Andrew Stilwell of distributor Bartholomews looks after a large area of potatoes and of that area he reckons that 60 to 70% are processing varieties prone to Alternaria infection.

“It seems to have got worse in the last two years. Alternaria manifests itself later on in the season and we try to integrate blight fungicides with a mancozeb component in a protectant situation early on in the programme, in order to keep this disease at bay. But you really need at least 1500 gms a.i./ha to be effective. We would then start to use a specialist fungicide such as Signum (boscalid and pyraclostrobin) at first signs of disease, usually from canopy complete onwards. It has a powerful knockdown effect and has been proven to be an important weapon in the armoury when used within a programme with other materials.”

In addition to good control of Alternaria, Andrew has also seen greening or added physiological effects on the canopy when using Signum in potatoes. “This doesn’t appear to be just a physical enhancement, but the product actually seems to maintain the green leaf for longer. It is important for the leaves of the potato haulm to “catch the rays” for as long as possible in order to enhance yield, so this effect is significant - especially so as it isn’t just a transient effect but can last two to three weeks. Such prolonging of green leaf could make a difference.”

This effect has also been reported by agronomist Martyn Cox. “I am convinced that there is more to Signum-type products that just Alternaria control. It seems to help to maintain a healthy canopy which helps to minimise other diseases and conditions.”

“Treated crops look better with more glossy leaves. In my view in potatoes it is the health, not just the size, of the canopy that matters. I am impressed how Signum-treated healthy plants look.”

Martyn is not surprised though as strobilurins in cereals are well known to produce physiological benefits. “So it will be in potatoes. Signum has to control the Alternaria obviously, but I am interested in how the crop reacts in addition to this. In agronomy many facets are related to each other, but in potato agronomy many things are looked at in isolation. For example the health of the canopy does affect other diseases such as black dot. Signum treated crops have a healthier and more even canopy and this is part of ICM for black dot. The control of tuber disease will be related to canopy health so this ought to be looked at in detail.”

The physiological, AgCelence, benefits of Signum are not insignificant and play an important role in maintaining the potato canopy greener for longer, says BASF’s Rob Storer.  “These benefits are well known in other speciality crops and it is interesting that two leading agronomists have observed them in potatoes, too.”

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Rob goes on to point out that when Alternaria is at high risk or when growing a susceptible variety such as Markies, a specific Alternaria treatment is advisable. Signum has an Extension of Authorisation (EAMU) for use against early blight or Alternaria. It has been given the top 3* rating for Alternaria control by Euroblight. With four applications possible at 0.25 kg/ha, the first treatment should be applied in a protectant situation at the start of the disease epidemic, when the weather conditions are conducive for disease development or as indicated using a Decision Support System.