Concern about blight fungicide resistance

UK - April 28, 2005

Nearly 80% of growers and advisors said that they were "very concerned or concerned" about blight fungicide resistance, according to a survey conducted by Dow AgroSciences, who market Electis and manufacture Dithane NT.

"Blight resistance continues to be a persistent worry for growers and their advisors and measures to minimise this risk should be considered carefully when planning an effective blight programme.  The survey indicated that 81% of growers and advisors were planning to alternate fungicide groups within a blight spray programme as an anti-resistance measure.  Over half will be adding multi-site products into their programme and 48% will be using products with more than one active ingredient," reports John Sellars, Marketing Specialist for Dow AgroSciences.

"Alternating products with different modes of action is a sound policy, not just as an anti-resistance measure, but also for a robust fungicide programme.  Electis is a useful product as it contains a unique active ingredient with a completely different mode of action together with a multi-site partner.  It works by disrupting microtubules and inhibiting nuclear division, stopping the spore germ tube from penetrating the plant.  This also inhibits the production of zoospores, which cause tuber blight.  Electis has no known resistance and it will control all known blight strains, including phenylamide resistant strains.  In the survey, Electis was ranked as the top fungicide, with regard to low resistance risk," adds John.

Products with multi-site activity such mancozeb, the active ingredient in Dithane, are incorporated into the majority of existing and development blight fungicides, including Electis.  This active ingredient is at low risk to resistance as the pathogen is controlled at different metabolic sites or at different stages in its life cycle.  Research shows that mancozeb works on six distinct metabolic sites within the fungus and, although it has been on the market for over thirty years, there is no known resistance to any disease in the world. 

"Electis performs well in a programme, alternating with complimentary fungicides with different modes of action such as Ranman TP.  In BPC trials, the Belchim/Dow manufacturers' programme, consisting of two treatments of Epok (mefenoxam (metalaxyl-M) plus fluazinam), followed by Electis alternating with Ranman TP (cyazofamid plus adjuvant) up until desiccation, gave the best control of foliar and tuber blight.  It is a sound approach to ensure that potato blight is kept at bay and that the risk of resistance is minimised," says Mr. Sellars.

"Other blight products such as phenylamides and QoI's are at higher risk of resistance developing.  Already 50% of blight spores show resistance to the phenylamides.  Consequently the number of phenylamide applications per crop per year is limited on the label.  The newer fungicides based on fenamidone and famoxadone, being QoI products (the same as strobilurin fungicides), also have label restrictions.  Products containing either of these two active ingredients should be used for no more than a total number of 6 applications in any crop or 50% of the spray programme, with a maximum of 3 consecutive sprays.  To further reduce the resistance risk, these actives are frequently formulated with another active ingredient such as mancozeb."     

Electis contains 8.3% w/w zoxium and 66.7%w/w mancozeb, formulated as a WDG.  It is recommended on all varieties of potatoes at spray intervals of 7 to 14 days.  It has a dose rate of 1.8 kg/ha in 200-600 litres of water per hectare.  It has a maximum number of treatments of 10 per crop and a harvest interval of 7 days.  It is packed in a 3-hectare pack (5.4 kg) and marketed exclusively by Dow AgroSciences.