Resistance management practice in potatoes

UK - March 12, 2004

Potato growers and advisers are concerned about resistance in potato fungicides and are actively taking broad measures to minimise this risk, according to a survey conducted by Dow AgroSciences, who market Electis* and supply Dithane* NT.

According to the latest results of the survey, around 61% of growers and advisors said that they were “very concerned or concerned” about blight resistance and 55% thought that the risk of blight resistance was increasing.

“Blight resistance appears to be a considerable worry for growers and their advisors and it is reassuring that they seem to be taking on various measures to minimise this risk. The survey indicated that 92% of growers and advisors were alternating fungicide groups within a blight spray program and 75% said that they also preferred to use products with two active ingredients as a method of reducing resistance risk. Around 70% of those interviewed said that they specifically used multi-site fungicides and just over half started their spray program earlier. In addition to these strategies, 56% were reducing their use of phenylamide fungicides, with 23% avoiding their use altogether.” reports Nick Gooch, Marketing Specialist for Dow AgroSciences.

Resistance is generally detected when a fungus becomes insensitive to a fungicide and that fungicide does not provide effective disease control. It arises through genetic mutation and so is inheritable. The risk of resistance development in any particular disease depends on the pathogen itself, its genetics and epidemiology, as well as the chemical class to which the fungicide belongs and its mode of action.

As a disease, potato blight (Phytophthora infestans) is one that is likely to suffer from resistance problems. This is because it has a number of generations per year, an increasingly shorter generation time and abundant spore production, all leading to the need for frequent fungicide use. The average number of fungicide sprays used in the UK is around 10 – 12 sprays in a season. These factors put potato blight in the high-risk resistance category.

Products with multi-site activity such as the dithiocarbamates (e.g. mancozeb in Dithane and one of the actives in Electis) are at low risk to resistance as the blight pathogen is being 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.

“This multi-site activity is vital for effective resistance management and explains why mancozeb is incorporated into two-thirds of all blight fungicides used in the UK, including Electis and many of the most recently launched products.” comments Nick.

“Other key products to integrate into an anti-resistance strategy include those with a unique active ingredient with a completely different mode of action, such as zoxium, the other active ingredient in Electis. It works by disrupting microtubules and inhibiting nuclear division, thereby stopping the spore germ tube from penetrating the plant and also inhibiting the production of zoospores, which cause tuber blight. It also 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 Nick.

Other products such as the phenylamides appear to be at higher risk of resistance. Resistance to this group of fungicides was first detected in the UK in 1981 and in recent years ADAS monitoring shows that the proportion of blight spores showing resistance to phenylamides has remained constant at around the 50% level. However, the samples tested are generally from crops where there has been a failure in blight control so may under-represent the national picture. In response to PA resistance, FRAC guidelines advise that when using phenylamides, they should be used only as a preventative spray early in the season when disease pressure is low and that the number of phenylamide applications per crop per year is limited on the label. They are also generally formulated with other products with different modes of action. This anti-resistance strategy has been successful in containing resistance to this group.

“Some newer potato fungicides run a higher risk of resistance; either because they are single active ingredients or they are in a group where resistance to other diseases has developed over a short period of time and so will need careful integration into a blight program. For instance, cyazofamid is an effective blight fungicide but it is a single active ingredient. Consequently, its label restricts its use to only 6 times per season in blocks of three which must be interspersed with a block of three sprays from another group. The mode of action of the newer fungicides based on fenamidone and famoxadone is also being closely linked to the mode of action of strobilurins and they should also be used carefully. Fenamidone and famoxadone containing products, being in the QoL product group, should be used for no more than 6 applications to any crop with a maximum of 3 consecutive sprays. These products should be alternated in a programme with fungicides from a different cross-resistance group such as Electis.” concludes Nick.