As a potato grower, few things are more concerning than walking out to your lush green potato patch and discovering black dots littering the leaves. While alarming, don’t despair. Learning to properly identify and manage common fungal and bacterial diseases behind these dark leaf spots can help safeguard your crop.
Key Causes of Black Spots on Leaves
Several potato pathogens can cause dark blemishes on foliage:
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Early blight (Alternaria solani) – Causes brown-black lesions with concentric rings giving a “bull’s eye” pattern. Spreads rapidly in warm, humid weather.
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Septoria leaf spot (Septoria lycopersici) – Small, circular spots with dark edges and gray centers. More common in warmer regions.
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Black dot (Colletotrichum coccodes) – Scattered black dot-like sclerotia on stems and undersides of leaves, Tubers may also be affected,
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Bacterial leaf spot (Xanthomonas campestris) – Small, water-soaked spots that become black and can merge into large lesions. Spread by wind-driven rain
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Late blight (Phytophthora infestans) – Irregular greenish-black greasy spots on leaves and stems during cool, wet weather.
While less common, environmental factors can also cause blackening of leaves:
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Frost damage – Leaves turn black from ice crystal formation within tissues.
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Sunscald – Direct sun exposure leads to blackened patches.
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Chemical burn – Overapplication of fertilizer or pesticides burns leaf edges and tips.
How to Identify the Cause of Black Spots
Carefully examine leaf spots and consider these characteristics:
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Pattern and size – Are spots small and scattered or large and merging? Do they have rings or dots?
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Color – Are the spots black, brown, gray, greenish?
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Texture – Are the lesions dry or watersoaked? Greasy?
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Distribution – Where are spots concentrated – edges, centers, upper, lower leaves?
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Weather conditions – Did spots arise after humid, wet weather or during a heat wave?
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Plant history – Have other solanaceous crops shown symptoms?
Sending samples to a diagnostic lab can provide confirmation, but assessing the spots’ appearance and progression helps narrow down causes.
Managing Black Leaf Spot Diseases
Once the culprit is identified, implement targeted management tactics:
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Improve airflow and avoid overhead irrigation to reduce leaf wetness.
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For late blight, destroy all infected plants promptly.
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Use disease-free certified seed pieces and rotate plantings.
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Apply protective fungicide sprays containing chlorothalonil, mancozeb or copper early in disease development.
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Remove healthy plants prior to leaf die-back to limit fungal spread to tubers.
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Plow under crop debris immediately after harvest to accelerate decomposition.
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Cull out spotted tubers and do not use for seedstock.
Rapid action is essential for managing fungal and bacterial diseases before they overtake plants. Be vigilant about scouting for symptoms and responding before problems get out of hand.
Key Questions About Black Spots on Leaves
If those dark blemishes pop up on potato foliage, here are some common questions:
What is the black ring pattern on my potato leaves?
Concentric rings indicate early blight, worsened by wet conditions. Improve airflow. Apply protective fungicides every 7-10 days if needed.
I see raised black dots on the undersides of leaves. What causes this?
Black dot sclerotia are the survival structures of the weak pathogen Colletotrichum coccodes. It is spread via affected seed tubers.
Can black leaf spots spread to my potato tubers?
Yes, early and late blight in particular can spread from leaves to tubers, rendering them unmarketable. Stop disease progression promptly.
Should I destroy plants with black spotted leaves?
For aggressive late blight, destroy entire affected plants right away. For other spots, removing affected leaves may suffice if action is swift.
Will crop rotation help reduce black leaf spots next season?
Yes, 3-4 year rotations to non-solanaceous crops limits pathogen carryover for most leaf diseases.
Stay vigilant for any black blemishes on potato leaves, and be prepared to act quickly to protect your crop. Proper identification guides your management approach. With attentive practices, your potato yields don’t have to be tarnished by black spots.
A Closer Look at Key Fungal Diseases Causing Black Spots on Leaves
Let’s explore some of the most likely fungal culprits behind black leaf spots in more detail:
Early Blight
Early blight, caused by the fungus Alternaria solani, is one of the most common and destructive diseases of potato. Yellowing and brown spots form on lower older leaves, expanding into large lesions.
Small dark lesions first appear at the leaf tips and margins. These spots enlarge into circular or angular dark brown patterns up to half an inch across. The lesions have concentric rings or a “bull’s eye” pattern that is distinctive of early blight.
Severely infected leaves turn yellow and perish early. Defoliation from the bottom up leads to reduced tuber size. Leaf and tuber infections are worsened by warm (80-90°F), humid conditions with frequent rainfall.
The fungus overwinters in crop debris and on infected tubers. Wind and splashing water spread spores. Tubers may show shallow, dark, dry, cracked lesions. Reduced storage life results from early blight infection.
Implementing a 4 year rotation, planting resistant varieties, and maintaining lush foliage growth are key preventive measures. Start protective chlorothalonil or mancozeb fungicide sprays at the first onset of disease.
Septoria Leaf Spot
Caused by the fungus Septoria lycopersici, septoria leaf spot is favored by warmer temperatures than early blight. Small, circular gray spots dotted with black pycnidia appear on lower leaves.
Lesions begin as tiny water-soaked spots. These develop into circular spots about 1/16 to 1/8 inch diameter with defined black margins. The centers turn an ashy gray color later on. During moist weather, the sexual spore-bearing structures called pycnidia form in the centers.
Septoria starts on the oldest foliage and progresses up the plant. Defoliation occurs but it develops slower than early blight. Yield impacts depend on crop stage affected. Diseased leaves are more prone to late blight infections.
The fungus persists on tomato and potato debris and is spread by wind and water splash. Long crop rotations, preventive fungicides, and destruction of crop residues help reduce septoria leaf spot problems.
Late Blight
The notorious Irish potato famine was triggered by the oomycete Phytophthora infestans, which continues plaguing potatoes today. On leaves, irregular wet greenish-black lesions first appear. A white fungal growth forms on the undersides in cool, wet conditions. Stems may also show lesions and cankers. Foliage quickly dies back, leading to poorly filled or rotting tubers if left uncontrolled.
Spread occurs rapidly by wind patterns over many miles. The pathogen requires living potato or tomato tissue and thrives in cool, wet weather with night temperatures below 55°F and humid days. Rain, fog, dew or overhead irrigation provide films of moisture needed for spore germination and infection.
Susceptible potato and tomato varieties will crash within a few weeks once late blight strikes without prompt action. Immediately destroy infected plants to limit spread. Apply targeted fungicides containing chlorothalonil, copper, mancozeb or mefenoxam.
Black Dot
Black dot disease, caused by the fungus Colletotrichum coccodes, occurs in most potato growing regions but with low incidence and severity. Still, it can contribute to potato plant decline and yield loss.
The most diagnostic symptom is the presence of small black sclerotia dots on stems and the undersides of leaves. Potato roots may also show black discoloration or rotting from infection. Raised black lesions can appear on tubers in storage.
The disease is spread via chlamydospores in infected potato debris and on seed tubers. Rain splash and insect movement also disperse spores. High humidity and temperatures around 70-80°F favor fungal development.
Crop rotation, planting resistant varieties, and fungicide application during leaf wet periods help control black dot. Cull out diseased tubers prior to planting. Promptly plow under crop debris after harvest to hasten decomposition.
Controlling Destructive Foliar Diseases of Potato
Don’t allow dark fungal spots to overshadow the potential of your potato crop. Scout plants routinely and follow these tips to enjoy healthy, high-yielding potatoes:
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Accurately identify spots based on appearance and plant distribution to guide management.
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Start preventive fungicide sprays early on susceptible varieties.
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Improve airflow and reduce leaf wetness duration.
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Maintain optimal fertility and soil moisture to avoid stressed plants.
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Eliminate cull piles and destroy crop debris promptly after harvest.
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Employ long crop rotations of 3-4 years for fungal disease prevention.
With quick action before fungal diseases run rampant, your potato leaves can stay lush and green, not tarnished by black spots. Diligent integrated disease management will pay off at harvest time with bountiful tubers.
Verticillium Wilt Verticillium spp.
- Leaves turn yellow then brown and wilted, stems remain erect
- Signs are usually only seen on one or a few stems, not the whole plant.
- Often only random plants scattered in the field are affected
- You can see brown rings or streaks just under the skin if you cut the stem close to the ground.
- If you cut the tuber flesh at the stem end, you can see dark brown to black rings or streaks.
- More information on managing plant diseases in the home garden
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Corky Ring Spot Tobacco Rattle Virus
- The leaves may not show any signs or may have yellow rings, spots, and arcs on them.
- Stems may be mottled green and yellow
- Tubers get brown rings and arcs on their skin and inside their flesh.
- Cracks and misshapen potatoes can happen if infections start too early.
- More information on Corky Ring Spot
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Q&A – What are the black spots on my potato leaves?
FAQ
How do you treat black spots on potato leaves?
What are the little dots on potato leaves?
What are the black specks on potatoes?
Why are my potato leaves turning yellow and black spots?
What is a black dot on a potato?
Black dot: Colletotrichum coccodes (Wallr.) Hughes. (Coelomycetes, Melanconiales) Black dot is a common disease of potato. It is most often observed on tubers but it can affect all parts of the plant. The disease has probably been underestimated in the recent past as the symptoms are similar to more common potato diseases.
What are the causes of black dots in the eyes?
Black dots in the eyes, often referred to as “floaters, ” are caused by tiny bits of gel or cells in the vitreous (the jelly-like substance in the eye) casting shadows on the retina. They become more noticeable when looking at bright backgrounds. While generally harmless, the sudden appearance of numerous floaters or flashes of light might signal a retinal tear or detachment, requiring prompt medical attention. If concerned, consult an eye care professional for proper evaluation and guidance.
How do you know if a potato has black spots?
Dark brown to black spots may also occur on the stems of the plant. Tubers are affected as well. The tubers will have dark gray to purple, circular to irregular lesions with raised edges. If sliced open, the potato flesh will be brown, dry, and corky or leathery.
What causes black spots on potatoes?
This virus is transmitted by species of potatoes and aphids. Mosaic brown patterns and black spots appear on the leaves of the potato. Once the plant is infected by this virus, the leaves shrink and become smaller through time, and then eventually the stems start to bent. Potato mosaic disease is characterized by the formation of black spots.