What is potato blight?
Potato blight is still one of the most devastating potato diseases in the UK. It is a fungus (Phytopthora infestans) that has the potential to completely destroy crops. It first infects the leaves, then the infection spreads to the tubers where they rot. Potatoes grown on a garden scale are just as susceptible to the disease as commercial crops.
How do organic growers stop blight?
Growers often practice damage limitation by growing varieties with low susceptibility to the disease or growing early crops that can be harvested before blight infection pressure gets really high in late summer. Neither of these is completely satisfactory. There are very few varieties that are completely resistant to the disease, and the Sarpo varieties that show good resistance are not always widely available. Additionally, not everybody might want to grow just early varieties, if they want to grow some maincrop varieties for storing.
Why use mesh netting to stop blight?
Some interesting work, done by Charles Merfield of the Future Crops Centre in New Zealand showed that covering potato crops with fine mesh netting could significantly reduce the incidence of potato blight. His work suggested that it was the exclusion of ultraviolet light by the netting that may be reducing the incidence of the disease. It is essential that fine mesh netting of less than 0.6 mm is used, otherwise the effects were not observed.
We often cover other crops such as brassicas with mesh netting, so this might work well, if we could use it to reduce blight on potato crops in gardens and allotments. We weren’t sure if this would work in the UK, as Charles found the netting was effective against early blight (Alternaria solani) in New Zealand whereas it is late blight (Phytopthora infestans) that is prevalent in the UK.
Did mesh netting reduce blight when we tried it?
We gave out fine mesh netting and Nicola seed tubers (Nicola is very susceptible to late blight) to gardeners around the UK and Ireland to see if the mesh netting had any effect against the disease. As luck would have it, there was very little blight around in 2020, and only 3 sites at County Wicklow, Lancashire and Dumfries & Galloway had more than moderate levels of infection. At 2 of these sites, the mesh netting resulted in a large reduction in blight infection but at the other site it had no effect at all.
Did the netting have any other effects?
The effects of the mesh netting on growth and yield of the crops was inconsistent, with it improving growth and yields in some cases but having a negative effect in others. A number of people noticed that rainfall did not penetrate this fine mesh netting and ran off the sides, so it is probable that the differences in growth and yield were down to different moisture levels. It is also possible that the reduction in blight at two sites may have been due to the netting preventing rainfall from wetting the leaves.
Would we recommend mesh netting as a way of preventing blight?
It is difficult to draw conclusions from these results, and the work would need to be repeated at a wider range of sites in a year with higher blight infection pressure to properly test whether mesh netting is an effective and practical way of reducing blight infection in UK potato crops.