Blight resistance must be a top issue in growers minds

UK - April 14, 2008

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 must be the top issue on growers minds this season, says Dow AgroSciences.

"Potato blight is a devastating disease which has become more aggressive.  At this year's Potato Council Blight Forum, experts reported that over 70% of blight populations tested in 2007 were of the new genotype 13 A2.  The A2 mating type remained rare in the UK blight populations until 2005 and now it is dominant.  With the evolution of genotype 13 it has become apparent that blight is now much more aggressive, it can grow faster, it has a shorter latent period and is possibly more active at lower temperatures.  So effective season-long control of this more aggressive pathogen must be planned with resistance management very much top of mind," says Andy Leader, Principal Biologist for Dow AgroSciences.

"The current chemistry available to growers is effective against these new aggressive blight strains, though care needs to be taken in which products to apply when and not over stretching spray intervals.  With fungicide resistance a significant concern for growers and advisors, measures to minimise the problem should be implemented in any disease control programme.  General advice is to alternate or block fungicide groups within a programme, to use multi-site products more frequently and to integrate products with more than one active ingredient.  To do this effectively growers and advisors need to go into a lot of detail on individual products, their active ingredients, their modes of action, their resistance status and their label recommendations," says Andy.

"For instance the protectant fungicide Electis will control all known blight races and strains, including A2 isolates.  It has intrinsically low resistance risk as it contains two complementary active ingredients, mancozeb and zoxium.  There is no known potato blight resistance to zoxium, which has a different mode of action distinct from all other potato blight products, or to the multi-site mancozeb which has been around over 40 years.  There has been no shift in resistance during testing of Electis since launch in 2001 and it shows excellent activity against phenylamide resistance strains.  Its label allows up to ten sprays in any one crop and growers have ultimate flexibility as to how they integrate this fungicide into their programmes."

"This is not the case for many other blight fungicides, particularly the more recently introduced products.  According to their labels, cyazofamid (Ranman) and benthiavalicarb-isopropyl + mancozeb (Valbon) can be applied up to 6 times to the one crop but with restriction on the number of successive sprays.  Fluopicolide + propamocarb (Infinito) can be applied up to a maximum of just 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," says Andy.

"With some products you also have to take into account restrictions based on fungicides 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.  Mandipropamid can be applied up to a maximum of 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 very complicated for some products - but not so for Electis," says Mr Leader.

Andy points out that it is also important to apply each blight fungicide at it most appropriate time as well as considering its resistance management contribution.  "Electis is one of the strongest protectant fungicides that is best used from stable canopy onwards.  In many years of successful commercial usage, it has proven excellent activity on both foliar and tuber blight and is categorised as one of the best protectant blight fungicides by independent European industry experts.  Where kick-back activity is needed such as last year, the addition of cymoxanil to Electis has proved to be successful.  Electis 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, an increasingly crucial issue," concludes Andy.