I have been bothered for some weeks by something that attacks coleus and impatiens.
It is like the whipped egg white we use in meringues.
It settles at the axles of the leaves and also mainly on the back of leaves.
It seems to suck the life out of the plants and they wilt and die.
I tried spraying with an aerosol white oil but this is too harsh for these soft plants and they die anyway.
Now I look every day for signs of the white sticky stuff and dip an artists brush in white oil and wipe the spots off.
These are mainly indoor plants but some outside also have the same problem.
I usually start new cuttings of these plants in a vase and when they root I plant them out thereby having the benefit of the coleus for many months.
Now I have this problem, it takes a lot of the fun out of gardening.
Meri, Silver Sands
Going off your descriptions, it sounds to me like either mealy bug, or powdery mildew.
Both of these afflictions thrive in warm, damp weather, which we’ve had quite a spell of this summer.
Powdery mildew is a fungus which looks like a fuzzy white film on the surface of leaves, first appearing as spots.
Mealy bug is a kind of scale that look like tiny fluffy slaters, and leave a spiderweb-like residue in the axles of plants.
Neither of these problems can be solved by white oil, and require specific management.
To manage powdery mildew you’ll need to remove the worst-affected leaves, and regularly spray a lime-sulfur or bi-carb soda based fungicide on the plants.
Fungal spores are spread by water splashing off the leaves, so take care when watering to direct the stream below the leaves, straight to the roots.
If your coleus or impatiens aren’t too precious to you, I’d even recommend just replacing them (if powdery mildew is the culprit), to avoid the disease spreading onto other plants.
Avoid composting infected plants, as the spores can stick around and spread.
Mealy bug can be controlled by either neem oil or soap-based pesticides, which you can purchase from most nurseries or hardware stores.
They’re attracted to excess water, fertilizer, and new growth, so if your gardening routines need tweaking that may be something to consider.
Do you have a gardening question for Jess? Send your queries to jess.cockerill@fairfaxmedia.com.au