Control blight with resistance in mind

UK - May 03, 2007

Potato growers and advisors need to plan a robust approach to potato blight control that alternates the best chemistry throughout the programme, not just for the sake of efficacy but also for resistance management, say Dow AgroSciences.

"Potato blight is a devastating disease which appears to becoming more aggressive.  At this year's Blight Forum, experts intimated that Continental research suggested that new blight populations appeared to be more aggressive, having a shorter latent period, that they may produce more spores and appeared to be active at more extreme temperature.  So effective control combined with resistance management must be the right way forward," says Andy Leader, Principal Biologist for Dow AgroSciences.

Andy points out that it is important to apply each blight fungicide at its most appropriate time.  "When the haulm is growing rapidly, many growers use a systemic fungicide so that the new growth is protected.  But from stable canopy onwards, they need a strong protectant fungicide with proven foliar and tuber blight activity, such as Electis.  Throughout the programme consideration must also be given to resistance management and multi-site fungicides such as mancozeb (as in Dithane and Electis) should be planned into the programme," says Andy.

"But during this the planning process, the maximum number of treatments that can be applied to the crop must be adhered to.  According to the labels, Electis can be sprayed up to 10 times, dimethomorph + mancozeb (Invader) 8 times with a maximum dose of 16 kg/ha per crop, and cyazofamid (Ranman) and benthiavalicarb-isopropyl + mancozeb (Valbon) up to 6 times.  Fluopicolide + propamocarb (Infinito) can be applied up to a maximum of 6.4 l/ha per crop, so at 1.6 l/ha has a maximum of 4 sprays.  Mandipropamid (Revus) can be used just 4 times, with no more than three consecutive sprays," reminds says Andy.

"You may also have to take into account restrictions based on fungicides that come from the same group.  For instance mandipropamid (Revus), benthiavalicarb-isopropyl + mancozeb (Valbon) and dimethomorph + mancozeb (Invader) are all from the carboxylic acid amide (CAA) group.  The new fungicide mandipropamid therefore has a maximum four sprays per crop, which can be applied up to a maximum of three applications in a block and should then be alternated with non CAA chemistry.  The planning process can be complicated for some products - but not for Electis," says Mr. Leader.

"Electis has low resistance risk as it contains two complementary active ingredients, mancozeb and zoxium.  There is no known potato blight resistance to zoxium, with a different mode of action, distinct from all existing and new potato blight products.  It will control all known blight races and strains, including phenylamide resistant strains.  Mancozeb is a highly effective protectant fungicide with multi-site activity and again has no known resistance, even after thirty years of use.  For this reason, it is still incorporated into nearly two-thirds of all blight fungicides used in the UK, including most of the newer products.  Because the Electis label allows up to ten sprays to any one crop, it means that growers have ultimate flexibility as to how they integrate this fungicide in their programmes, not the case for other fungicides."

"Fungicide resistance is an increasing concern for growers and advisors, whether it is in potatoes or cereals, and measures to minimise the problem should be implemented as part of an effective disease control programme.  General advice is to alternate fungicide groups within a programme, to use multi-site products more frequently and to integrate products with more than one active ingredient."," advises Andy.

"Electis is a strong protectant fungicide that is best used from stable canopy onwards.  In many years of successful commercial usage, it has shown excellent activity on both foliar and tuber blight and has been categorised as one of the top foliar blight fungicides as well as one of the best protectants by independent European industry experts.  It is very effective against both foliar and tuber blight and is a highly suitable product to integrate into any blight programme, appearing to enhance the programme and help in minimising any resistance risk," concludes Andy.