Cambs grower considers how new late blight strains put pressure on fungicides

The spread of late blight (Phytophthora infestans) strains resistant to some widely used fungicides has added cost and complexity to fungicide strategies. For Myles Forber, technical manager at Chatteris-based grower A & EG Heading Ltd, this has increased the agronomic risks at a time when business confidence is already under pressure.

“Late blight poses a significant crop protection challenge, but while it is not the biggest threat we face, the loss of fungicides and the spread of resistant strains make managing it more difficult and expensive,” Mr Forber says.

It’s the nature of the disease too that causes concern, especially when seeking to balance fungicide applications within a standard seven-day irrigation schedule. EU_36_A2, the dominant strain of late blight in England, can produce more than 800,000 spores per square cm of lesion every seven days. A latent period – the period between when a host is infected and symptoms appear, becoming infectious – of just four days means once an infection is created, it quickly produces spores.

“Blight control during the tuber bulking period is the most challenging because of the crop’s demand for water. We can’t afford to stop irrigating to spray crops, so applications during this period need to be well-timed and robust,” he says.

“There is also the need to recognise that other crops around us can serve as a reservoir for infection. There is a large area of irrigated potatoes on the Fen and blight spores have been known to be carried great distances on the wind, so we cannot afford to think of our own crops in isolation,” he adds.

Of particular concern is the resistant strains EU_46_A1, which was found in Wales and Scotland in 2024 and sensitivity screening revealed resistance to the OSBPI mode of action group featuring oxathiapiprolin, as in Zorvec®. Another concern is EU_43_A1, which is yet to be detected in Great Britain but is resistant to CAA fungicides, such as mandipropamid, as in Revus®.

Managing the threat posed by new and established strains comes down to having a balanced programme that ensures no one mode of action is exposed.

“I’m fortunate to have enough mancozeb to serve as a mixer with straights such as Revus® or those active substances with single site modes of action, notably QiI fungicides, such as amisulbrom and cyazofamid,” he says.

“What we use in its place after this season remains uncertain. Privest® (ametoctradin + potassium phosphonates) may fill the space in the early season, but after this, we don’t know,” he adds.

There are cost implications of having to add a non-CAA mode of action such as mancozeb to Zorvec® Entecta (oxathiapiprolin + amisulbrom) and Revus®, but the need to protect these modes of action for future seasons means that many growers recognise it as necessary.

A positive of this, however, is that there is scope to incorporate more applications of Ranman Top® (cyazofamid) or other QiI fungicides in alternation with Infinito® (fluopicolide + propamocarb) in the final stages of the season when the anti-sporulant properties of propamocarb are useful in limiting the risk of tuber blight.

“I’m already using the maximum number of four Infinito® sprays. With no known resistance this has the advantage of not needing a third mode of action which also makes it financially competitive. In contrast, Ranman Top® needs to be mixed with partner products, such as mancozeb, cymoxanil, fluazinam or propamocarb to guard against resistance,” he says.

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Infinito® contains fluopicolide and propamocarb. Privest® contains ametoctradin and potassium phosphonates. Ranman® Top contains cyazofamid. Revus® contains mandipropamid. Zorvec® contains oxathiapiprolin. Infinito is a registered trademark of Bayer. All other brand names used are trademarks of other manufacturers in which proprietary rights may exist. Use plant protection products safely. Always read the label and product information before use. Pay attention to the risk indications and follow the safety precautions on the label. For further information, including contact details, visit www.cropscience.bayer.co.uk or call 0808 1969522. © Bayer CropScience Limited 2025