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Irrigation to control common scab

13 June 2018

By Nicola Dunn

Recent dry weather will mean that more and more growers will be thinking about irrigation, which is a vital time for common scab control.

Dr Mark Stalham, Senior Scientist at NIAB CUF whose research, combined with demonstrations held at our Strategic Potato Farm network provided the most up-to-date guidance on common scab control, advises:

“Despite late planting, emergence from late April and early May plantings has been rapid and has occurred over a very short period in 2018.  Tuber initiation will occur in most varieties 14-18 days after emergence and this is the key timing for scab control irrigation.

“We would estimate that soil moisture deficit (SMD) would be around 20-25mm at this stage of the season (15-18 mm in the ridge), unless irrigation has taken place.

“Well-timed and evenly distributed water applied at three- to seven-day intervals is as beneficial to the crop as daily irrigation, and offers potential water savings and an effective control measure against common scab.”

In order to produce high yielding, quality potato crops it’s important to maintain consistent soil moisture throughout the growing season.

“However, with many early-planted seedbeds on heavier soils suffering from shallow compaction, the key is not to apply large doses of water during scab control.  Target 12-18 mm applications for boom and gun irrigators.”

Irrigation scheduling services provide valuable insights to help growers plan irrigation to keep within the guideline SMDs.

It’s also worth keeping an eye on the Environment Agency (EA) and Scottish Environment Protection Agency (SEPA) websites for their regular water situation reports.

The reports use MetOffice data to provide calculated average SMDs by area across the UK. You can find the reports by following the below links:

https://www.gov.uk/government/publications/water-situation-local-area-re...

https://www.sepa.org.uk/environment/water/water-scarcity/.

Please note: These reports do not substitute local data and a good irrigation scheduling system. Any irrigation already applied to the crop should be taken into consideration when estimating your SMD.

How long should I use control scheduling for?

  • For Maris Piper, with a typical commercial emergence period, 31 days after start of tuber initiation (TI)
  • Maintain the correct SMDs for varieties and soil types (see below table) for three to four weeks after TI
  • Potato varieties for the salad market are at risk from common scab infection for much longer owing to the small size of tubers. Scab control for six weeks is sufficient, even in susceptible Group 1/2 varieties such as Maris Peer or Charlotte and as short as four to five weeks in more resistant Group 3/4 varieties such as Perline, Regina or Juliette
  • Ceasing irrigation two weeks after TI allows pathogens to multiply rapidly and can increase scab infection unless rain falls
  • Avoid overwatering soils during the scab control period, particularly in the first week after TI. Excessive irrigation or soils kept above field capacity for substantial periods can aggravate other disease problems and impede root growth, leaving the crop more susceptible to drought later in the season
  • Uneven or protracted emergence lengthens the control period required. This increases costs and water use, and risks higher levels of scab. Seed management, soil cultivation, planting and agronomy can all increase crop uniformity and improve scab control
  • The allowable SMD during the scab control period can be increased to one suitable for preventing yield loss (eg 30mm in sands, 50mm in silty or clay soils) immediately after the period is over. Unless over-watered, there is little evidence that frequent irrigation makes the crop more susceptible to drought later in the season

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