Black Spots On Hibiscus Leaves and Buds: What And Why?

Black spots on your hibiscus leaves or buds? Take a breath. Most of the time this is a cosmetic problem you can clear up, not a death sentence for the plant.

Black spots on hibiscus usually come from one of two things: a leaf spot disease growing inside the leaf, or sooty mold growing on top of it. They look alike from across the yard. Up close, they’re very different problems with very different fixes.

Here’s how to tell which one you have, how much it matters, and what actually works.

What Causes Black Spots on Hibiscus?

Two very different things leave black marks on hibiscus:

• Leaf spot diseases. Fungi or bacteria infect the leaf tissue itself, leaving dead spots in the leaf.
• Sooty mold. A black fungus grows on the sticky honeydew that sap-sucking insects drip onto leaves, buds and stems. The leaf underneath is fine.

Leaf Spot Diseases

Clemson Cooperative Extension says leaf spots on hibiscus may be caused by several fungi. Its hibiscus factsheet covers both the hardy types, such as rose mallow (Hibiscus moscheutos) and rose of Sharon (Hibiscus syriacus), and tropical Chinese hibiscus (Hibiscus rosa-sinensis).

Bacteria cause leaf spots too. The University of Hawaiʻi describes a bacterial leaf spot of tropical hibiscus caused by Pseudomonas cichorii. Its spots are 2 to 10 mm across with a tan center, a black border, an outer purplish-red edge and a yellowish halo, though the purplish tinge can be missing on some cultivars and on the leaf underside. Spots often turn angular where they meet leaf veins, and the dead centers sometimes fall out, leaving a shot-hole look. That disease doesn’t touch flowers, petioles or stems, and the same publication notes that in Florida these bacterial leaf spots have been most common in nurseries during propagation and develop less often in the landscape.

What every leaf spot has in common is water. The University of Minnesota Extension says most leaf spot diseases need water on the leaves or very high humidity for a prolonged period (12 to 24 hours) to start an infection, and that spores are spread by wind and by splashing rain or irrigation. The hibiscus bacteria spread mainly by splashing or windblown rain. The Missouri Botanical Garden says most fungal leaf spots are active during wet times of the year, such as spring or early fall.

Sooty Mold From Sap-Sucking Pests

Sooty mold is the black, soot-like coating you can see on top of leaves and stems. The University of California Statewide IPM Program (UC IPM) says sooty mold doesn’t infect plants. It grows on honeydew, a sweet, sticky liquid that sap-sucking insects excrete while they feed. Aphids, mealybugs, soft scales, whiteflies, leafhoppers and psyllids all make honeydew.

On hibiscus, the usual suspects are aphids, whiteflies and mealybugs. UF/IFAS Extension says these piercing-sucking insects are usually found in clusters on leaves and flower buds, and suggests scouting for them by flipping leaves or by noticing sooty mold growing on the honeydew. So if your buds look black and sooty, check them closely for insects.

Hibiscus aphids and the pink hibiscus mealybug are two to know. Pink hibiscus mealybug excretes honeydew that encourages black sooty mold, and its toxic saliva curls and contorts leaves; buds may not flower.

More on Hibiscus Pests:

• White Fly on Hibiscus
• Spider Mites on Hibiscus

How To Tell Leaf Spot From Sooty Mold

• Where it sits. Sooty mold is a coating on the surface; it can be washed off plants with a strong stream of water or soap and water. Leaf spots are dead patches in the leaf itself.
• Sticky leaves. Honeydew is sticky. Sticky leaves with a black coating point to sooty mold.
• Insects. Flip the leaves and check buds and new growth for clustered aphids, whiteflies or cottony mealybugs.
• Ants. Ants feed on honeydew and protect the insects that make it, so ants marching up the stems point to a honeydew problem.
• Spot shape. Leaf spots come in many shapes, sizes and colors and occur randomly on the leaf surface. On tropical hibiscus, the bacterial kind has a black border and a yellowish halo.

Do Black Spots Hurt the Plant?

Usually not much. Most leaf spot diseases affect only a small part of a plant’s leaf area and are a minor stress, and they will not seriously harm your plants. Leaves may fall if the problem is severe, but these pathogens rarely cause long-term damage. Most fungal leaf spots cause only cosmetic damage. Heavy bacterial spotting on hibiscus can make leaves drop early.

Sooty mold is also mostly cosmetic, with one catch. Although sooty mold doesn’t infect plants, a coating thick enough to keep sunlight off the leaves reduces photosynthesis, which can stunt growth, and coated leaves may age and drop early. The bigger worry is the insects underneath. Large aphid or mealybug numbers do their own damage, from curled new growth to buds that don’t open.

How To Treat Leaf Spot on Hibiscus

Start with the simple stuff, because it’s what the experts lead with. In most cases leaf spot infections can be tolerated, and fungicide treatment is generally not warranted.

• Pick off and pick up. Cleaning up plant debris and removing infected leaves provides adequate control in most cases. Remove fallen leaves and debris promptly; the hibiscus bacteria can survive on fallen leaves.
• Keep water off the leaves. Water hibiscus at the soil. Water at the base, avoid splashing the leaves, and use a drip or soaker hose instead of sprinklers.
• Water early in the day. Avoid overhead sprinklers and irrigate early in the day so foliage dries more quickly.
• Open it up. Prune trees and shrubs to let in light and improve air circulation, and don’t overcrowd plants. Good soil drainage, wide spacing, trimming weeds and avoiding heavy shade all cut leaf wetness and humidity around hibiscus.
• Don’t push fertilizer. Don’t fertilize shrubs with leaf spot diseases unless a soil test calls for it.

Weather drives leaf spot. Infections need long wet periods, and the cycle of new spots repeats whenever conditions are favorable. That’s why spotting tends to flare in rainy spells.

Thinking about a spray? Fungicides are protective: they need to go on before symptoms appear, not after. Use one when leaf spots are going to reduce the plant’s value or health, applied before infection begins. Neem oil isn’t a cure-all here; neem oil products act mainly against insects and may also suppress certain fungal diseases when used in combination with fungicides. For the bacterial leaf spot, foliar bactericide sprays usually won’t control it in high-rainfall areas. If you do spray, use a product labeled for hibiscus and the disease, and follow the label.

Related: Why Are My Hibiscus Leaves Turning Yellow?

How To Get Rid of Sooty Mold on Hibiscus

You can’t fix sooty mold by attacking the mold. Control begins with managing the insect creating the honeydew; once the insects drop, honeydew production slows and sooty mold gradually decreases.

• Check new growth. Many sucking insects prefer new, tender growth. Look at buds and leaf undersides, and check at least twice a week when plants are growing rapidly.
• Hose them off. A strong stream of water can knock off the insects in some situations.
• Prune out the worst. Careful pruning can remove most of the infested plant parts.
• Stop the ants. Ants protect honeydew-producing insects from predators and parasites. Put a sticky band around the trunk of a tree-form hibiscus, trim branches that touch buildings, or set ant stakes under the plant.
• Feed and water sensibly. Fertilize and water properly so plants stay healthy but don’t grow excessively, since soft new growth draws sucking insects.

If you need a spray, aphids, whiteflies and mealybugs on hibiscus caught early are easily controlled with insecticidal soaps or horticultural oils. Buy a commercial product: homemade soap recipes are more likely to injure plants. Soaps and oils only kill the insects they cover, so wet the tops and undersides of leaves and the stems, and repeat as the label allows. Don’t spray water-stressed plants, in direct sunlight, or above 90°F, and spray early in the morning or late in the evening. Test a small area first and wait 24 hours to check for damage.

Skip the heavy artillery. Read labels carefully, because you should never use the pesticide malathion on hibiscus. Broad-spectrum sprays also kill the natural enemies that keep aphids in check.

Natural enemies help too. Lady beetles, lacewings, syrphid fly larvae and tiny parasitic wasps are among the insects that eat aphids. Buying ladybugs? Released lady beetles may give some temporary control, but most will disperse from your yard within a few days.

Once the pests are under control, clean up the leaves. Sooty mold can be washed off plants with a strong stream of water or soap and water, and insecticidal soap is an effective leaf wash for honeydew and sooty mold. As honeydew production drops, the sooty mold will gradually decrease.

Which one turned up on your hibiscus, leaf spot or sooty mold? Tell us in the comments.