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Optimising irrigation for potato scab control

Understanding varietal susceptibility is crucial when deciding on irrigation strategy to prevent common scab in potatoes, says Mark Stalham, senior research associate at NIAB-CUF


Heather   Briggs

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Heather   Briggs
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Irrigation practices need to be perfect to control common scab on the most susceptible varieties
Irrigation practices need to be perfect to control common scab on the most susceptible varieties

Effective management of the disease in Maris Piper calls for perfect timing and perfect irrigation. The good news is growers can afford to be more flexible when it comes to varieties with lower susceptibility to the pathogen.

 

Quality standards for skin finish are tight in both the pre-pack and processed sectors, and common scab is estimated to cost the ware industry around £3 million a year. As tolerances to common scab are declining, getting the irrigation strategy right becomes crucial to achieving market requirements.

 

Timing

Many growers of the most susceptible variety, Maris Piper, can be caught out as they do not realise how soon scab gets into the crop, says Dr Stalham, who has just finalised an AHDB-Potato Council-funded study on early irrigation.

 

“They wait until they see the first tubers have been formed, but by the time they get over the field they are too late.”

 

Another common mistake is to water for a couple of weeks and then stop, which can result in worse scab than not irrigating at all.

 

“Understanding when to start and stop is crucial to getting scab control right,” says Dr Stalham. “Irrigation practices need to be perfect on the most susceptible varieties, such as Maris Piper.”

 

Maris Peer also appears in this category because of the sensitivity to blemishing in the market place with regard to salad potatoes.

 

“You need to start at tuber initiation and continue until the tubers have grown out of the susceptible phase, which is four weeks in a main crop and six to eight weeks in a salad crop. This is because tubers in salad crops grow more slowly and therefore remain susceptible for longer.

 

“Starting irrigation early and finishing it too soon can actually exacerbate the problem with these susceptible varieties,” adds Dr Stalham.

 

Greater resistance

But this is for the worst possible scenario, he adds, and growers with varieties with greater resistance to the pathogen can look at different strategies as starting date and duration are not so critical.

 

The pathogens which cause common scab do not start to build up on the surface of the tuber until two to three weeks after initiation in less susceptible varieties, which gives growers more of a window.

 

“Nevertheless, the crucial thing is to wet soil up, but it’s not about starting exactly at tuber initiation,” says Dr Stalham.

 

In addition, with these less-susceptible varieties, the soil does not need to be as wet as for Maris Piper.

 

Varietal influence

“Although there is certainly still an important need to know exactly when tuber initiation occurs and when the soil needs wetting, the research is really about the variety. Those that don’t develop scab have low levels of pathogens, so there is a clear link between the two.”

 

It is not yet known whether these low levels are due to suppression or the absence of attractants released by the tuber, or even the presence of antagonists on the less susceptible varieties. “What is clear is that the good bacteria develop rapidly under wet conditions on resistant varieties,” he says.

 

Growers’ concerns about wetting the soil at tuber initiation and scab often results in over-irrigation, particularly when rain-guns are used.

 

“The poor uniformity of water distribution causes growers to over-water to avoid dry areas. When you over-water early in the season, the soil has a limited capacity for water and you can end up with over-wet patches that can cause problems such as powdery scab, blackleg or tuber cracking. These are all equally as disfiguring as common scab and result in serious rejections.”

 

For more details go to www.potato.org.uk/sites/default/files/publication_upload/1%20Early-season%20irrigation.pdf

 


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Variety resistance to common scab

 

Susceptible

Intermediate susceptibility

 

Resistant

 

Maris Piper

Charlotte

Bute

Maris Peer

Desiree

Electra

 

Estima

Elfe

 

Exquisa

Jelly

 

King Edward

Lanorma

 

Maribel *

Orchestra

 

Melody

Perline

 

Nectar

Regina

 

Rooster

Vales Sovereign

 

Sylvana

Volare

 

Safari *

 

 

Venezia

 

 

Vivaldi

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: * Tentative results for Safari and Maribel

Source: Mark Stalham, NIAB-CUF

Top tips for scab control

Top tips for scab control
  • The critical Soil Moisture Deficit (SMD) ranges from 10mm to 31mm depending on the soil type and variety
  • Always look for tuber initiation in the most advanced plants to judge the start of the irrigation regime.  And remember the time frame of scab infection is extended where crops don’t emerge evenly
  • Maris Piper – always start your irrigation at the start of tuber initiation. For other varieties you can delay by a week, but the irrigation system must be capable of completely wetting the ridges across all the area under scab control
  • Once you've started your common scab control regime don’t stop until the risk period is over.  There’s evidence that stopping before the end of the critical period allows a rapid build-up of the scab organism and high risk of severe scab.  The critical period to maintain wet soil is 1-3 weeks after tuber initiation
  • How long is the risk period?  New evidence shows it’s possibly not as long as you thought. Look in the guide for details of main crops and salad varieties

Source: PCL/NIAB-CUF

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