Poinsettia makes for a colorful houseplant that’s prized for its festive bloom and look. But you may not feel so festive when your poinsettia leaves start turning black.
Your poinsettia leaves are likely to turn black because of bacterial or fungal diseases, especially Alternaria leaf spots. They start as small black or brown spots, then expand and cover the entire leaf. Other culprits include heat stress, low humidity, improper watering, and salt build-up.
In some cases, you may be able to salvage your poinsettia. And, in other cases, the leaf blackening problem cannot be reversed. This makes prevention the best strategy.
I’ll show you how to identify, treat, or prevent each potential cause of poinsettia leaves from turning black.
Causes of Poinsettia Turning Black
Poinsettia is susceptible to fungal and bacterial diseases. These include bacterial canker, fungal infections, powdery mildew, scab, and more. The three below cause extensive leaf spot damage, resulting in leaf blackening.
– Alternaria Leaf Spot
The most obvious sign of the Alternaria leaf spot is the presence of blackish or dark brown spots on the foliage. Early on, they’re small, and over time, can expand to cover the whole leaf.
In severe cases, the black leaves will wilt and drop off. This disease is rarely fatal but, if left untreated for a long period, can cause your poinsettia to die.
– Bacterial Leaf Spots
Bacterial leaf spots are a more serious and likely fatal disease than Alternaria. If you spot yellow flecks underneath leaves, you may be dealing with this foliage disease. If left untreated, they’ll turn from yellow to red streaks, which eventually lengthen and turn black.
Yellow leaves with stem cankers are other symptoms of the disease. The affected stems and leaves will ultimately blacken and shrivel or curl up. Dark brown or black spots on leaves with yellow halos is the sure sign of bacterial leaf spot disease.
Also called poinsettia scab, anthracnose causes black spots on the midrib, blade, and the leaves’ veins. The affected stems develop pale buff lesions that turn black over time, covering them with a canker.
The affected leaves will wilt, roll inwards, and take on a puckered appearance before falling off prematurely.
The first step when you spot these diseases is to isolate the affected plant. From there, prune away and destroy any infested leaves, stems, and other plant matter.
The next step is to spray your poinsettia with a fungicidal spray at weekly or ten-day intervals. Neem oil, captan, copper-based fungicides, and horticultural oils often do the trick.
Space out your houseplants, improve aeration, and avoid overhead watering to prevent further spreading of the disease vectors.
Bract Edge Burn on Poinsettias
Bracts on your poinsettia are rapidly growing. They need plenty of calcium and other nutrients for cell-wall building and to sustain this robust growth.Bract edge burn results when there’s calcium deficiency.
This causes the bract edges or margins to turn brown or blackish. When Botrytis infection sets in, you’ll see tiny, necrotic brown lesions along the bract edges. They eventually coalesce and turn the whole bract black.
Don’t wait until the damage is visible. Instead, give your poinsettia a dose of calcium when you spot any signs of bract edge burns.
You can use a nitrate-based fertilizer for root application. Or, you can opt for a calcium chloride-based foliar spray, applying every week for at least four weeks.
Poinsettias prefer warm, humid conditions since they’re native to tropical climates. Low humidity will cause stress to your poinsettia. Leaves will turn brown, black, or fall off as a response to the stress.
Low humidity also accelerates the loss of leaf moisture via transpiration and respiration. As a result, after a prolonged period, the leaves of your poinsettia will become dry, crisp up, and appear scorched black.
Your poinsettia will appreciate high relative humidity levels. First, however, you must get rid of heavily affected or fallen material to avoid a disease outbreak.
Place a water tray with pebbles near your poinsettia to improve humidity. Then, of course, you can use a humidifier.
Heat stress can cause your poinsettia leaves to turn black. Poinsettias are tender tropicals that begin to wilt, dry up, and scorch when exposed to too much heat. This becomes even worse when combined with underwatering and low humidity.
Heat stress can cause heat build-up in the root system. This hampers the take-up of water and nutrients, causing the leaf to turn brown, yellow, or black.
Move your poinsettia away from the source of heat stress. These can be fireplace, heat vents, radiators, or opened windows during summer. Avoid spots that subject your poinsettia to rapid changes in temp.
Your poinsettias will thrive in temperatures in the ideal range of 60-70°F (15-21°C). Unfortunately, both heat and cold drafts will subject your plant to temperatures outside this bracket. This will possibly cause leaf damage and turn leaves black.
This is especially true when cold drafts drive temperatures below 60°F (15°C). Prolonged exposure to extreme drafts will cause leaves to appear burned, wilt, droop and fall off. You may also notice some leaves yellowing, browning, or fading.
- Your poinsettia doesn’t like drafts, so move it away from them.
- Cold drafts include poorly insulated rooms, AC vents, entryway doors, and open windows during winter.
- Move from hot drafts like space heaters, heat vents, radiators, fireplaces, and so on.
Exposing your poinsettia to too much direct sunlight will scorch and damage the leaves. Sunburns manifest in the form of dry, crispy black or dark brown leaf tips. This is usually easy to diagnose because of scorched black spots on the leaves.
Move your poinsettia away from the scorching spot. Find a nice spot where it’ll receive bright, indirect light. Ideally, it should be a west-facing window, where it’ll benefit from the mild morning sun and get afternoon shade.
The black leaf spots and sunburned leaves will likely not recover, so trim them. However, mildly sunburned foliage should remain, especially if the damage to the rest of the leaves is extensive.
Root Rot and Stem Rot
Your poinsettia leaves can turn black because of stem rot and root rot. You should watch out for Pythium root rot which causes stunting, leaf browning, and wilting. It may result in premature leaf dropping and flowering.
Rhizoctonia root rot may also cause leaf blackening. It usually spreads through infected cuttings. When the roots are damaged, your plant will no longer be able to absorb nutrients and water, hence leaf damage and turns black.
Rhizoctonia stem rot is also common among poinsettias. The rot spreads rapidly to the crown and other parts of the plant. As a result, they droop and feel soft, mushy to the touch.
- Root and stem rot disease is exacerbated by overwatering and over-fertilizing. Avoid them.
- Firstly, you must isolate affected plants and trim away diseased or dead roots and stems. Then, if the rot damage is too severe or widespread, consider propagation.
- Treat with fungicides or hydrogen peroxide, then repot with a fresh potting mix.
Like most tropical plants, your poinsettia is prone to a range of pests, including mealybugs, thrips, aphids, whiteflies, spider mites, and fungus gnats. Therefore, you should double-check under the leaves, specifically for aphids and whiteflies.
Most of these pests suck the sap out of the leaves, leaving brown spots encircled with a yellow halo. Over time, these spots will enlarge and turn black.
Take your infested poinsettia to the shower, bathtub, or outdoors. Give it a good water spray to wash off the bugs. Don’t forget to get rid of heavily affected or diseased parts.
You should spray your poinsettia with an insecticidal spray like neem oil or miticide. Repeat this routine every 7-10 days until signs of insect infestation are gone.
Overwatering leads to leaves turning black in a variety of ways. First, most fungal and bacterial diseases are favored by overwatering (and sometimes over-fertilizing).
Excess water in the soil causes the roots to suffocate, rot, and die. This is a perfect recipe for nutrient deficiency, wilting, and leaf damage, all of which cause leaves to blacken.
If you see wilting and dropping of lower/older leaves, drooping, and large black patches on the leaves, you should ease up on the watering can. (Source: Iowa State University)
- Stop watering immediately. Prune away diseased and affected plant parts.
- Check the roots for root rot and treat accordingly.
- Let the top two to three inches of soil dry out before watering next.
Over-Drying the Soil
If you let the soil dry out excessively, this will cause the leaves of your poinsettia to turn black. This happens indirectly, as dry soil makes it hard for roots to “breathe” or absorb nutrients.
It’s nutrition deficiency, dehydration, poor root absorption, and root damage that will eventually result in the blackening of leaves. This usually occurs due to exposure to too much light, heat drafts, or low humidity.
Provide indirect light to avoid compacting the soil. To loosen up the soil, use a fork or stick to dig up a little. Now, mix in some small pebbles, gravel, organic vermiculite, or perlite to promote drainage.
Consider repotting with fresh, well-draining soil. Water the soil until the liquid comes out of the bottom holes, then empty out the excess run-off.
When your poinsettia is growing rapidly, you may be tempted to use too much fertilizer. This is detrimental to your plant’s health. This is especially seen during warmer spring and summer weather.
Your poinsettia will take up too many nutrients faster than it can handle. This will lead to fertilizer burn and cause leaves to turn black.
Overfertilizing also causes salts to accumulate in the soil. This results in the tips, edges, or the whole leaves turning black.
- Remove white salt scabs from the surface of the soil—flush water through the soil to remove excess salts.
- If the salt build-up is severe, you should re-pot with a fresh batching of potting mix.
Lack of Nutrition
Feeding your poinsettia can be tricky. Giving too much fertilizer will cause salt burns. On the other hand, lack of nitrogen, potassium, and other nutrients will cause stunted or degenerative growth.
Nutrient deficiency is often seen in the form of leaf yellowing and black spots. Leaves may also curl up, wilt, and fall off.
Start feeding your poinsettia with half-strength houseplant fertilizer in early spring. Don’t give fertilizer during the blooming phase. Otherwise, fertilize every 3-4 weeks until the signs of malnutrition disappear.
Salt Build-up in Soil
Salts may start to build up in the potting mix due to softened water or poor water quality. Again, salts cause burns and leaf damage. This affects the take-up of water and nutrients. In the meantime, the leaves will start scorching, wilting, and may drop.
The tips, edges, or entire leaves will start turning black and shrivel. In addition, you may notice white scab on the soil surface indicating salt build-up.
Scrape off white salt scabs from the soil surface. If the build-up isn’t too severe, douse the soil with water to flush out excess salts.
For severe cases of salt accumulation, you have no choice but to re-pot with a new, high-quality potting mix.
Low light, cold draft, and overwatering are often interrelated. Photosynthesis (which helps your poinsettia make food) and respiration rely on sunlight exposure.
Even if you water correctly, the soil will remain moist or wet for long because of low light. This will lead to yellow, dark blotches on the foliage.
Inadequate sunlight also leads to the decrement of green pigment chlorophylls. Without them, the leaves will discolor then turn black.
Move your poinsettia to a spot where it’ll sit in bright, indirect light.
How to Prevent Blackening of Poinsettia
- Avoid overhead irrigation
- Ensure your poinsettia gets plenty of bright, indirect, or filtered light
- Keep your plant off hot and cold drafts
- Make sure topsoil dry out between waterings
- Trim away any black, yellow, brown, or otherwise disease parts
- Provide ideal temperature; 60-70°F (15-21°C)
- Immediately isolate any houseplant that shows signs of disease or pests
- Fertilize your poinsettia every 3-4 weeks.