It looks like a scale insect infestation. I would treat with a systemic containing imidacloprid. Others might suggest multiple applications of an oil such as neem.

Hi Thanks for your reply. Indeed, they can be rubbed off, and the leave looks intact after removal. They are on both surfaces and the the leave has an exudate that can be seen if you zoom in on the photograph. Iochroma, (see above) suggested a scale insect. What is your diagnosis tapla?

Same as Iochroma's. The questions were only to eliminate the physiological disorder, oedema, as a potentiality.

If it was my plant and I wanted to rid it of scale, my preferred method of treatment would be to spray with Bayer 3-in-1 for Insect, Disease, and Mite control, which contains Imidacloprid, but this must be sprayed outdoors and the plant must remain there long enough for the spray to dry. Second choice would be to use a 1.46% solution of Imidacloprid mixed with water as a soil drench. Add 2 tsp of the product (1.47% Imidacloprid) to a quart or liter of distilled water, along with 2 tsp white vinegar, then water it in making sure you cover the surface of the soil evenly until the solution is exiting the drain hole. It's also helpful to mix the Bayer product with distilled water + white vinegar at 2 tsp vinegar per qt/ltr of water. The reason is, Imidacloprid's effectiveness is diminished significantly in alkaline water (alkaline hydrolysis). It's best if water pH is in the 5.0-7.0 range, but no higher than 7.0. Last resort would be using a granular product mixed into the soil or broadcast on the soil surface. I have had poor results using these granular products due to root damage, which is why I don't like them. It might or might not be an innate issue with the product. It's possible, because I use such porous media, that the granules are clogging macro pores and increasing water retention, robbing me of the wide margin for error that allows me to water on a schedule.

Al

Thanks to Iochroma and tapla, I ordered from Amazon, you guys know your plant science!

@tapla (mid-Michigan, USDA z5b-6a), I didn't know that Imidacloprid's effectiveness is diminished significantly in alkaline water (alkaline hydrolysis). Is water from air conditioners ok instead of distilled water? PS: I have a bottle of Imidacloprid from Bayer, but unfortunately it has been banned here in Europe because it is harmful to bees. Is it still sold in the USA?

Once in a while I see a few scales on a plant, usually Schefflera, a couple times they were on a palm. Misting with 91% rubbing alcohol once a week for about 3 weeks has always gotten rid of them. Complete defoliation can also help (but not an acceptable option for a palm.) Short of that, cutting off every leaflet that has a scale will enable you to know if any new ones show up.

Condensate from air conditioners and humidifiers works well for watering plants, preparing insecticide solutions, should be close to neutral pH, lack carbonates, and have little buffering capacity. In the US, we can purchase small 5-stage reverse osmosis filtration systems which deliver 300-800 L of water with 0 ppm dissolved solids for a cost of $240 - 264 USD (currently). I use one for all plants in winter and for all drinking/ cooking water. They cost little to maintain. LINK

Imidacloprid is widely available here in the US.

Al

If the plant is not too big you can effectively remove scale by rubbing it off every leaf and stem with baby wipes. I have done it successfully with citrus. The method was recommended to me by a professional citrus grower.

@tapla (mid-Michigan, USDA z5b-6a), regarding imidacloprid, is the alkaline pH only a problem if you leave imidacloprid in the water for a long time before watering or also if you water immediately after adding it to the water?

Air conditioner water doesn't contain Chlorine or Fluorine either and I can use it to water my Dracaena marginata, right?

One can mechanically remove conspicuous adult scale, but that leaves all other stages of the pest in play.

pH is based on a logarithmic scale from 1 to 14. Where a pH of 7.0 is neutral, a pH of 6.0 is 10 times more acidic than a pH of 7.0, and pH at 5.0 is 10 times more acidic than 6.0. Many insecticides are sensitive to pH levels because of something called hydrolysis, which is a chemical process whereby water breaks molecular bonds. During the process, larger molecules with insecticidal properties are broken (cleaved) into smaller (ionic) particles that may totally lack any insecticidal properties when they recombine with other smaller particles. How much effect hydrolysis has on insecticides depends on the insecticide's chemistry, water pH, water temp, exposure to sunlight, and how long the mixture has been in the spray container. A few insecticides are susceptible to acid hydrolysis.

Hydrolysis starts immediately, as soon as imidacloprid is mixed into alkaline water, which is why insecticide solutions should be used asap and not retained for future use. You can see another example of alkaline hydrolysis in the scum left behind from soaps mixed with hard tapwater. The scum forms immediately after coming in contact with water.

Al

Hydrolysis starts immediately, as soon as imidacloprid is mixed into alkaline water, which is why insecticide solutions should be used asap and not retained for future use.

This thing is clear to me. But even if you water immediately after adding imidacloprid to the water, is it good to lower the pH of the water? Also in the growing medium does imidacloprid lose its effectiveness if tap water is used?

This thing is clear to me. But even if you water immediately after adding imidacloprid to the water, is it good to lower the pH of the water? From the moment it is mixed into alkaline water, its insecticidal properties are being diminished; so yes, it's best to use distilled water and acidify it before mixing in the insecticide. Most insecticides are degraded by water alkalinity - with neem oil, it's a significant issue.

Also in the growing medium does imidacloprid lose its effectiveness if tap water is used? If using it as a soil drench, it would depend on the pH of the soil solution and its buffering capacity. In most cases, your tapwater is alkaline but you would be using an acid forming fertilizer and the organic fraction of the grow medium produces organic acids as it breaks down. So in most cases it's not likely to be an issue, but it is still best to use distilled water (or its equal) which has been acidified. 2 tsp to a tbsp of white vinegar in a gallon of distilled water is a good way to go.

Al

Using wipes carefully removes all stages. You wipe the entire leaf and stems. You don’t just poke at the adult scales.