Most of the leaves on this plant are infected. I don’t know if this is a fungal disease or insect damage. What can I do to save it? Thanks
Thanks for contacting the Toronto Master Gardeners. It’s difficult to diagnose disease or insect issues by looking at a photo, but I think what you have is likely an insect problem. I suggest that you check your plant thoroughly with a magnifying glass, especially the undersides of the leaves. In addition to the insects themselves, look for webbing (an indication that spider mites might be present), or a sticky substance on the leaves (this is called honeydew and could be evidence of scale or aphids or mealy bugs). If you don’t see any webbing or stickiness, then my guess based on the damage to the leaves is that your plant has thrips.
Thrips are tiny, so really hard to see. They have narrow, elongated bodies with fringed wings, and some species are brown or black. Some species leave sooty black spots on the leaves. You can purchase sticky traps at a home and garden store and place them near your plant, to confirm the presence of these and any other winged insects. You can also try blowing on the damaged leaves, and watch for any movement as they run about.
The first thing you need to do is isolate this plant, since thrips can move easily from plant to plant. You should also check any other plants that might be nearby and isolate them as well if they appear to be infected. Thrips are difficult to get rid of because they lay eggs inside the plant and pupate in the soil, so multiple treatments to the leaves and stems as well as the soil with insecticidal soap or horticultural oil for indoor plants are needed. With persistence, hopefully you will be able to totally eradicate the thrips from your plant. To limit or reduce any further damage, it will help to increase humidity and not let the plant fully dry out.
You can find more information about thrips at the link below. Please note that the information about pesticides (chemical controls) in this article applies in the US and not in Canada. The use of pesticides in Ontario for cosmetic purposes in home lawns and gardens is strictly controlled by law. The second link below contains an Allowable list of active ingredients that can be used in Ontario.
The link below has information about common problems in indoor plants that are caused by insects, diseases, nutrient deficiencies and environmental conditions that you might find helpful.
Good luck with your plant !