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General Information

More and more crops of potatoes in Ireland are being irrigated. This is mainly to control common scab on our washing variety Rooster. In addition, irrigating to increase yield is also getting more popular.

Irrigation should commence at tuber initiation. This is when the tip of the first stolon is twice the diameter of the stolon itself.


(a) Reduces Common Scab.
(b) Reduces risk of second growth.
(c) Reduces risk of cracked tubers.
(d) Increases yield.


(a) High capital cost
(b) High labour cost in moving equipment from place to place.
(c) Increases the risk of powdry scab especially in susceptible varieties like Rooster, Cara, Saturna and Lady Rosetta.
(d) Makes blight control more difficult.
(e) Can cause machinery mobility problems during harvesting on heavy land or land with poor soakage.

Crop Responses (Per Mm Of Water Applied

CropAverage Yield response - T/Ha. per mm
Cereals 0.018
Peas - vining 0.04
Peas - dried 0.04
Potatoes - Early 0.08
Potatoes - Maincrop 0.08
Sugar Beet 0.13

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 attack 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:

With maincrops, early irrigation is to control common scab and late irrigation (mainly July) is to increase yield. Irrigation may have to commence before tuber initiation if soils are dry and continue for the full period of initiation. Our most recent information gives us the number of days taken from 50% emergence to 50% tuber initiation for four varieties;

  1. Cara - 27 days
  2. Russet Burbank - 23 days
  3. Estima - 19 days
  4. Lady Rosetta - 18 days

(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