Potato growers and advisers should not be complacent about managing the build up of blight fungicide resistance, particularly with some of the newer blight products being at high resistance risk, warned Interfarm UK Ltd.

According to a recent survey conducted by Interfarm, 70 per cent of growers and advisers said they were 'very concerned' or 'concerned' about blight resistance. But this was much less than a similar survey the previous year when 88 per cent were in this category, and was surprising because 54 per cent of growers and advisers said that resistance was increasing while only four per cent said this risk was decreasing.

Dr David Stormonth, technical manager for Interfarm, said: 'We were interested that potato growers and advisers appeared to be less worried about resistance to late blight, yet they all employ several methods that are aimed at minimising this risk.' The focus group survey indicated that 94 per cent of growers and advisers were already alternating fungicide groups within a blight programme, as a means of minimising the risk of resistance. More than 77 per cent said that they also used products with more than two active ingredients as a method of reducing resistance risk, and 71 per cent said that they specifically used multi-site fungicides. In addition to these strategies, 44 per cent were reducing their use of phenylamide fungicides, with 21 per cent avoiding their use altogether.

Stormounth continued: 'The components of Electis and Dithane NT have different modes of action and are important potato fungicides for use in blight resistance management programmes. Electis is based on a unique active ingredient, zoxium, which has a completely new mode of action, distinct from all other potato blight products.

'It works by disrupting microtubules and inhibiting nuclear division, thereby stopping the spore germ tube from penetrating the pant and also inhibiting the production of zoospores, which are the cause of tuber blight. It also has no known resistance and it will control all known blight strains including phenylamide resistant strains. This is why it is approved for use up to 10 times per crop, a feature not found with many other blight products. In the survey, Electis and tin-based fungicides were ranked the top fungicides with regard to low resistance risk.' He went on to say: 'Electis also contains mancozeb, which is the active ingredient of Dithane NT. Mancozeb is recognised as an excellent protectant fungicide and research shows that it works on six distinct metabloic sites within the fungus. This multi-site activity is important to any resistance management and explains why this active ingredient is incorporated into two-thirds of all blight fungicides used in the UK, including the most recently launched product.

'Potato blight is a disease that is highly conducive to resistance problems, as it has a number of generations per year, a short germination time and it produces many spores, all leading to the need for frequent fungicide use. Products with multi-site activity such as the dithiocarbamates are at low risk to resistance and they have never been known to experience resistance, even after 30-plus years of use. Other products such as phenylamides and cyazofamid are single site and are at high risk. This is why the FRAC guidelines indicate that when using phenylamides, they should be used only as a preventative spray early in the season when disease pressure is low and why the number of phenylamides applications per crop per year is limited on the label. This is also why cyazofamid can be used only six times per season in blocks of three which must be interspersed with a block of three sprays from another group and why famoxadone can only be used in 50 per cent of the spray programme. The new fungicides based on fenamidone and famoxadone are also being closely linked to the mode of action of strobilurins and they should also be used carefully.'