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Growing salad potatoes is a specialist activity. If the specifications are not achieved the crop will have little value.  Besides specifications for size the other main criteria relate to quality. This means virtual freedom from blemishes.  Common scab is the most important cause of blemish and quality loss.  To prevent infection from the bacteria that cause common scab (Streptomyces spp.) adequate soil moisture should be present during the critical period of tuber initiation and for several weeks afterwards.  For salad crops, it is desirable to have the availability of irrigation to counter the risk of inadequate soil moisture during the critical period.

Populations of pathogenic Streptomyces bacteria on the surface of tubers increase rapidly after tuber initiation, with the increase faster in dry soils than wet soils. These pathogenic populations can be suppressed by antagonistic micro-organisms but these antagonists are favoured by moist soil conditions.  Thus soil moisture in the critical 2-4 weeks after tuber initiation must be maintained below a maximum soil moisture deficit level.

AHDB Potatoes in the UK have published guidelines for maximum soil moisture deficit for common scab control. An adapted table is shown in Table 1.

Table 1. Maximum soil moisture deficit (SMD, mm) for common scab control in different groups of varieties








Soil texture

Example salad varieties

Maris Peer (5)

Charlotte (4)

Exquisa (7)








Loamy sand





Sandy loam





Sandy silt loam





Silt loam





Clay loam/clay





The SMD values are for the top 25cm of ridge and stone-free ridge profile. Values in brackets are published ratings for common scab resistance in the AHDB Potatoes variety database (https://potatoes.agricrops.org/), a low figure indicates greater susceptibility to common scabAdditional factors to optimise the impact of irrigation

  • Ensure salad crops are grown in soil with a fine tilth. Where soils are cloddy, voids or air spaces exist in the soil and uniform moisture on the surface of developing tubers is difficult to achieve. In such soils a lower soil moisture deficit may need to be maintained
  • Aim to avoid over-irrigation and periods of soil saturation as this may lead to other effects on skin quality. When irrigating, aim to return the soil to field capacity. Thus if the soil moisture deficit is 18mm, avoid applying more than 18mm irrigation
  • Where salad crops are planted evenly and emerge evenly tuber initiation will also occur evenly. This will reduce the critical period for susceptibility to common scab. If emergence is uneven, maintaining SMD below the target will be required for a longer period.
  • To maintain adequate soil moisture, irrigation scheduling or an accurate water balance sheet should be used
  • Tuber initiation is defined as when the swollen tip of the stolon is twice the width of the stolon. In practice, to avoid missing the start of the critical period of tuber initiation, maintaining the soil moisture below the maximum SMD should start just before tuber initiation begins

After the critical period for common scab, the requirement for irrigation in salad crops is much less and much higher soil moisture deficits can be tolerated