Common scab control: reducing the irrigation water requirements and understanding the effects of beneficial soil microorganisms

Results from experiments examining the correct length of control period for salad potatoes, gave no indication that irrigation for 8 weeks was more successful in preventing scab than irrigating for 6 weeks.

Trials to assess the effects of the cloddiness of the ridge structure at planting demonstrated little effect on common scab incidence and severity and that irrigation regime was the over-riding factor in determining the level of control of scab.  Aggregate size distribution may have less effect on scab development than previously thought.

A substantial amount of new information on the microbial communities on tubers from different sites and seasons was obtained. Controlled environment studies showed that in the absence of microorganisms other than the scab pathogen, irrigation level alone was shown to have no effect on populations of pathogenic Streptomyces or on development of disease, indicating that an intact soil microflora is required to mediate control by irrigation.  However, across the tuber samples there was no clear correlation between particular microbial groups and a reduction in scab severity. As such, it was not possible to identify any potential antagonists of pathogenic Streptomyces that warranted further study.

Information on the detrimental effects of overwatering (during tuber initiation and the scab control phase) in relation to tuber cracking and nitrogen uptake is provided. Nitrogen uptake by plants was reduced by over-watering during the 3 weeks after TI, possibly as a consequence of nitrogen leaching below rooting depth and of denitrification. This reduced canopy size, canopy longevity and yield. Over-watering later in the season had relatively little effect on nitrogen uptake or crop performance, indicating greater sensitivity of the rooting system to waterlogging early in the crop’s life.

An experiment in 2011 showed that the cloddiness of the ridge structure at planting had little effect on common scab incidence and severity and that irrigation regime was the over-riding factor in determining the level of control of scab. Eight trials in commercial fields examining the aggressiveness of declodding on common scab showed no effect of web pitch, star spacing, rotor or forward speed on scab incidence and severity, indicating that ped size distribution may have less effect on scab development than previously thought.

Practical recommendations

• Irrigation regimes for common scab could be adapted according to varietal susceptibility
• For all varieties other than Maris Piper, delaying start of irrigation until 1 week after initial TI would produce equally good control of common scab to commencing irrigation at TI. Delayed-start irrigation timing should be based on initial TI as using the date of 50 % TI in variably-emerging fields could lead to more scab infection
• A 6-week period for scab control is sufficient in susceptible varieties such as Maris Peer and probably a 4-5 week period in less susceptible varieties such as Regina or Venezia
• Over-watering during TI and the scab control phase should be avoided as this increases the incidence of tuber cracking and rotting diseases and reduces nitrogen uptake and promotes early senescence in some varieties
• Growers should not be producing overly-fine seedbeds as this does not improve control of common scab