Managing late blight: what reduced sensitivity and product losses mean for growers and advisers

Great Britain’s late blight (Phytophthora infestans) population has seen significant changes in recent years, with many advisors reporting that the spread of new strains has made crop protection more complex and costly to deliver.

This trend seems set to continue as new strains with reduced sensitivity to fungicides continue to emerge. Forecasts produced by Scottish Agronomy suggest growers can expect a typical 13-spray programme to cost £350/ha in 2022, a rise of 25% on the year earlier. Some of this cost price rise can be attributed to product price inflation. The need to follow best practice in resistance management by not applying products containing only a single mode of action alone or in sequence, has resulted in the use of a wider range of products which in turn has contributed to the increase in overall cost of a late blight fungicide spray programme.

At the heart of the issue is grower reliance on a limited number of active substances, especially mancozeb. Grower reliance on mancozeb is highlighted by statistics from the FERA Pesticide Usage Surveys (see table below). Potatoes were last surveyed in 2020 during which 126,704 hectares of ware potatoes were grown in the United Kingdom. On average, this crop received 10 fungicides, three herbicides and two insecticides.

Of the popular active substances applied for late blight protection, mancozeb and cymoxanil continue to be the most widely used. In contrast, growers have largely heeded the message over fluazinam. Since the emergence of the fluazinam resistant strain 37_A2 in 2016, the use of this active substance has fallen 89%.

Product withdrawal and disease resistance has led to greater use of fewer active substances

Source: FERA Pesticide Usage Survey 295, arable crops in the UK 2020

For nearly a decade two such strains, 6_A1 and 13_A2 (described as genotypes in the technical jargon) 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 present across all regions, the threat of 37_A2 marked the loss of another popular fungicide for late blight control.

The arrival of 36_A2, also in 2016, changed the population dynamic again. Also resistant to fluazinam, it spread rapidly, and testing revealed that it posed a greater threat to crops than genotypes that preceded it. As of 2021, the late blight population across much of England is dominated by 36_A2: in East Anglia it accounts for 60% of samples submitted for testing by blight scouts. Outside England, the situation is most aptly described as ‘regional’.

In Wales, 37_A2 has a small but notable presence. Given its poor fitness relative to other strains, its existence suggests fluazinam is still being applied alone rather than in a mix with a product belonging to another mode of action group.

In Scotland, the situation is more complex as there is a greater mix of strains. That 36_A2 has not established itself in the country is confounding, but this does not mean that crops in the country are at less risk. The arrival of 41_A2 in Fife in 2021, another strain with fluazinam resistance, should sharpen attention. It is reportedly more aggressive than other strains on varieties considered to have good resistance to late blight. Evidence from Belgium suggests 41_A2 expresses the same reduced sensitivity to phenylamides, such as metalaxyl-M as in Fubol Gold, as 13_A2.

Growers will feel the impact of these developments most come the end of the season, warns Eric Anderson, Scottish Agronomy senior agronomist.

“The consequences of more aggressive strains will be felt most acutely during the rapid canopy stages of crop growth because of the amount of new material emerging between sprays. Fortunately, the products and modes of action available for protection during this time will provide adequate protection,” Mr Anderson says.

“The same cannot be said for control towards the end of the programme. For the final three sprays of the season, growers will need to ensure good anti-zoospore activity to protect against tuber blight. In practice, this means alternating between Infinito (propamocarb + fluopicolide) and Ranman Top (cyazofamid) in a mixture with Enervin (ametoctradin) or mancozeb. To limit the cost, Infinito should be the first and third spray in this sequence with Ranman Top + Enervin or mancozeb as the second spray,” says Mr Anderson.

Resistance management

The confirmation of a second strain of Phytophthora infestans resistant to fluazinam – and probably phenylamides – is a reminder of the pathogen’s immense capacity to evolve. It also serves as a reminder that resistance management deserves to be taken seriously.

In support of efforts to protect crops in the face of new, possibly more threatening strains, in 2018 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 did 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 except for 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, apart from 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. Whether 41_A2 establishes itself in regions where 36_A2 dominates will determine its exact threat to crops.

“The industry has not done the tests to establish its [36_A2] aggressiveness relative to the other strains in the UK, however, results from tests conducted in France at INRA have shown 36_A2 isolates can 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 advisors.

“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.

It is becoming more important to understand that crop protection and disease management strategies, extend far beyond fungicide applications.

IPM strategies such as planting varieties with better resistance, destroying early sources of blight such as outgrade piles to slow the epidemic, and monitoring and removing volunteers are all valid means of reducing the disease pressure facing crops.