From the number of questions and positive comments about potatoes we get here at GrowVeg, it’s clear that you’re really into these tasty tubers. And I don’t blame you—they’re easy to grow, and there’s nothing better than pulling those tasty nuggets out of the ground when it’s time to harvest! Power to the potato I say!.
The trouble is potatoes are notorious for one particularly devastating disease: potato blight. Unfortunately for us, blight, like all good diseases, changes as quickly as breeders try to figure out how to stop it, always staying one step ahead of the game.
Blight in potatoes is caused by a fungus that goes by the Latin name Phytophthora infestans. Its signs are easy to spot: the leaves get small brown-black spots surrounded by a pale halo, and when it rains, the underside of the leaves may turn white and fluffy. These are the hyphae that the fungus uses to colonize. Blight can spread with impressive speed, causing the complete collapse of foliage within a few short days. In severe cases the tubers underground can become infected, giving rise to sunken patches and a brown rot. As a result, other bacteria and fungi often get into the potatoes and make them stink awful while turning them into a mushy mess.
Infections are most common during Smith Periods, which are two days in a row when temperatures don’t drop below 10°C (50°F) and humidity stays above 90% for most of the day. Spores can come from miles away, so it’s pointless to try to keep them from getting to your plants.
In ideal conditions, infection is almost certain if the leaves that are infected are wet, which makes it easy for the harmful spores to get into the plant’s vascular system. During rainy days, blight-damaged leaves will release spores that will fall to the ground and make their way to the valuable tubers.
In order to avoid blight in the first place, there are a number of things that can be done. If it does happen, the first thing that you should do is stay calm and stop the infection. You may have caught it just in time. Clean your pruner blades with soapy water between cuts to keep the disease from spreading from plant to plant. Then pray for dry weather.
If more than 10% of the leaves are infected, you’ll have to cut them all off. This is the point where they can’t be saved. This will clearly stop the tubers underground from growing any further, and it will also stop the disease in its tracks. If you let it go on its own, it would have destroyed your whole crop. Once the foliage is removed, leave well alone for three weeks before excavating your tubers. They should have a thicker skin by now and be ready to store. Any blight spores that were still alive should have died. Thoroughly inspect the tubers for any signs of blight, discarding any that have succumbed. Dry the sound tubers (never, ever wash them clean!) before storing in a cool, dark and dry place.
Infected foliage is fine for composting in a thriving compost heap. Put the leaves in with other things that you want to compost, and keep the process going smoothly by turning the stuff around to keep it hot. Since the spores won’t be active for long, it’s very unlikely that the finished compost will infect a new crop of potatoes. The spores can, however, overwinter on potatoes, so keep blighted tubers well clear of your compost heap.
My grandmother always told me that it’s better to avoid problems than to fix them, so the best thing to do is to avoid blight altogether. There are three steps to take: planting outside of the blight danger period, picking varieties that are resistant to blight, and being very careful with the growing and cleaning.
When potatoes are grown outside of the danger period, they are grouped by when they were planted and when they are ready to be picked. The earliest to latest spuds are in this order: first earlies, second earlies and maincrops. If planted early enough, most early varieties and some of the first maincrops to mature will be picked well before high summer, when blight is more likely to happen. To make sure you have a lot of potatoes before the blight period, you could plant potato growing bags full of potatoes under cover in early spring, making sure to keep them safe from frost. By late spring, you could move them outside. This will almost certainly give a crop by early summer. See my blog entry on growing potatoes in containers for more on this.
Blight-resistant varieties: Some varieties display a reasonable amount of resistance to blight. However, this is a constantly changing picture as varieties once resistant fall susceptible to the ever-adapting fungus. Pick a variety that is known to have both resistant foliage and tubers. Suitable varieties include those in the Sarpo range – such as Sarpo Mira or Sarpo Axona. North American gardeners should also keep an eye out for Defender.
Cultivation techniques and hygiene: Only ever plant fresh, certified disease-free seed potatoes. Trying to save your own tubers is a waste of money and makes blight and other diseases much more likely.
When you water plants during a dry spell, make sure you only wet the bases of the plants and not the leaves. Do this first thing in the morning so that any water on the leaves can evaporate as the day gets warmer. Take the time to properly till the soil and cover it with organic matter mulch to keep the water in the ground. Extra-thick mulches, like straw, will give any spores that fall to the ground a long way to travel, protecting the growing tubers from getting infected.
Never allow potato volunteers – plants that spring up from old tubers – to remain. Grub them out so you dont inadvertently carry over problems from one year to the next. In the same vein, ensure you harvest every last potato at harvest time. This will give blight nowhere to hide during the winter or inactive periods.
It may feel like a military operation to do all of these things, and most years you wouldn’t get blight at all. But knowing about potato blight ahead of time can help you avoid it. With a little knowledge and careful gardening, you should never have to worry about it.
Black spots on potato leaves are a common issue that can significantly affect your potato crop if left untreated. These dark lesions are usually caused by fungal diseases like early blight or late blight. While black spots can appear alarming, there are several effective ways to treat and prevent their occurrence in your potato patch.
What Causes Black Spots on Potato Leaves?
The two most common fungal culprits behind black spotting on potato foliage are early blight (Alternaria solani) and late blight (Phytophthora infestans).
Early blight thrives in hot, dry conditions and first appears as small, dark brown spots with concentric rings on older leaves near the base of the plant. As the fungal infection worsens, the spots grow larger and more numerous, eventually causing entire leaves to yellow, wither and die.
Late blight favors cooler wetter weather and begins as pale green water-soaked spots on leaves that quickly enlarge into large blackish-brown lesions. A white fungal growth may appear on the undersides of infected leaves in very humid conditions. Late blight can spread rapidly and devastate entire crops within just a few weeks.
Both fungal diseases survive winter in infected tubers, plant debris and soil. Wind, rain splash and garden tools easily spread spores to infect new foliage in spring and summer Healthy, vigorous plants can better resist infection, but stressed tubers and weakened foliage are highly vulnerable
How to Prevent Black Spots on Potato Leaves
An ounce of prevention is truly worth a pound of cure when it comes to fungal diseases on potatoes. Here are some effective ways to help avoid those telltale black leaf spots:
-
Choose disease-resistant potato varieties – Look for early maturing potatoes as well as cultivars described as resistant to early or late blight.
-
Allow for good airflow – Avoid overcrowding and prune lower leaves to encourage airflow.
-
Water at the base of plants – Use soaker hoses or drip irrigation to keep water off foliage.
-
Allow foliage to dry – Water early in the day so leaves dry quickly.
-
Practice crop rotation – Don’t plant potatoes in the same spot for at least 3 years.
-
Remove weeds/ debris – Eliminate places fungal spores can overwinter.
-
Stake plants if needed – Keep foliage off the soil to avoid wet leaves.
-
Apply preventative organic fungicides – Try copper, sulfur or bacillus subtilis products.
-
Scout regularly – Catch issues early before they intensify.
How to Treat Existing Black Spots on Leaves
If preventive measures have failed and those dark black leaf spots appear, prompt action is needed to save your crop:
Remove Infected Leaves
Prune off any foliage with black spot lesions, especially lower leaves near the soil. Sterilize pruners between each cut with rubbing alcohol to avoid spreading spores. Removing the infected leaves prevents fungal spores from spreading.
Improve Air Circulation
Allow more space between potato plants by thinning overloaded sections. Gently prune and trim lower leaves to improve airflow. Avoid working amid wet plants – wait for leaves to fully dry first.
Apply Organic Fungicide Sprays
Organic fungicidal sprays can help stop fungal spread when applied at first sight of leaf spots. Look for OMRI-listed products with active ingredients like copper, hydrogen peroxide, sulfur, neem oil or bacillus subtilis bacteria. Cover leaf tops and undersides thoroughly and reapply as directed on the label.
Consider Synthetic Fungicide Application
For severe black spot outbreaks, targeted application of synthetic fungicides may be warranted as a last resort to save the remaining crop. Chlorothalonil and mancozeb products are labeled for use on edible plants. Strictly follow all safety directions and never apply these chemicals right before harvest.
Stop Overhead Watering
Avoid using sprinklers or any system that wets potato foliage. Instead, water only at soil level with drip irrigation or soaker hoses to keep leaves dry. Water in the morning so plants dry out during the day. Wet foliage encourages fungal diseases.
Destroy Severely Infected Plants
If black leaf spots cover more than 30% of the foliage, the potato plants are likely too far gone to save. Pull up the entire diseased plants and put in the trash, not the compost pile. Removing the infected potatoes is critical to prevent fungal spore spread.
Allow Tubers to Cure in Soil
If potato tops must be removed early due to excessive leaf spotting, leave the tubers underground for 2-3 weeks to allow skins to thicken before gentle harvesting. Curing makes them less prone to skin damage and disease entry.
Disinfect Garden Tools
Clean and sanitize all gardening tools after use around infected plants. Soak pruners, hoses, stakes etc. in a 10% bleach solution for 10 minutes, then rinse clean before storing or reusing in the potato patch.
Monitor Potato Storage Carefully
Check harvested potatoes frequently for any signs of fungal infection like wet rot or mushy spots. Remove and destroy affected tubers immediately to prevent disease spread. Store only clean, intact potatoes from disease-free plants.
Acting quickly at the first sight of black leaf spotting is key to saving your potato crop. Focus on improving plant health and reducing fungal spore spread. When possible, rely on organic approaches first before considering synthetic fungicides as a last resort. With proper treatment and preventive care, you can still harvest a bountiful potato crop despite some early spotting on leaves.
Plants Related to this Article
If you need help designing your vegetable garden, try our Vegetable Garden Planner.
Blight: 5 Ways to Control Potato Blight (Late Blight)
FAQ
How do you treat potato leaf disease?
What is the best fungicide for potato blight?
How do you get rid of dark spots on potatoes?
How do you treat black spots on leaves?