Q What is potato blight?
A Potato blight (Phytophthora infestans) or 'late blight' is a devastating fungus disease that spreads rapidly in wet weather. Like a mildew disease, to which it is related, it attacks the foliage, but can get washed down to damage the tubers. Blight also attacks tomatoes, and has been recorded on aubergines. Related plants, such as peppers, seem immune.
Caption: Cut off the foliage at ground level if blight strikes and wait two weeks before harvest
Q How do I recognise potato blight?
A The first signs are small, dark spots, often on the edges of leaves. If the weather is wet, a white mould surrounds these spots, usually on the underside of the leaf. This shows that spores are being produced. The spots grow, covering the whole leaf. They also spread onto stems.
Infected tubers have brown and purple skin blotches that go into the flesh of the tuber. The flesh is brown/red and granular. Infected tubers may shrivel and dry, but are often infected by rotting organisms and liquefy in storage, smelling strongly and contaminating other tubers.
Caption: Potatoes infected by blight often rot and liquefy in storage
Q Could I mistake potato blight for anything else?
A Early blight also blackens leaves, but with concentric rings of darkened leaf. It usually attacks older leaves and does little damage. Viral diseases can also speckle leaves with black spots and stunt growth.
Tubers can be infected with scab, spots, rots and gangrene. The spots and rots can be difficult to tell apart from blight as the end result, a liquefied potato, is the same. Fortunately, the same counter-measures apply.
Q When should I expect a potato-blight attack?
A Following recent research by the James Hutton Institute, blight alerts for potato and tomato growers are now being issued under the Hutton Criteria, which has replaced the Smith Period.
The minimum temperature needed by the disease, 10°C over two consecutive days, is unchanged, but it's now known that relative humidity only needs to be above 90% for six hours on both consecutive days for the disease to spread, rather than 11 hours, as previously thought. In these conditions, the disease spreads rapidly. In hot, dry weather it temporarily 'dries up', but breaks out again if conditions become favourable. This typically happens during early summer in the west and late summer in the east.
Q How can I avoid potato blight?
A Dig out as many tubers as you can when you gather the potatoes. At the end of the season, remove all potato tubers, even tiny ones, and destroy them. Digging over the plot so frost can kill any remaining tubers will help. Throw away, burn or bury deeply any leftover or rotting tubers from storage, so they can’t grow the following year. Protect new crops by earthing up well. A layer of soil will protect tubers from spores falling from infected foliage.
Q Can I compost plants that are affected by potato blight?
A Potato blight persists on living material so you can safely compost the stems and leaves as they are dead, provided the temperature in the compost is high enough, such as in a Hotbin. If you're using an open compost bin it's unlikely to get hot enough to destroy the blight spores, so it's best to burn affected plants or put them in the council green waste bin as their composting facilities run at higher temperatures. There is a possibility that blight spores could overwinter on other plant material but this hasn't been tested scientifically yet. Don't compost the tubers as they are alive and will carry the blight spores; put these in your council green-waste bin.
Q Can I reuse compost that potatoes were grow in?
A Yes, you can but you must be careful to remove all of the tubers, including tiny ones. Also carefully check for the c-shaped grubs of vine weevils. As with any compost that you reuse, add controlled-release fertiliser before you replant to replace the depleted nutrients.
Q Can potato blight be sprayed?
A No, there are no longer any chemical controls available for potato blight.
Q Can I rescue plants infected by potato blight?
A Cutting off infected foliage can prevent spores from reaching the tubers. Leave at least two weeks between removing the foliage and lifting the tubers, so viable spores lurking on the soil surface don’t contaminate the tubers as you lift them. Store the tubers in dry, cool conditions to reduce the disease’s activity and subsequent rotting. Check potatoes every month; get rid of rotting ones.
Q Can potato blight be prevented?
A Using certified disease-free seed potatoes and good garden hygiene will help to prevent blight, but the spores of the disease are carried on the breeze, so few gardens will escape. You could try covering the crop with an open-ended polythene tent to keep the foliage dry. Avoid high-nitrogen fertiliser, as this stimulates growth of soft, blight-susceptible foliage.
To avoid the worst of blight outbreaks you could also grow early or second-early varieties and lift before August.
Q Are there any varieties that are resistant to potato blight available?
A 'Sarpo Axona' and 'Sarpo Mira' were both extremely resistant in our trial.
Caption: 'Sarpo Mira' potatoes are resistant to potato blight