Green Mold Spreading Across Citrus and Olive Trees: Identification and Treatment

I have two orange trees and a lemon tree. One orange started with a small amount of green mold, and now all three citrus trees—and even my olive tree—appear to be affected. The problem seems to be spreading. I can't find reliable information on what this is or how to save my trees. Please help.

Understanding "Green Mold" on Trees

True green mold (Penicillium digitatum) is primarily a post-harvest disease of citrus fruit, rarely seen on living trees unless fruit is overripe or damaged. What you're observing on the foliage and branches is more likely one of the following:

• Sooty mold: A black or dark green fungal growth on honeydew excreted by sap-sucking insects (scale, aphids, whiteflies, mealybugs). This is very common on citrus.
• Algae or lichen: Green, crusty growth on bark in humid, shaded conditions—harmless but unsightly.
• Fungal leaf spot: Various pathogens causing greenish-brown lesions on leaves.

Recommended Action Plan

1. Inspect closely: Use a magnifying glass to check for insects on leaf undersides, stems, and branch crotches. Look for bumps (scale), tiny soft bodies (aphids), or white fluff (mealybugs).
2. Treat the underlying cause: If insects are present, apply a systemic insecticide like imidacloprid (Bayer Advanced Tree & Shrub) as a soil drench. This provides season-long control.
3. Fungicide application: For fungal issues, spray with copper fungicide or neem oil (which has both fungicidal and insecticidal properties). Apply according to label directions, covering all surfaces.
4. Cultural practices: Improve air circulation with selective pruning. Avoid overhead watering. Clean up fallen debris.

The olive tree may have a separate issue—olives are susceptible to peacock spot (Spilocaea oleagina), which causes dark circular lesions with yellow halos. Copper sprays are effective for this as well.

Start with a thorough inspection. Identifying whether insects are driving the problem will determine the most effective treatment.