Judging by number of queries and enthusiastic feedback relating to potatoes we receive here at GrowVeg, youre clearly mad for these totally tasty tubers! And I dont blame you – theyre easy to grow and theres nothing more satisfying than unearthing those nuggets of goodness from the soil at harvesting time. Power to the potato I say!
The trouble is potatoes are notorious for one particularly devastating disease: potato blight. Unfortunately for us, like all good diseases blight has a habit of evolving as quickly as the breeders efforts to outwit it, always remaining just one step ahead of the game.
Blight in potatoes is caused by a fungus that goes by the Latin name Phytophthora infestans. Its symptoms are unmistakable: small brown-black spots appear on the leaves, often surrounded by a pale halo, while the underside of the leaves may take on a white, downy appearance in wet weather – these are the hyphae by which the fungus colonises. 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. This usually leads to secondary infection by other bacteria and fungi to give a most unpleasant stink as your prized potatoes turn into a mushy mess.
Infection reaches a peak during Smith Periods – defined as two consecutive days when temperatures fail to dip below 10°C (50°F) and humidity remains above 90 per cent for the majority of the day. Spores can blow in from literally miles away, so trying to stop the spores from reaching your plants is futile.
Infection in ideal conditions is almost guaranteed if the recipient leaves are moist, which allows for efficient transfer of the troublesome spores into the plants vascular system. If it happens to be raining then the spores from blight-bashed foliage will drip down onto the soil to wheedle their way towards those precious tubers.
Theres a lot you can do to avoid blight in the first place, but if it suddenly strikes the first task is to remain calm – dont panic just yet! Cut out the infection as its possible you may have caught it just in the nick of time. Wipe the blades of your pruners with detergent between cuts to avoid inadvertently spreading the disease from plant to plant. Then pray for dry weather.
If more than 10 per cent of the foliage is infected Im afraid youll have to cut away the foliage completely as this is the point of no return. This will clearly stop the tubers underground from growing any further but will equally stop the disease in its tracks (left alone it will have inevitably destroyed your entire crop). Once the foliage is removed, leave well alone for three weeks before excavating your tubers. By this time they will have developed a thicker skin and will be ready for storage while any lurking blight spores should have perished. 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. Bury the foliage among other material and keep the composting process progressing smoothly by turning the material to keep it hot. The spores wont remain active for long, so the chances of the finished compost infecting a new generation of spuds is exceedingly remote. The spores can, however, overwinter on potatoes, so keep blighted tubers well clear of your compost heap.
As my granny always used to tell me, preventions better than cure, so the ideal solution is to sidestep blight altogether. Theres a three-pronged plan for this: growing outside of the blight danger period, choosing blight-resistant varieties, and meticulous cultivation and hygiene.
Growing outside the danger period: Potatoes are grouped according to planting date and the time they take to reach harvest time. The earliest to latest spuds are in this order: first earlies, second earlies and maincrops. When started off early enough in the growing season, most early varieties and some of the earliest-to-mature maincrops will be harvested well before high summer when the risk of blight increases. One idea to guarantee a potato haul ahead of the blight period is to plant potato growing bags of spuds under cover in early spring, taking care to protect against frost, before moving them outside by late spring. 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. Saving your own tubers is a false economy and serves to dramatically improve the prospects for blight and other diseases.
If watering during a dry spell, apply the water to the base of plants to prevent wetting the leaves. Do this in the morning so that any moisture left on the foliage evaporates during the warm of the day. Take the time to properly earth up and mulch with organic matter to lock in the moisture at ground level. Extra-thick mulches, for example with straw, will give any spores dropping to the soil surface much further to travel, insulating the developing tubers against infection.
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.
All of the above may feel like a military operation and in most years you probably wouldnt get blight at all. But forewarned is forearmed with a little knowledge and careful cultivation there is no reason why potato blight should ever dampen your efforts.
Black spots on potato leaves are a common issue that can significantly impact your potato harvest if not treated properly. These dark lesions are typically caused by fungal diseases like early blight or late blight. While the black spots may look worrying 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 frequent fungal culprits behind the black spotting on potato foliage are early blight (Alternaria solani) and late blight (Phytophthora infestans)
Early blight thrives in hot, dry conditions and first shows up as small, dark brown spots with concentric rings on older leaves near the base of the plant. As the fungal infection intensifies, the spots enlarge and multiply, eventually causing entire leaves to yellow, wither and die.
Late blight favors cooler wetter weather and starts as pale green water-soaked spots on leaves that rapidly expand into large blackish-brown lesions, A white fungal growth may emerge on the undersides of infected leaves in very humid conditions Late blight can spread quickly 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:
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Choose disease-resistant potato varieties – Look for early maturing potatoes as well as cultivars described as resistant to early or late blight.
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Allow for good airflow – Avoid overcrowding and prune lower leaves to encourage airflow.
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Water at the base of plants – Use soaker hoses or drip irrigation to keep water off foliage.
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Allow foliage to dry – Water early in the day so leaves dry quickly.
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Practice crop rotation – Don’t plant potatoes in the same spot for at least 3 years.
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Remove weeds/ debris – Eliminate places fungal spores can overwinter.
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Stake plants if needed – Keep foliage off the soil to avoid wet leaves.
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Apply preventative organic fungicides – Try copper, sulfur or bacillus subtilis products.
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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 required 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.
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Blight: 5 Ways to Control Potato Blight (Late Blight)
FAQ
What fungicide is used for potato blight?
To manage potato blight, a combination of preventative cultural practices and fungicide applications is recommended. Several fungicides are effective against both early and late blight, including those containing chlorothalonil, azoxystrobin, and copper-based compounds.
How to treat black dots on leaves?
Good garden hygiene is important for treating black spot. Remove garden litter, ensure plants have plenty of sunlight and good circulation. During the growing season, overhead watering should be avoided. To control blackspot spray with PLANThealth Fungus Control or PLANThealth Spectrum.
Why do potatoes turn black on spots?
Black dot disease of potato, caused by the fungus Colletotrichum coccodes, is generally considered to be a weak root pathogen of potato.
What causes potato leaves to turn black?
What is potato blackleg? Blackleg is a disease of potatoes caused predominantly by the bacterium Pectobacterium atrosepticum.