Work on a much-needed rapid identification system for alternaria in potatoes is to get under way this year, as Louise Impey discovers

Difficulty in identifying which strain of alternaria is attacking a potato crop makes the disease complicated and sometimes costly to manage.

So the need for some form of analysis or quick test is vital to help agronomists and growers make more-informed decisions early, says Agrii’s potato specialist Barrie Florendine.

“At the moment, it’s extremely hard to know if lesions are due to one of the two strains of alternaria, ozone damage or some other cause.”

Alternaria alternata tends to occur throughout the growing season, at varying levels, but the more damaging Alternaria solani usually comes in later, he explains.

The importance of knowing which is the culprit was apparent last year, when expensive blight fungicides were employed at the first sign of disease symptoms, he recalls. “The assumption was that it was active A solani, so products such as Olympus and Signum were applied.”

In fact, spore trapping showed that there was very little A solani present in 2011. “There was just the odd spore,” notes Mr Florendine. “That was unexpected and went against our previous monitoring work. It also meant that the money spent on sprays was wasted.”

While the very dry conditions may have been responsible for the lack of A solani spores last year, he points out that the climate in the potato-growing areas of the USA is much drier and yet the disease is a major problem there.

“When it gets into the upper canopy, there can be significant yield loss. US growers are coping with a much worse situation. So we mustn’t make assumptions.”

The unpredictable nature of the pathogen means that an integrated approach to its control is the best course of action, he believes.

“Certain varieties are more susceptible and Markies is particularly bad. So the agronomy of Markies is very important, as other varieties being grown in close proximity to it are put under extreme pressure.”

Nitrogen management, avoiding crop stress and increased use of mancozeb and chlorothalonil can all help, he says.

Decision-support systems have a role too, he stresses. “Used with spore trapping, they can help to identify the most appropriate chemistry and spray timings. Waiting until A solani has been identified allows the right control programme to be initiated.”

That’s why the development of a rapid test, which can be done in the field or turned around very quickly by a laboratory, is so important. “You do need to know what you are dealing with.”

He has been working with Howard Hinds of Dacom’s Forecast Xtra for the last three years to forecast when conditions are conducive for the disease and to refine the use of fungicides according to disease pressure.

The good news is that the fungicide armoury exists to manage the disease, he advises, with Olympus and Signum, Roxam/Electis, Tanos and higher rates of Invader all being effective against A solani.

“There are also some dual-purpose fungicides, such as Consento, which combine alternaria activity with late blight control. They can be boosted by adding an A solani active material, if need be.”

There is some evidence, however, that there is variation between fungicide groups and how well they work on alternaria, he remarks.

“The suggestion is that there’s been a fall-off in activity from some chemistry. It appears that the strobilurins aren’t as effective as they used to be in the USA.”

As a result, fungicides will also be under the spotlight in his future work, as Mr Florendine intends to conduct in-vitro studies on them with disease inoculum. He will also be investigating variety susceptibility further, having noticed differences in the field.

“There are no signs that alternaria is going to disappear,” he points out. “And as long as we continue to grow Markies, it’s likely to become a greater problem.”

The disease is more prevalent in the eastern part of the UK and is favoured by warm weather. “A dry, warm summer with intermittent showers is perfect for it.”

Active ingredients

• Consento – fenamidone + propamocarb hydrochloride
  
  
  
• Electis/Roxam – mancozeb + zoxamide
  
  
  
• Invader – dimethomorph + mancozeb
  
  
  
• Olympus – azoxystrobin + chlorothalonil
  
  
  
• Signum – boscalid + pyraclostrobin
  
  
  
• Tanos – cymoxanil + famoxadone