One of the most head-scratching things for gardeners is when their tomato plant leaves start showing signs of a problem. It’s easy for someone to become overwhelmed when reading through all of the conditions that could be the culprit of black spots.
What Can You Immediately Rule Out?
If you see black leaves in your garden, it is possible to immediately rule out mosaic virus, nutrient deficiencies, and blossom end rot.
- Tomato mosaic virus — closely related to tobacco mosaic virus — appears as mottled leaves (a mosaic appearance), stunted eaves, curled leaf edges, and uneven ripening of fruits.
- Nutrient deficiencies show as yellowing of the leaves or stunted growth of the vegetables.
- Blossom end rot appears as black, leathery patches on the bottom of fruit. The leaves and stems are unaffected.
What Do You Need to Consider?
Are you planting tomatoes and noticing black spots on their leaves? Are you struggling to figure out what is causing the demise of your plants? Don’t worry. Chances are the answer to your woes is one of the following pathogens.
Causes of Black Leaves
Black spots on leaves, stems, and fruit are distinguishing features of a bacterial or fungal disease. These airborne or soil-borne pathogens infect the tissue, causing harm and even plant death. The most common diseases are late blight, early blight, Alternaria stem canker, bacterial canker, and Septoria leaf spot.
Late blight is the least common type of blight but undoubtedly the most destructive. This blight occurs when prolonged wet weather or high humidity is accompanied by cooler air temperatures (60 to 78℉). Airborne fungus-like Phytophthora infestans spores infect healthy vegetables, and the decimating late blight symptoms appear in three to five days.
- Small irregularly-shaped lesions appear first on the older foliage (lower leaves) and then move to the stem and fruit.
- White fungal growth appears, covering affected parts of the leaves, stems, and fruits.
- The area around lesions looks water-soaked or bruised and the spot may turn yellow or grey-green color.
- Spots quickly grow in size as the late blight pathogen reproduces.
The soil-borne fungi Alternaria tomatophila and Alternaria solani cause early blight. Both fungus types prefer warmer temperatures (75 to 80℉ and above) and high humidity levels (above 90%). This fungal disease is characterized by dark lesions that display concentric circles. Early blight, like late blight, spreads rapidly through plants after infection.
- Dark brown, round lesions with concentric circles appear first on the lower leaves.
- Stems show dark brown spots that are sunken and dry, displaying the characteristic concentric circles.
- Tomato fruit exhibit leathery, sunken, black spots with concentric circles.
- Severe infestations cause leaves to yellow or brown and drop off the vegetation.
Alternaria Stem Canker
Alternaria alternata, a soil-borne fungus, causes Alternaria stem canker and is favored by warm temperatures (75 to 84 °F), high humidity levels, and wet weather. It often occurs in young seedlings or plants after pruning. Infected vegetables are often girdled, unable to transport nutrients upwards from the roots to the leaves.
- Dark brown to black spots appear on stems near the soil line, moving upward through the plant.
- The fungus creates a toxin, killing plant tissues and creating brown streaks in the vascular system above and below the cankers.
- Lesions on the leaves cause leaf edges to curl, eventually killing the leaf.
- Fruit lesions are circular or oval and appear yellowish-green on mature tomatoes.
- Enlarged cankers girdle the stem, causing the death of the plant.
Unlike the other diseases, bacterial canker is caused by a bacteria called Clavibacter michiganensis. Once infected, symptoms don’t appear for several weeks, and it is difficult to diagnose. Bacterial canker occurs during warm weather (75 to 90°F), with high moisture or relative humidity. The resilient bacteria survive in the soil for up to three years.
- Leaf edges turn brown, with yellow streaking radiating outward.
- Leaf veins darken and sink into the leaf tissue.
- Severe infections trigger wilt and leaf drop.
- Stems split open and display dark brown to black streaks.
- Fruits show small, raised circular spots of light colors, surrounded by yellow halos.
Septoria Leaf Spot
The fungus, Septoria lycopersici, causes Septoria leaf spot. Symptoms appear as small circular lesions on lower leaves after the first fruit set; stem and fruit infection is rare. Septoria leaf spot is particularly severe where wet, humid weather persists for extended periods. Severe disease cases impact the plant’s ability to photosynthesize, leading to death.
- Small circular grey lesions (1/16 to ¼” in diameter) appear on leaves with dark borders and small black fruiting bodies. The tiny black dots are Septoria leaf spot spores.
- Septoria leaf spot spreads upward through plants from oldest to youngest leaves.
- Leaves turn yellow, then brown, and wither up.
Step By Step Guide to Treating Diseases
Unfortunately, once the leaves on your tomatoes start turning black, the infected areas are typically not treatable. However, it is necessary to address the issue quickly, preventing the pathogens from causing further damage. Treatment helps slow spread which leads to death and also spread to neighboring vegetation.
Treating diseases follows a similar approach regardless if they are infected with blight or septoria leaf spot. The answer to your problem requires utilizing cultural and chemical control methods, with both organic and synthetic options available. When treating diseases, make sure to thoroughly disinfect garden equipment before and after use and avoid touching healthy tomatoes to prevent the spread of the fungus.
- Remove all diseased tissue from the infected plants.
- Clean up all debris and fallen fruits in the surround area.
- Dispose of the debris in the garbage. Do not compost it or allow it to sit in your yard to decompose. Most spores survive these conditions.
- Apply mancozeb, chlorothalonil, or a copper-based fungicide. Follow the product label directions for mixing rates and timing, applying the product to the entire stand of tomato plants and surrounding soil.
Since the treatment of an attacking pathogen doesn’t “heal” affected areas, preventing infection, therefore, becomes critical to growing healthy, high-yielding plants. As it is always said, “prevention is better than cure.” The following cultural practices help minimize conditions favoring diseases, hopefully resulting in fewer leaves turning black.
- Choose varieties resistant to disease if ones are available.
- Practice good crop rotation, planting tomatoes (or other nightshade family members) every second growing season in the same garden spot.
- Sanitize all gardening equipment before and after use using rubbing alcohol or a dilute bleach solution.
- Control volunteer weeds in the garden area, especially those in the nightshade family and the areas close to the tomatoes.
- Avoid overhead watering. Instead, water the soil around the plant’s base or use drip irrigation, keeping the foliage dry.
- Water vegetables in the morning, giving the water time to evaporate off of the leaves before nightfall.
- Apply a couple of inches of mulch on the soil around the base of vegetables, making sure not to touch the stems.
- Make sure there is good air circulation between and through the center of the plants. Accomplish this through adequate spacing and proper pruning.
- Remove bottom branches and suckers to prevent spores from splashing onto them from the soil surface.
- Discard all debris, whether healthy or diseased, into the trash.
- Treat pest problems quickly. Damage to the tissue from insects increases the chance of disease infection through open lesions on the foliage or stems.