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Great Britain’s late blight (Phytopthora infestans) population has seen significant changes in recent years, with many advisers reporting that the spread of new strains has made crop protection more complex and costly to deliver.  This trend seems set to continue as concerns over the future availability of mancozeb force growers to review programmes: forecasts produced by Scottish Agronomy suggest growers can expect a typical 13-spray programme to cost roughly £280/ha, a rise of 3-5% on 2020. Without mancozeb in the programme, however, this increases to about £330/ha, a rise of 18%.

Grower reliance on mancozeb is highlighted by statistics from the DEFRA Pesticide Usage Survey (see table below). Potatoes were last surveyed in 2018 during which 127,859 hectares of ware crop received on average 10 fungicides, three herbicides and two insecticides. Mancozeb was most often applied in a mix with cymoxanil and together this combination alone accounted for 33% of the formulation area treated.

The five most used formulations applied to ware potatoes in 2018

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Source: FERA, Pesticide Usage Survey report 284, arable crops in the UK 2018

For nearly a decade two such strains, described as genotypes in the technical jargon, (6_A1 and 13_A2) dominated populations, but the arrival of 37_A2 challenged the status quo. This genotype spread rapidly and was later found to demonstrate a level of insensitivity to fluazinam that effectively brought about an end to its use as a blight fungicide.  With the metalaxyl-resistant 13_A2 still a significant presence across all regions, the threat of 37_A2 marked the loss of another popular fungicide for late blight control.

In 2016 the situation changed again with the arrival and subsequent rapid spread of 36_A2. It was reported to represent a greater threat to crops than genotypes that preceded it and may have a level of aggressiveness beyond that seen with previous strains. As of 2019, 6_A1 and 36_A2 dominate populations across all regions of Great Britain (see chart) though both 13_A2 and 37_A2 remain a significant threat and can be found in potato crops across the country.

 

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Source: AHDB, 2019

 

Resistance management

The loss of fluazinam was a stark reminder that resistance management deserves to be taken seriously. Growers have largely heeded this message but adopting it has necessitated the use of a wider spectrum of products belonging to different modes of action and an end to the practice of applying products containing a single mode of action alone or in sequence.

The situation is set to be repeated with mancozeb. While it remains authorised for use in Great Britain, the issue is one of product supply. There is potentially sufficient quantity for the 2021 season in the supply chain, but concerns arise should this season be one of high pressure where intervals have to be reduced or if supply is limited.

In support of efforts to protect crops in the face of new, possibly more threatening strains, AHDB Potatoes commissioned the James Hutton Institute (JHI) to test the performance of a range of fungicides belonging to different groups against both 36_A2 and 37_A2.

The results identified minor differences between genotypes when growth was tested on fungicide-treated leaves in the laboratory. Aside from confirming the resistance of 37_A2 to fluazinam, the results do not suggest a decline in the protection offered from fungicides when applied at field rates or that the new strains are a significantly greater threat to crops than either 6_A1 or 13_A2.

“The results must be seen in context,” says Dr Faye Ritchie, plant pathologist with ADAS which contributes input on resistance management strategies to the AHDB/JHI project.

“The tests identified differences in the growth of strains exposed to different active ingredients, but they were incredibly small and should not be considered to impact field performance negatively because the rates being tested were so low, in many cases a tenth or hundredth of the full label rate.”

The results have received attention across the industry, but Dr Ritchie is careful to explain that with the exception of 37_A2 and fluazinam, such minor differences in the laboratory do not compare to resistance in the field.

“The EC50 test is the standard test to monitor fungicide sensitivity, the result being the concentration of the fungicide that reduces growth by 50% relative to a non-fungicide treated control. We're talking different orders of magnitude; for example, where you see problems with fungicide resistance in the field, you might see sensitivity differences between strains of 30-fold in laboratory based tests whereas these tests have identified a difference of just two- or three-fold, within the range for natural variation.”

What these results confirm is that the fungicides we use today, with the exception of fluazinam and metalaxyl, for which there is known resistance in certain genotypes, continue to offer effective protection.

The rapid spread of 36_A2 has caused concern in some quarters and there is evidence to support claims that it is aggressive.

“The industry has not done the tests to establish its aggressiveness relative to the other strains in the UK, however, results from tests conducted in France at INRA have shown 36_A2 isolates to produce large lesions and plentiful sporangia and that this has allowed it to establish alongside 6_A1,” says Dr Ritchie.

 

Industry communication

Ensuring the industry understands these results and their implications for crop protection strategies is now the focus of experts and advisers.

“We might think we have a wide range of products for late blight control, but limitations on the number of times a product or mode of action can be applied in a programme or season mean it is not as great as it may at first seem. Add in the need to ensure modes of action are not applied repeatedly in sequence and that they are alternated or mixed with different modes of action it is clear why control has become more complex and costly.  This may appear unnecessary or precautious, but it is what is needed to ensure we retain the effectiveness of the products we have for future use,” says Dr Ritchie.

Crop protection and disease management, however, extend beyond just fungicides.

“With an early start to the epidemic, you get more generations in a season and you will select more strongly for aggressive, virulent and resistant strains. We can use IPM strategies such as using varieties with better resistance, destroying early sources of blight such as outgrade piles to slow the epidemic, which in turn slows the selection for those fungicide resistance and aggressive strains,” she adds.

 

Advice into practice

For Dr John Keer, trials and research services officer at Richard Austin Agriculture, designing and implementing an active resistance management strategy is reasonably easy to achieve in practice.

“Resistance management deserves to be taken seriously. As long as products containing a single active substance are mixed with a product belonging to another mode of action and the same active or mode of action is not applied in consecutive applications, it is easily achieved.”

He shares Dr Ritchie’s observation that there isn’t the range of products available that there once was but says there is still enough to comply with resistance management guidelines.

“We still have enough products to alternate modes of action while applying that which is most appropriate to the crop growth stage and the disease pressure facing the crop,” he says.

If there is a pinch point that has the potential to cause concern it is in the final weeks of the season when the need to protect crops from tuber blight is made harder by a protracted haulm destruction.

“This is perhaps the one area of the programme that poses a difficulty. My preferred action here is to alternate Ranman Top (cyazofamid) + cymoxanil with Infinito (propamocarb + fluopicolide). There are enough permitted applications here, even allowing for protracted haulm destruction brought on by the loss of diquat, to keep within the restrictions of a resistance management programme and still apply products with good zoospore activity.”

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