Something a lot of home-growers don’t realize they have to look for is potato blight, the same blight that caused the Irish Potato Famine. There’s a resurgence of the fungus along the eastern side of the US, but as this fungus is airborne and because some sellers of seed potatoes may not know they are selling diseased seed potatoes, it could pop up anywhere.
This blight is officially called Phytophthora infestans. It’s also called late blight, tomato blight, and potato rot. It affects tomatoes and potatoes, but is not transmissible via tomato seeds, only established plants. This fungus is known as an obligate parasite, which means it survives only in a living host. This is important to know.
It flourishes in high humidity, heavy dew, and/or wet weather coupled with moderate to cool temperatures: 50ºF – 80ºF ( around 20ºC or less). This, too, is important to know.
The blight starts as small, irregularly shaped light green to gray lesions on leaf tips. It spreads rapidly to form large black rot spots on the leaves, leaf stems, and the stems themselves. It will kill the plant if left untreated. If you see the early blight, the disease is already 2 – 3 weeks old and may affect neighboring plants, in an area as wide as 100 feet. For home-growers, if immediate action isn’t taken, you can lose your entire crop to the blight. It is carried and spread through tomatoes, potatoes, peppers, eggplant, and hairy nightshade. You’ll need to check all of these.
The fruits of the potato and tomato are also affected. The potato tubers will develop sunken areas that are a darker brown or purple color. If the potatoes are harvested and stored, the affected tubers will have dry, sunken, light brown spots that are not otherwise distinguishable from unaffected potatoes. This is how blighted seed potatoes can still be sold – it’s very hard to tell when the blight only lightly affects the tubers. Badly blighted potatoes will develop purplish flesh and be the consistency of cork and those are easy to spot.
There are ways to reduce and even prevent blight from affecting your home crops. The best way is to ensure your plants are dry or will be dry before sunset. The fungus loves moisture. Plant your potatoes where they will get wind to dry them, use drip irrigation methods or water them only in the mornings on sunny days and not at all on highly humid days. Hill the potatoes high and make sure the soil drains well. If you are growing your potatoes in bags (as I do), make sure you punch drainage holes along the bottom so water doesn’t accumulate to rot the tubers. Set your bags in a sunny, windy location so prevailing winds will dry them during the day. If you need to, use fans to help move air around them to dry the leaves by sunset.
Tomatoes planted in high tunnels with drip irrigation are less affected by blight. "Topsy turvey" tomatoes (planting them upside down in containers) will also reduce the likelihood of blight gaining a foothold in them. Ventilation in greenhouses is important. Good air movement and a reduction of wetness are important – and far easier for home-growers than for large commercial crops. Staking and pruning tomatoes will also reduce susceptibility to blight.
Keeping foliage off the ground helps a lot, so planting in very high hills, domes, and ridges helps a lot.
Keep susceptible plants away from host plants that can carry the blight without being affected (eggplant, peppers, hairy nightshade, volunteer tomatoes and potatoes), in shaded areas under trees or near buildings that cast a shadow over them, or near the cull piles of tomato and potato plants, especially if the culled plants are blighted.
Do not oversupply the tomatoes and potatoes with nitrogen via fertilizers or nitrogen fixing interplantings. Both potatoes and tomatoes need nitrogen, but they don’t need an excess of it.
If a blight starts anyway, action depends on the point during the harvest cycle. Early in the growing season and very early in the blight (when only 1 or 2 plants may show early signs of blight), you can spray with a fungicide or a copper sulfate solution. Pick and destroy affected leaves after the dew has completely dried. You’ll need to spray the copper sulfate solution frequently so new foliage is protected. My potato plants can grow several inches a day, so I’d recommend daily spraying when potatoes are at their fastest growing and no less than weekly once the potato plants appear until 2 weeks before harvest.
If it’s mid-season, and the infestation is just starting, you can try the picking infected leaves after they are completely dry and spraying with fungicide. Copper sulfate won’t work as well at this point.
Late in the season, if it’s near harvest anyway, go ahead and kill back the potato foliage to allow the tubers to ripen. You can do this in several ways: flail the plants, spray out with approved herbicides, or burn the living foliage. Burning is effective for large crops when there is no interplanting, not so good for patio potatoes. Flailing works best for small-croppers and home-growers.
Store diseased potato tubers separately from healthy ones. Potatoes should be stored dry and as cool as possible without freezing to discourage spore growth. Waste potatoes from culls can be fed to animals, buried 2 or more feet deep, composted after freezing, or, if they aren’t badly blighted, peeled, the blighted parts cut off, and eaten by people. Just don’t use blighted potatoes as seed potatoes.
For tomatoes, never cull or harvest when the plants are wet. The fungus sporulates during periods of dampness and can spread on people, tools, clothing, gloves, and other equipment. When you cull blighted fruits or leaves, carry them far from the plants to destroy. Do not eat blighted tomato fruits or store them with healthy tomatoes – the blight can spread during storage. Unblighted tomatoes from a blighted foliage plant can still be eaten, but I’d eat them fresh and not process them for storage.
If you have a serious infestation, sprays of metalaxyl and carbamate compounds or Cymoxanil and Mancozeb combinations or fungicides containing chlorothalonil work well – not organic, but effective if you have a sudden serious infestation and you are relying on those potatoes and tomatoes to feed you.
Here are some pictures of blight to help you identify it:
Peppers, eggplants, squashes, pumpkins, and melons are also affected by this blight. For peppers, it can attack any part of the pepper plant at any time. The first sign is usually a wilting of the plant just as it reaches fruiting stage. Stem lesions occur at the soil line. Stems discolor, collapse and become woody. Infected fruits develop dark watery patches with white mold on it. Pepper seeds will carry the blight.
For the squash family – the fruit will develop tan or brown banding lesions or circular spots. They may develop white moldy spots, and are susceptible to rotting from other causes faster than the blight.
Treatment is the same as for tomatoes and potatoes.
Tomatoes with brown spots on them may have portions that are edible fresh – use only the firm red portions and cut away the rot spots. Preserves made with blighted tomatoes will spoil so only eat lightly blighted tomatoes fresh.
The same applies to peppers if they blight after they are ripening – cut away the rot, eat the good part fresh. On peppers, it appears to be safe to freeze the good portions. Tomatoes don’t freeze well.
Salvage what you can from your plants, if it’s early enough, treat the plants to end the blight, and enjoy later blight-free crops. If the blight is too advanced, destroy it, treat the soil, and plant blight resistant varieties of tomatoes, potatoes, peppers, and other blight-prone vegetables.
Check each spud for signs of blight. Only use clean spuds as seed. When you plant next season, plant in high hills, or plant in large trash cans or bags (you can usually plant 3 – 5 seed potatoes per trash can or bag) in clean soil so you can control the moisture and temperature to some extent, use fans to blow air around them to keep the foliage dry since blight enters through wet leaves, use a fungicide, be vigilant in checking for early signs of blight, protect the foliage with a copper sulfate solution.
You may not be able to grow as many potatoes this way, but you should be able to at least preserve your heirloom seeds.