Root Awakening: Sooty mould on pomelo leaves due to a sucking pest infestation

Sooty mould on pomelo leaves due to sucking pest infestation

I have been growing this pomelo plant from seeds for several years. The underside of the leaves has a white layer and some leaves have a black layer as well. I have been pruning the affected leaves, but the problem seems to be getting worse. What is causing the white and black layers and how do I remove them? The plant’s leaves also attract many caterpillars – how can I prevent this?

Lyn Tan

The black patches on your plant could be due to sooty mould. This fungus grows on the sugary excrement of sucking insects and, when it covers large sections of the leaves, can reduce the photosynthetic capacity of the plant.

These patches can be removed, with some effort, using a dilute solution of soapy water and gently rubbing the affected leaves between fingers.

Sucking insects that might affect woody plants include scale insects, mealy bugs, aphids and whiteflies. Check for their presence and take appropriate action by applying low-toxicity pesticides like pyrethrins, castile soap solution and summer oil. Rotate the pesticides and apply them at regular intervals to manage the pests’ populations.

The white spots on the underside of leaves could be a past infestation of whiteflies.

The caterpillars could be those of the lime and common Mormon butterflies. Instead of killing them, you can rear them or give them to schools to teach students about the life cycles of insects.

To rear caterpillars sustainably yet not affect your plant, keep them in a tank where you ration their food by feeding them plucked leaves.

Caterpillars can be managed by spraying the plant with Dipel, which is a pesticide based on Bacillus thuringiensis subspecies kurstaki. The pesticide needs to be sprayed preventively and is more effective on young caterpillars. Reapplication is required as the insecticide is washed away by heavy rain.

Young chlorotic leaves due to nutrient deficiency

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