Maintaining Scotland’s high health status for potatoes

17 April 2019

New pest and disease threats and pesticide losses through legislation and resistance development are cross cutting issues across all our crops at the moment and potatoes are no exception. Our cooler climate means a reduced risk of aphids and hence a reduced risk of viral infections in seed potatoes produced in Scotland. But there are plenty of other indigenous and exotic potential threats to potato health. The long list of potential invaders on the UK plant health risk register includes things like root-knot nematodes, Zebra chip, brown rot or Colorado beetle which are subject to extensive statutory plant health checks and legislative measure to reduce the chances of them ever establishing in the UK.

Some of our indigenous problems are also subject to statutory control measures to try and minimize their spread – virus in seed crops being one risk managed through statutory inspection schemes to maintain the high health status of Scottish seed crops. Potato cyst nematodes (PCN) is another and is one of the very major issues which the Scottish potato industry wrestles with currently. Land tested positive for PCN cannot be used for seed production so as to prevent the spread of the nematodes to other fields but despite this the problem spreads annually and it is a concern how rapidly clean land in Scotland is diminishing. Ware can still be grown in infected land but great care should be taken not leave behind or spread a worse problem with PCN. Many varieties grown as ware crops, such as Maris Piper have good resistance to one of the PCN species (Globodera rostochiensis) but not to the other species (Globodera pallida) so management is only partial. The use of nematicides such as Vydate and Nemathorin can also reduce the risk of feeding damage but is expensive and carries user and environmental risks so their use is subject to careful stewardship measures and approvals are under constant review.

Potatoes have also been affected by recent pesticide withdrawals because of safety or environmental concerns. Several actives are in final use up periods. Slugs are a major threat in potatoes so Ferric phosphate will be the only option for slug management from next season now that metaldehyde and methoicarb are going or gone respectively. The news last week that chlorothalonil will be withdrawn also affects potatoes where it is used in mix with cymoxinil to manage potato blight. While we have a number of fungicide groups to manage blight it is a pathogen with past history of developing resistance issues so the use of a diverse range of actives and in particular the use of a multisite to aid efficacy and protect the partner active against resistance development were positive features.

New blight strains with reduced sensitivity to fluazinam have been picked up over recent seasons amongst the new strains of blight being detected. Genotypes 33_A2 (Green 33) and 37_A2 which show reduced sensitivity were first found in The Netherlands six to ten years. The 33_A2 strain is less competitive than others so tends to decline rapidly where fluazinam use is avoided. The more aggressive 37_A2 strain seems to be associated with reduced sensitivity to fluazinam as well so in areas where 37_A2 is present, the effectiveness of fluazinam will be reduced and fluazinam is not recommended for late blight control where resistance has been confirmed. Fluazinam should only be used in two- or three-way mixtures with a partner(s) with a different mode of action and with comparable persistence. The use of fluazinam at any point in the season will potentially select for resistant strains. FRAG-UK advise that its use at the start of the season may select for a high proportion of such strains if 37_A2 is present in the seed and its use at the end of the season could leave a legacy of resistant strains of this genotype persisting as tuber blight and so 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.

As with all fungicide resistance issues, a reduced reliance on pesticides and the uptake of Integrated Pest Management practices can help. Varietal resistance reduces the risk of blight if the end-market allows. Outgrade piles remain a key source of infection each year and need to be managed, as do other sources of infection like volunteer potatoes which should also be controlled where they appear. Once blight spray programmes start they should make full use of fungicides with different modes of action and label restrictions on total and sequential numbers of treatments must be adhered to. The risk of additional fungicide–resistant P. infestans genotypes being produced in the UK is higher if crops are infected by soil-borne oospores of blight and longer crop rotations can substantially reduce this risk.

Fiona Burnett, SRUC for the Farm Advisory Service

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