My potato plants are showing signs of blight, with some of the leaves starting to discolour and drop off. Is it too late to save them? I don’t really want to spray them unless it’s necessary.
Michael D, Galway
Unfortunately, early summer’s rainy, humid weather created exactly the sort of conditions suitable for the spread of potato blight, the famously destructive fungal disease that can badly damage and even destroy potato plants as well as the tubers developing underground. It can also strike tomato plants, with similarly depressing consequences.
Controlling it is challenging, especially if you’re growing your plants in an area with other potato plants growing nearby – for example, on an allotment site – where the risk of the disease leapfrogging from one crop to another is high.
The conventional approach is to spray plants with a protective fungicide several times during the growing season to prevent infection. Some of these conventional fungicides are non-systemic, acting only as a protective layer, while others are systemic, being absorbed deep into the tissue of the plant. Neither is effective as a treatment for already infected plants, nor are they considered organically acceptable.
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Instead, organic gardeners favour a combination of planet-friendly methods that include choosing potato varieties with proven high blight resistance; growing “early varieties” that are ready to harvest much more quickly and are thus less vulnerable to the disease than slower-to-mature main crop varieties; practising good cultural controls such as crop rotation and generous spacing between individual plants; removing old tubers that have overwintered in the ground as well as “volunteer” plants in spring; and using natural biostimulants such as Herfomyc, which boost the plants’ ability to resist infection.
You can also slow the spread of the disease in infected plants by handpicking and carefully binning individual infected leaves. But once it has spread to roughly a quarter of the crop, you should cut back all the plants to just above the ground and then carefully bin burn/ dispose of all the afflicted top growth. This will slow down the spread of blight to the tubers themselves, which can be left in the ground for up to several weeks to ripen before being harvested.
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Some gardeners like to combine this method with a layer of plastic sheeting or horticultural fleece spread over the bare soil after the stems have been cut back. The thinking here is that this also helps to prevent the fungal spores from infecting the tubers themselves.
