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Don't Underestimate Compaction:

Few potato soils are over bedrock which confine rooting, but compaction in the profile can change the effective soil depth on all soils, light and heavy. In seasons where there is moderate, evenly-spaced rainfall, crops grown without compaction can yield close to similar crops grown with irrigation. With compaction, especially where it is shallow, potato crops yield poorly. The target for minimizing the need for irrigation is clear; to create as deep a root system as possible, and the key is the quality of the soil conditions into which the crop is planted.

Too Much Water:

At full maturity, potato plants lose much mater through transpiration.

  • August the plant requires a regular supply of moisture for optimum yield and growth.
  • During the months of July and August the crop requires 3½ mm of water per acre every 24 hours (almost 25mm per week). Whether this is supplied by rainfall or irrigation makes little difference to the crop.
  • Do not apply in excess of 30 mm of water at any one time as this may result in restrictions to growth caused by temporary waterlogging, physical damage to both the soil and canopy and lead to quality defects such as erupted lenticels.
  • Identify the "trigger" soil moisture deficit (approx. 35mm).
  • Apply 25mm (do not bring the soil to full water holding capacity - subsequent rainfall will induce water logging, run off or leaching).
  • Watch the frequency of repeat applications in relation to crop growth rate/canopy size. (Large canopies remove water rapidly through the transpiration system. Small canopies - water is lost from the soil surface through evaporation).

Irrigation and Disease Control

a) Powdery Scab

If powdery scab is present in either seed or in the soil, then this disease can be increased by large fluctuations in soil moisture during the period following tuber initiation. Cold soil temperatures and low oxygen concentration as a result of waterlogging delays suberization of lenticles, thereby increasing the susceptibility of infection by both forms of scab.

(b) Common Scab (Streptomyces scabies)

This disease causes serious loss of skin quality in potatoes, particularly in susceptible varieties. Research has shown that tubers are attacked if the soil is dry as new tissue is forming. Young skin over new tissue contains small pores called stomata: as the skin matures these develop into larger pores called lenticles. The organism responsible for common scab can infect tubers through stomata and newly forming lenticles, but not mature lenticles.

Each successive layer of new tissue on a tuber is susceptible to attach for a period of ten to fifteen days only, and it then becomes immune for the rest of the season. As the tuber grows fresh areas of new tissue at the rose end are vulnerable, but the pattern of growth is such that most of the transition from stomata to lenticles is completed during the first six weeks after tuber initiation. The major concern, therefore, is to control common scab during the initial six weeks. Irrigation prevents infection by the common scab organism. So growers using irrigation to control common scab must be prepared for a 6 weeks irrigation programme.

Common scab can be reduced by paying careful attention to irrigation for six weeks immediately after the first visual signs of tuber initiation.

When irrigating to control common scab note the following:

  1. Identify the crops where skin finish is a priority.
  2. Start irrigation in time - i.e. at tuber initiation - this stage must be identified by lifting plants and examining stolens (physical ageing will reduce the period from planting to tuber initiation.
  3. Apply small amounts of water - 12 15 mm.
  4. Depending on soil moisture, applications may have to be made every 7 days

Applying the Water:

As a general rule of thumb, a little and often is a better approach to applying water, than applying large volumes of water infrequently. Each application should contain no more than 25 mm of water. In high Summer the soil can loose up to 7 mm per day, so multiple applications may be needed on a weekly basis.

(c) Potato Blight (Phytophthora infestans):

The greatest need for irrigation occurs in dry and sunny periods that are associated with blight spread. However, there is often a need to apply some irrigation to crops even in dull damp weather, as rain falls during such periods is often inadequate.

If a crop is irrigated, it should be considered as being in a “high-risk” situation for potato blight.

The best protection of foliage should result from the continued presence of an adequate level of fungicide. Thus rainfast fungicides and those with systemic or partically systemic components should be most effective. Crops with obvious blight in the foliage should not be irrigated as this could increase the risk of tuber infection.

Fungicides should be applied according to recommendation, but after irrigation whenever possible in order to prevent their being washed off. On heavier soils, it may not be possible to travel after irrigation. In this case, spraying a few days earlier would be a better option


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. (https://potatoes.ahdb.org.uk/publications/irrigation-and-effective-early-season-water-management). 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 (http://varieties.ahdb.org.uk/), a low figure indicates greater susceptibility to common scab

Additional factors to optimise the impact of irrigation

  1. 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
  2. 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
  3. 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.
  4. To maintain adequate soil moisture, irrigation scheduling or an accurate water balance sheet should be used
  5. 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