Late blight tech update for 2025: New blight strains cause concern

The spread of late blight (Phytophthora infestans) strains resistant to some of the most frequently used fungicide groups and the risk these pose to crop protection programmes is causing concern among researchers and manufacturers. 

Fungicide resistance is not a new phenomenon but earlier instances, such as the spread of fluazinam-resistant 37_A2 (‘Dark Green 37’) and metalaxyl-resistant 13_A2 (‘Blue 13’), were contained with reasonable ease through changes in application practices. The emergence of strains with resistance to CAA fungicides, such as mandipropamid, and OSBPI fungicides, such as oxathiapiprolin, pose a challenge of a different magnitude.  

Newer genotypes, EU_43_A1 and EU_46_A1 have demonstrated resistance to both CAA and OSBPI fungicides, and sub-populations of some newer genotypes are more virulent, able to overcome what have been identified as major resistance genes of cultivars. This is concerning because these genes are central to breeding varieties with improved resistance. 

Breeding innovation and plant protection product development are long-term programmes with progress in one area often reliant with that in another. The loss of resistance genes seen as integral to cultivar development undermines efforts to preserve fungicide efficacy for future seasons.  

We will need to find new ways to control late blight, including through the wider use of decision support systems and in-crop monitoring tools, while continuing to practice responsible resistant management.  

To highlight the threat these strains pose and the response needed to contain them, Bayer hosted a seminar for growers and agronomists. The role of infected seed in enabling the spread of these new strains was among the topics covered.  

Research in 2011 (Zellner et al.) found that of the seed potato batches tested, on average 11% carried a latent late blight infection. EU_46_A1 has been detected in Poland and given that about 80% of the seed potatoes planted in Poland are supplied by Germany and the Netherlands, transmission of late blight in seed partly explains how these new strains spread so far, so quickly other than by air-borne spore transmission. 

In the UK, we should be cognisant about seed transmission and continue to manage primary inoculum sources, by controlling volunteer potatoes and destroying or covering out grade piles. Ensuring crops remain protected in the face of these new threats means mixing modes of action and using active substances recognised to give sufficient protection.  

For instance, suggestions that solely including cymoxanil with mandipropamid as a means of combating CAA-resistant 43_A1 is not responsible, as although cymoxanil is another mode of action with activity on late blight, it will give only two to three days protection which is insufficient. 

It is similarly important to maintain dose rates. The QiI amisulbrom is a popular mixing partner for CAA and OSBPI fungicides but be sure to maintain the full application rate. Bayer glasshouse trials indicate that amisulbrom at 60% dose will give 20-25% control of EU_43_A1 isolates with resistance to both CAA and OSBPI fungicides. At the full rate, this increased to over 70%. 

Resistance management guidelines produced by Aarhus University have resulted in a dramatic fall in the frequency of EU_43_A1 since 2023.  

Up until 2022, growers in Denmark made regular applications of mandipropamid, often up to six times in the programme. They have since limited its use to just two applications and always in a mix with a reliable partner. Also, there have been fewer applications of mandipropamid at the start of the season, to reduce early selection for EU_43_A1.  

Propamocarb unaffected 

To protect the efficacy of Infinito® (fluopicolide + propamocarb) and other propamocarb products, Bayer has published updated guidance for the 2025 season.  

Propamocarb continues to be effective against late blight with no reports of resistance. To reduce the chances that this might change, propamocarb should always be applied in mixture with another fungicide with sufficient activity on P. infestans and in alternation with fungicides from a different cross-resistance group,” Mr Astill said. 

Bayer advises that no more than 6060g propamocarb is applied per ha per crop in 2025. 

Pictured: Tom Astill

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