White Spots on the Crown of a Thorn Plant

Mealy Bugs

• Mealy bugs are small, mobile insects up to 1/5 inch long. They tend to be covered in white dust and look like little bits of cotton wool on plant stems and leaves. They gather on fresh leaves and shoots and can cause weak or stunted growth. Mealy bugs also produce a sweet substance called honeydew, which encourages mold and attracts ants. Treat mealy bugs by dabbing them with a paintbrush dipped in rubbing alcohol.

Scale Insects

• Scale insects are similar to mealy bugs but the adults are immobile. Several species, such as the cochineal bug and oyster shell scale are white. They look like raised bumps on the surface of stems and leaves. Treat them as with mealy bugs or spray with an insecticidal soap.

Sap

• Crown-of-thorns produce a white, milky sap when their leaves or stems are damaged. This sap causes an allergic reaction in sensitive people. Dried drops of sap look like little white balls or spots on a crown-of-thorns plant and can appear after wind or contact damage. Plant crown-of-thorns well away from paths and use gloves when pruning. Do not grow crown-of-thorns near ponds because the sap kills fish if it gets into the water.

Water Marks

• Pale white spots on the leaves of the crown of thorns may just be a mineral deposit left behind by evaporating water drops. These spots are most common in hard water areas. Avoid watering crown-of-thorns from above, especially during the winter, to avoid rot and mold.