Ways to tackle potato blight
On the Wednesday 18th September the Soil Association will be holding a workshop to explore alternatives to spraying copper on potatoes to deal with potato blight. Copper was not authorised for use in organic farming in 2019. But many organic farmers are already leading the way to find practices to tackle blight without using chemicals.
Organic farming advisor Paul Flynn has given us a list of some of the agroecological approaches that you can use on your farm:
Agroecological way of tackling blight without using copper
- Developing blight resistant varieties that pass the taste test. In 2015 there was an Innovative Farmers field lab looking into this, examining growth, resistance, taste and yield for 11 blight resistance varieties that are all suitable for organic systems.
- Purchasing clean seed and avoiding reusing home-saved.
- Incubating potato seeds at 15C for two weeks to identify subclinical blight before sowing. And carrying out a full disease analysis.
- Good crop rotation. The spores of the fungal pathogen phytophthora infestans that causes late blight, can last for up to four years. So a good rotation where you don’t plant any potatoes in a bed for at least 5 years can be a good disease control.
- Providing good crop nutrition including N,P,K and Sulphur can give the potato plants strong resilience to infection.
- Using green manures/winter cover to retain nutrients, particularly nitrogen can also give the potato plants strong resilience to infection.
- There's an Innovative Farmer's field lab investigating if mesh covers can reduce potato blight spores significantly and lower the susceptibility of the crop to disease.
- Earlier sowing (for earlier harvest), but only where possible as this does give rise to soil erosion risks.
- Chitting can help quick establishment of the crop, and allow slightly earlier harvest
- Early harvest of younger potatoes. This is not suited to longer term storage and this does compromise overall yield.
- Planting a mix of varieties in the same field can delay the spread blight across the field, but complicates harvest with additional handling required.
- Wider spacing of rows to delay canopy closure.
- Growing more erect varieties –this avoids the canopy closure and increases air flow.
- Using trickle irrigation in preference to rain gun or boom irrigation to keep leaves drier.
- The ideal conditions for blight are measured as Smith Period. This is a 48 hour period in which the minimum temperature is 10 C or more and the relative humidity exceeds 90% for at least 11 hours during the first 24 hours and for at least 11 hours again during the final 24 hours. Growers can use predicted Smith Periods to react more quickly to the threat of blight.This can be sent as an automated warning to growers via blightwatch.co.uk
- Planting rows in to the predominant wind direction will increase air flow.
- Excellent plant hygiene. Removing any trace of previous crop residues, volunteers and particularly any waste material and out-grades from post harvesting and packing.
- Removing single plants that show initial focus of infection.
If you have any other practices that you use on your farm please let us know and we’ll add it to the list.
The workshop on the 18th will be an opportunity for farmers and growers to discuss these practices and create shared roadmap for the future of Organic UK Potato production.
Our work looking into alternatives to copper is part of part the Europe wide research project, Organic-PLUS, which seeks to minimise—and eventually phase out—certain contentious inputs occasionally used in certified organic agriculture.