampled weekly from one to five weeks after initiation to determine Streptomyces populations, and at harvest to measure yield and scab level.

Results

Control of scab was most effective when plots were maintained at field capacity following tuber initiation, although over-irrigation was detrimental to canopy duration and yield. In dry soils, populations of pathogenic Streptomyces increased between three and four weeks after tuber initiation. Levels were lower in irrigated soils. The critical period for control of scab by irrigation was between one and three weeks following tuber initiation.

DNA sequencing was used to identify groups of microorganisms which may be responsible for suppression of scab where low populations of pathogenic Streptomyces (and hence low levels of scab) have been observed. Although no single group of microorganisms was shown to be prevalent at all of the diverse sites studied, some groups of bacteria were found to be associated with suppression of scab in more than one trial.