There have still been no confirmed cases of blight in Scotland this year – and very few throughout the whole of the UK. Prolonged dry conditions have helped in that and although irrigated crops clearly present more humid and suitable conditions for blight development than non-irrigated equivalent crops, a general lack of inoculum in the area is a major plus. Scottish weather being what it is however we can expect more ‘normal’ weather to resume at some point and so the need to protect potato crops against tuber blight remains a high priority in designing blight programmes.
Maintaining effective fungicide protection until the foliage and stems are dead reduces the risk of tuber blight. Field trials on tuber blight control have shown that effective control of tuber blight is related to the effectiveness of foliar blight control. As crops start to close over and move into a period of slower and more stable canopy growth foliar blight control can be very significantly improved through, for example, by using a fungicide with a higher EuroBlight rating. Other measure to boost efficacy can also help such as adding an additional fungicide to the spray tank or tank mixing certain combinations of anti-drift agents and fungicides. Using a forecasting scheme to time fungicide applications to shortly before a high-risk weather period is a very useful and integrated measure that can be taken to improve the efficacy of blight programmes.
In terms of preventing tuber blight, keeping blight out of the crop for as long as possible is key but fungicides with good ratings for anti-sporulant activity can also contribute to reducing the inoculum available for tuber infection. Products containing fluopicolide, amisulbrom or cyazofamid tend to be amongst the most effective for control of tuber blight. Fluazinam products also have a high rating, assuming the P. infestans is fluazinam sensitive. Growers will recall that there were issues in 2017 with strains of reduced sensitivity to this active so it is sensible not to be over reliant on it in programmes in 2018.
In the absence of fluazinam the most effective control of tuber blight will depend on only two modes of action – fluopicolide or a QiI. Amisulbrom and cyazofamid both fall in the same ‘QiI’ activity group. This clearly presents an enhanced risk of resistance development and so additional, alternative modes of action in mix and alternation in blight programmes would boost resistance management. Examples include zoxamide and ametoctradin containing products. These two examples would add two different zoospore-active FRAC modes of action to the two that fluopicolide, amisulbrom and cyazofamid provide despite having not quite as high a degree of efficacy as the these leading examples.
Other integrated management methods to reduce the risk of tuber blight still further include avoiding the cracking of ridges. A good covering of soil goes some way to protecting tubers and preventing zoospores directly landing on exposed tubers. And, clearly, ensuring good haulm desiccation to prevent blighted re-growth is as important as ever.
Fiona Burnett and Ruairidh Bain for the Farm Advisory Service
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