Demystifying Black Spots on Potato Leaves

As a potato grower, few things are more worrying than walking out to your lush green potato plants and seeing black dots scattered across the leaves. While it’s alarming at first glance, try not to panic. Learning to properly identify and manage the common fungal and bacterial diseases behind these dark potato leaf spots can help safeguard your crop.

What Causes Black Spots on Potato Leaves?

Several different potato pathogens can lead to the development of dark blemishes on foliage

• Early blight (Alternaria solani) – Results in brown-black lesions with concentric rings giving a “bull’s eye” pattern. It spreads rapidly in warm humid weather. This was mentioned 4 times.

• Septoria leaf spot (Septoria lycopersici) – Causes small, circular spots with dark edges and gray centers It’s more common in warmer regions This was mentioned 2 times.

• Black dot (Colletotrichum coccodes) – Leads to scattered black dot-like sclerotia on stems and undersides of leaves. Tubers may also be affected. This was mentioned 3 times.

• Bacterial leaf spot (Xanthomonas campestris) – Creates small, water-soaked spots that turn black and can merge into large lesions. It spreads via wind-driven rain. This was mentioned 2 times.

• Late blight (Phytophthora infestans) – Forms irregular greenish-black greasy spots on leaves and stems during cool, wet weather. This was mentioned 5 times.

While less frequent, environmental factors can also induce blackening of leaves:

• Frost damage – Leaves turn black due to ice crystal formation in plant tissues.

• Sunscald – Direct sunlight exposure results in blackened patches.

• Chemical burn – Overapplication of fertilizer or pesticides burns leaf edges and tips.

How to Accurately Identify the Cause of Black Spots

Carefully examine the leaf spots and look for these characteristics:

• Pattern and size – Are the spots small and scattered or large and coalescing? Do they display rings or dots?

• Color – Are the lesions black, brown, gray, greenish?

• Texture – Are the spots dry or water-soaked? Greasy?

• Distribution – Where are the spots concentrated – leaf edges, centers, upper or lower leaves?

• Weather conditions – Did the spots appear after humid, wet weather or during a heat wave?

• Plant history – Have other solanaceous crops shown similar symptoms?

Sending samples to a diagnostic lab can provide confirmation, but assessing the spots’ appearance and progression will help narrow potential causes.

Managing Destructive Black Leaf Spot Diseases

Once the culprit is pinpointed, put targeted management tactics into action:

• Improve airflow and avoid overhead irrigation to reduce leaf wetness.

• For late blight, immediately destroy all infected plants to limit spread.

• Use disease-free certified seed potatoes and rotate plantings.

• Apply protective fungicide sprays containing chlorothalonil, mancozeb or copper early on.

• Remove healthy plants before leaf dieback to restrict fungal spread to tubers.

• Plow under crop debris right after harvest to accelerate decomposition.

• Cull out spotted tubers and don’t use for seedstock.

Swift action is essential for getting fungal and bacterial diseases under control before they overtake plants. Be vigilant about scouting for early symptoms and take action before issues escalate.

Key Questions About Black Spots on Potato Leaves

If those dark blemishes appear on potato foliage, here are some common questions:

What causes the black ring pattern on my potato leaves?

Concentric rings indicate early blight, exacerbated by wet conditions. Improve airflow. Apply protective fungicides every 7-10 days if needed.

What are the raised black dots on the undersides of leaves?

Black dot sclerotia are the survival structures of the weak pathogen Colletotrichum coccodes. It spreads via infected seed tubers.

Can black leaf spots spread to my potato tubers?

Yes, early and late blight in particular can spread from leaves to tubers, making them unmarketable. Halt disease progression right away.

Should I destroy plants with black spotted leaves?

For aggressive late blight, eliminate the entire affected plants immediately. For other spots, removing the affected leaves may be enough if action is prompt.

Will crop rotation help reduce black leaf spots next season?

Yes, 3-4 year rotations to non-solanaceous crops limits carryover of most leaf disease pathogens.

Stay alert for any black blemishes on potato leaves, and be ready to take action quickly to protect your crop. Correct identification guides your management approach. With attentive practices, potato yields don’t have to be ruined by black spots.

A Closer Look at Key Fungal Diseases Causing Black Spots

Let’s explore some of the most likely fungal culprits behind black potato leaf spots in more detail:

Early Blight

Early blight, prompted by the fungus Alternaria solani, is one of the most widespread and destructive potato diseases. Yellowing and brown spots form first on lower older leaves, expanding into large lesions.

Small dark lesions initially show up at the leaf tips and margins. These spots enlarge into circular or angular dark brown patterns up to half an inch wide. The lesions have concentric rings or a “bull’s eye” pattern that is characteristic of early blight.

Severely infected leaves turn yellow and die off early. Defoliation from the bottom up results in reduced tuber size. Leaf and tuber infections worsen with warm (80-90°F), humid conditions and frequent rainfall.

The fungus survives in crop debris and on infected tubers. Wind and water splash spread spores. Tubers can display shallow, dark, dry, cracked lesions. Early blight infection decreases storage life.

Implementing a 4 year rotation, planting resistant varieties, and maintaining lush foliage growth are key prevention measures. Start protective chlorothalonil or mancozeb fungicide sprays at disease onset.

Septoria Leaf Spot

Triggered by the fungus Septoria lycopersici, septoria leaf spot thrives in warmer temperatures than early blight. Small, circular gray spots dotted with black pycnidia appear on lower leaves.

Lesions start as tiny water-soaked spots. These develop into 1/16 to 1/8 inch circular spots with defined black margins. The centers later become an ashy gray. During wet weather, sexual spore-bearing pycnidia form in the centers.

Septoria begins on the oldest leaves, moving up the plant. Defoliation occurs but more slowly than with early blight. Yield impacts depend on the growth stage affected. Diseased leaves become more prone to late blight infections.

The fungus persists on tomato and potato debris, spreading by wind and water splash. Long rotations, preventive fungicides, and destroying crop residues help reduce septoria leaf spot issues.

Late Blight

The notorious Phytophthora infestans oomycete triggered the Irish potato famine and continues plaguing potatoes today. On leaves, irregular wet greenish-black lesions first emerge. A white fungal growth forms on the undersides in cool, wet conditions. Stems can also show lesions and cankers. Foliage rapidly dies back, resulting in poorly filled or rotting tubers if uncontrolled.

Spread happens quickly via wind over many miles. The pathogen needs living potato or tomato tissue, thriving in cool, wet weather with night temps below 55°F and humid days. Rain, fog, dew or overhead irrigation provide the moisture films required for spore germination and infection.

Susceptible varieties will crash within weeks if late blight hits without prompt action taken. Immediately destroy infected plants to restrict spread. Apply targeted fungicides containing chlorothalonil, copper, mancozeb or mefenoxam.

Black Dot

Black dot disease, prompted by the fungus Colletotrichum coccodes, occurs in most regions but with low incidence and severity. Still, it can contribute to potato plant decline and yield loss.

The most diagnostic symptom is small black sclerotia dots on stems and leaf undersides. Roots can also show black discoloration or rotting. Raised black lesions may appear on tubers during storage.

The disease spreads via chlamydospores in infected debris and seed tubers. Rain splash and insects also disperse spores. High humidity and 70-80°F temperatures favor fungal growth.

Crop rotation, resistant varieties, and fungicide application during wet periods help control black dot. Cull out diseased tubers before planting. Promptly plow under crop debris after harvest to decompose.

Controlling Destructive Foliar Diseases of Potatoes

Don’t let dark fungal spots overshadow your potato crop’s potential. Routinely scout plants and follow these tips to enjoy healthy, high-yielding tubers:

• Accurately identify spots to guide management based on appearance and distribution.

• Start preventive fungicide sprays early on susceptible varieties.

• Improve airflow and reduce leaf wetness duration.

• Maintain optimal fertility and soil moisture to avoid stressed plants.

• Eliminate cull piles and promptly destroy crop debris after harvest.

• Use long 3-4 year crop rotations to prevent fungal diseases.

With quick action before fungal diseases become rampant, your potato leaves can stay lush and green, not tarnished by black spots. Diligent integrated disease management will pay off at harvest with plentiful tubers.