The challenges in managing potato blight have always been the main focus on in-season fungicide programmes for potato growers. As a mobile and aggressive pathogen, it has readily adapted itself to new varieties over the years and we’ve been used to seeing it shift and slide. But in recent seasons the emergence of particularly worrying strains has been a feature and last season’s data confirms that the rise of three different issues in the new blight strains being commonly detected. Firstly there are strains that are more aggressive (i.e. bigger and faster lesions), then there are strains that can overcome established varietal resistances and finally there are new strains which are harder to control and carry reduced sensitivity to fungicides. This means that blight programmes for 2019 will require different approaches to previous seasons, and the need to mix and alternate fungicides to make it harder for new strains to survive is much greater than previously.
The two strains of most concern to the industry are the so-called 36_A2 strain which is highly aggressive and the 37_A2 strain which has reduced sensitivity to fluazinam, and both have increased to frequency in population testing. Other strains of great concern are the 13_A2 and 41_A2 strains which are able to overcome many of the varietal resistance genes present in commercial potato varieties. The rise of these aggressive strains has lead to the tightening of blight spray intervals in recent seasons and their ability to infect more resilient varieties is one of the reasons why growers find it hard to adapt blight spray programmes very much to reflect varietal ratings. The 41_A2 strain is the more recent of the two and in addition to being able to overcome common varietal resistances, it appears to be even more aggressive which is an unwelcome development.
In practical terms, the rise of the 37_A2 strain which has shown reduced sensitivity to fluazinam which has been associated with reduced performance of fluazinam in some field trials and commercial crops in several areas of the UK needs careful management this season. The Fungicide Resistance Action Group for the UK advise that fluazinam should be used with caution and should not form a major component of blight control programmes. In areas of England where 37_A2 is present, the effectiveness of fluazinam will be reduced so fluazinam is not recommended for late blight control. For Scotland, fluazinam should only be used in two- or three-way mixtures with partners with a different mode of action and with comparable persistence. Mode of action information is characterised by FRAC codes which indicate if a fungicide is in the same or a different chemical family. There are 13 different FRAC codes or chemical families in play for managing potato blight which is great news in terms of being able to reduce reliance on any one family. Repeated use of one chemistry, particularly when blocked together in a programme makes it really easy for any resistant blight strains in the crop to survive the first spray and then multiply rapidly, and with impunity, to subsequent sprays of the same chemical type. Mixing in an alternative active makes it harder for them to survive, and alternating different chemistry at the next spray means that if you don’t get them on the first pass you’ll get them on the second.
The use of fluazinam at any point in the season will potentially select for resistant strains. Its use as a sequence partner in late blight control programmes should be limited, and it should always be alternated with a product containing fungicides from a different group. Multisite fungicides like mancozeb are very low risk when it comes to resistance development and offer a cheap and effective mixing partner. But don’t choose between mixing and alternating chemical groups – do both!
Fiona Burnett SRUC, for the Farm Advisory Service
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