The humble potato is an essential part of our western diets, and it’s quickly replacing grain as a major crop worldwide. And for good reason – potatoes grow faster and do well in worse conditions than grain, not to mention that they’re far more nutritious (think half an adult’s daily dose of vitamin C for one medium-sized tuber).
Potatoes are everywhere, commercially grown, in our fast foods, in our pantries. And of course, in our gardens. But it’s only in our gardens that we start to notice problems when we grow the potatoes ourselves. Namely, the most difficult disease to fight: blight.
Here’s the problem with blight: once it hits your potatoes, there’s not much you can do about it to stop it, you can only slow it down. Or, if you’re lucky, prevent it. But since blight is so widespread and difficult to fight (it’s now resistant to multiple fungicide applications), you can’t help but wonder what happens to our beloved potatoes when they become blighted.
Are blighted potatoes still safe to eat or do they produce disease in humans? Does blight on foliage translate into blighted tubers? If you’re curious, read on.
The 70s’ Potato blight health scare
Blight is sneaky. You may think you have a perfectly healthy supply of potatoes in storage, only to find some of them rotting on a weekly basis. So is that something you should be worried about? Well, UK scientists in the 70s certainly thought so.
British scientist James Harrison Renwick started investigating the link between eating blighted, blemished potatoes during the first trimester of pregnancy and giving birth to children with neural tube defects, particularly spina bifida and anencephaly.
The UK government took this information seriously and advised pregnant women to refrain from eating potatoes until further safety studies were carried out, but years later, there was no conclusive data to prove this to be true.
So if you see advice on potato food safety and pregnant women, it might be based on this event and not on actual hard evidence. However, pregnant women aside, there are no reports of people developing serious medical conditions after eating blighted potatoes.
The final verdict on eating blighted potatoes
If your potato crop has been affected by blight, you can still eat the tubers, but keep some things in mind:
- You’ll possibly get fewer and smaller tubers because the affected foliage didn’t allow them to grow to maturity;
- Blight spores can trickle into the ground and affect your potato tubers, which will only show later, in storage;
- Healthy potato tubers can get secondary infection with blight spores if you’re not careful during harvesting and you create lesions on their skins;
- Blemishes caused by early blight are more forgiving and tend to heal when cured, compared to late blight, which can rot your entire crop of potatoes if you’re not vigilant.
Once you see your crop has been affected by blight, the best thing you can do is to use the tubers as soon as possible, as long as they don’t show obvious defects. Blight doesn’t cause tubers to rot immediately – it’s the weak secondary bacteria that causes potato tissue to break down in storage.
Eating blighted potatoes is generally considered safe if you cut away the brown, damaged part. But often, the entire potato is damaged and not a pretty sight anyway, so it’s best to toss it. Check your stored potatoes regularly for signs of decay to stop disease from spreading to healthy tubers.
How blight looks on potato foliage and tubers
The initial symptoms of blight on potato plants are small, dark lesions on the leaves, usually on the margins, surrounded by a halo of light green tissue. These brown lesions tend to spread and affect the entire leaf, as well as descend down the stems.
In humid summer conditions, blight moves fast and can quickly overtake an entire plant in a matter of days. Aside from killing foliage, blight spores can be washed down on the soil and make their way towards the potato tubers.
Blighted foliage doesn’t necessarily mean that your tubers have been affected, as the spores move from the top down, and if you act fast, you can still save your crop. But if the tubers do suffer from blight infection, look for brown or purple discoloration on the surface. If you cut the potato open, you’ll notice reddish-brown granular markings in the tuber flesh.
Some tubers may look healthy, only to succumb to bacterial soft rot in storage. In this case, there’s not much to salvage, and they’re no longer good for consumption.
Early blight in potatoes – how to handle tubers
Early blight is not as prevalent in the UK, and usually affects early potato crops. Spores of Alternaria solani, the causal agent of early blight, move through the air and spread between fields, where they drop on the soil and survive on infected tubers or just below soil levels.
Even though spring may be rainy and cold, which can generally encourage blight, the rate of early blight infection is low early in the season, and increases after the potato plants start flowering. Early blight becomes more rampant right when it’s time to harvest.
This is good news for most early potato crops:
- Early blight doesn’t destroy essential potato foliage until the very end of the season, allowing for tubers to mature and bulk up;
- If careful not to injure potato tubers when harvesting, you’ll get healthy potatoes for storage, safe from secondary infection.
Early blight potato lesions aren’t as aggressive for the tubers as late blight, and the infection tends to limit itself during curing.
If your crop of earlies has been affected by blight, here’s what you can do:
- Kill the potato vines two to three weeks before harvest time.
- Be careful not to injure the potato skins during harvest.
- Store tubers in conditions that promote skin healing – fresh air, temperatures between 55 and 60F – for 2 to 3 weeks. Following curing, store in a dark, well-ventilated, cool and dry place, such as a root cellar..
Early potatoes are a good choice if your early summers are dry and you only tend to have blight problems in July and August (which is when your tomatoes may suffer from blight as well). By sowing early potatoes, you’re more likely to escape blight infection, as you’ll harvest them before the weather gets warm and humid enough for the rampant spread of blight.
Late blight in potatoes – how to handle tubers
For home gardeners dealing with temperate, damp weather conditions, late potato blight is almost a given. Late blight is a fungal infection caused by Phytophthora infestans, which spreads by spores in warm, humid weather.
Late blight usually hits from July onwards, especially in areas where it rains a lot. The best thing you can do if your potato crops tend to suffer from blight is to look for signs of this fungal infection every other day, as it spread very quickly.
If you manage to prevent late blight from destroying the potato plant leaves, they won’t get to the tubers. This means you’ll have to stay vigilant and prune affected foliage every day. Cut away all parts of the plant that are affected, even if that means sacrificing the whole plant. It’s better to have some tubers than no harvest at all.
According to RHS, if more than 25% of the foliage of one plant is affected, you should cut the plant to ground level, stems and all, clear all the foliage and stems away, and wait for the tubers to mature and harden their skins in the ground for 2 weeks. Don’t allow more than 2 weeks to pass, though, or your tubers will get eaten by slugs and other critters in the soil.
Potato late blight can defoliate an entire crop, but if it hits late enough in the season and your tubers get the chance to fatten up, you’re all good.
When it’s finally time for harvest, make sure the stems are dried and completely dead. Don’t harvest on a humid day and stop harvesting altogether if it’s raining.
With late blight, if you leave potatoes for 2 to 3 weeks to cure, you’ll soon notice which potatoes are blemished and which are fine to move to storage. As a home gardener, you’ll probably be more forgiving with your crop and eat around the blemishes, but the current recommendation with commercial growers is not to store potatoes that show more than 5% blight damage or tuber rot.
Tips for preventing blight
We’ve covered how to fight blight, but is there any way to prevent it from attacking? While these methods are not guaranteed, here are some measures you can take:
- Buy your seed potatoes from a reliable source – blight spores can lay dormant in tubers from one season to another.
- Choose a blight-resistant variety. This may prove more difficult in recent years, as new strains of blight are becoming more aggressive.
- Mound your potato plants. The extra soil will act as a barrier from the blight spores on foliage.
- Mulch your potato plants to reduce the need for watering.
- When watering, be careful not to splash the leaves. Just like with tomatoes, blight affects damp, poorly ventilated foliage.
- Don’t leave any potatoes in the ground and practice crop rotation if you have repeated blight problems.
As home gardeners, we all have to deal with blight from time to time, and while blighted tomatoes quickly rot and go bad, making them impossible to eat, we can’t say the same thing about potatoes. In fact, I remember my family frequently dealing with blight and soft rot, and growing up I saw them happily cut around the damage and use the potatoes regardless of their flaws.
So don’t be afraid to eat your blighted potatoes, but also don’t be afraid to take immediate, aggressive action as soon as you see early or late blight on your potato plants’ foliage.