How to Treat Black Spots on Potato Leaves

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:

• 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.

• Crop rotation means you shouldn’t plant potatoes in the same spot for at least three 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 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 you have to cut off the potato tops early because of too many leaf spots, leave the tubers underground for two to three weeks to let the skins get thicker before carefully harvesting. After being cured, they are less likely to get skin damage or diseases.

Disinfect Garden Tools

Do not use any gardening tools near infected plants until they are clean and germ-free. Soak pruners, hoses, stakes etc. put them in a bleach solution for a few minutes, then rinse them clean before putting them away or using them again 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.

To save your potato crop, you need to act quickly as soon as you see black leaf spotting. Focus on improving plant health and reducing fungal spore spread. Synthetic fungicides should only be used as a last resort and only when nothing else works. Even if the leaves get spots early on, you can still harvest a bumper crop of potatoes if you treat them right and keep them healthy.