That doesn't look like any spider mite damage I've ever seen. However, some of the leaves on the schefflera look chlorotic. What fertilizer have they received in the recent past?

Deanna

Looking at the deformations on the other leaves makes me wonder about some kind of mites. IDK what caused the damaged spots on the leaf in question.

Thanks everyone for answers. I googled and it looks it was spider mites. Unfortunately I didn't have time last weeks so I took action only today. Since I was late with action plants were damaged little too much as I would like, so I made them "soap bath" and then rinsed with water. I'll let them dry for couple days and maybe then I'll repeat the procedure.

It's oedema. Something I wrote a while ago:

Oedema is a physiological disorder that can affect all terrestrial plants. It occurs when the plant takes up more water than it can rid itself of via the process of transpiration. The word itself means 'swelling', which is usually the first symptom, and comes in the form of pale blisters or water-filled bumps on foliage. Under a variety of circumstances/cultural conditions, a plant's internal water pressure (turgidity) can become so elevated that some leaf cells rupture and leak their contents into inter-cellular spaces in leaf tissue, creating wet or weepy areas. Symptoms vary by plant, but as the malady progresses, areas of the leaf turn yellow, brown, brown with reddish overtones or even black, with older damage appearing as corky/ scaly/ridged patches, or wart/gall-like bumpy growth. Symptoms are seen more frequently in plants that are fleshy, are usually more pronounced on the underside of leaves, and older/lower leaves are more likely to be affected than younger/upper leaves.

Oedema is most common in houseplants during the winter/early spring months, is driven primarily by excessive water retention in the soil, and can be intensified via several additional cultural influences. Cool temperatures, high humidity levels, low light conditions, or partial defoliation can individually or collectively act to intensify the problem, as can anything else that slows transpiration. Nutritional deficiencies of Ca and Mg are also known contributors to the malady.

Some things that can help you prevent oedema:

* Increase light levels and temperature

* Monitor water needs carefully – avoid over-watering. I'd heartily
recommend a soil with drainage so sharp (fast) that when you to water
to beyond the saturation point you needn't worry about prolonged
periods of soil saturation wrecking root health/function. Your soil
choice should be a key that unlocks the solutions to many potential
problems.

* Avoid misting or getting water on foliage. It slows transpiration and
increases turgidity.

* Water as soon as you get up in the AM. When stomata close in
preparation for the dark cycle, turgidity builds. If you water early
in the day, it gives the plant an opportunity to remove (for its own
needs) some of the excess water in the soil.

* Put a fan in the room or otherwise increase air flow/circulation.
Avoid over-crowding your plants.

Al

tapla(Mid-MI z-5b-6a) this sounds interesting!!! Thanks for into. I found some pictures on internet that looks exactly the same as damage on my plants, but the only thing: I have that problem with all the plants on one window sill. In the same room on other window sill plants are ok. The temperature and humidity conditions are the same for all plants. I also wouldn't say that I am the one who overwaters plants (actually other way round - my fault could be to underwater them). And I didn't find any blister on plants, just those brown/yellowish spots... I washed all plants and I hope they won't get more spots..

It's entirely possible that you can't help but over-water; or, a better way of organizing that thought might be to say, if you're watering correctly and your soil holds an excessive volume of water when watering chores are done, the stage is set for bouts of Oedema, which is clearly being made manifest in the plants in the image.

My suggestion would be to continue to water correctly, but start using any or several of a variety of techniques that will limit the amount of excess water your soil can/will hold. Then, as your plant begins to use what water remains in the soil, monitor it carefully with a tell. More on that later.

Something as simple as tipping the pot at a 45* angle after you water can eliminate a large measure of excess water. Compare line drawing B to drawing A in the image below to see the difference that makes:

Something I wrote about using a 'tell':

Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor.

In many cases, we can judge whether or not a planting needs watering by
hefting the pot. This is especially true if the pot is made from
light material, like plastic, but doesn't work (as) well when the pot
is made from heavier material, like clay, or when the size/weight of
the pot precludes grabbing it with one hand to judge its weight and
gauge the need for water.

Fingers stuck an inch or two into the soil work ok for shallow pots, but not
for deep pots. Deep pots might have 3 or more inches of soil that
feels totally dry, while the lower several inches of the soil is 100%
saturated. Obviously, the lack of oxygen in the root zone situation
can wreak havoc with root health and cause the loss of a very
notable measure of your plant's potential. Inexpensive watering
meters don't even measure moisture levels, they measure electrical
conductivity. Clean the tip and insert it into a cup of distilled
water and witness the fact it reads 'DRY'.

One of the most reliable methods of checking a planting's need for water
is using a 'tell'. You can use a bamboo skewer in a pinch, but a
wooden dowel rod of about 5/16” (75-85mm) would work better. They
usually come 48” (120cm) long and can usually be cut in half and
serve as a pair. Sharpen all 4 ends in a pencil sharpener and
slightly blunt the tip so it's about the diameter of the head on a
straight pin. Push the wooden tell deep into the soil. Don't worry,
it won't harm the root system. If the plant is quite root-bound, you
might need to try several places until you find one where you can
push it all the way to the pot's bottom. Leave it a few seconds, then
withdraw it and inspect the tip for moisture. For most plantings,
withhold water until the tell comes out dry or nearly so. If you see
signs of wilting, adjust the interval between waterings so drought
stress isn't a recurring issue.

You'll find more about dealing with water-retentive soils if you click me.

Al

Those yellow spots are definitely mechanical and sucking damage from invertebrate pests. This may exacerbate or be exacerbated by watering issues as well but that's definitely spider mite damage on the scheff. I wouldn't doubt if the spots were a little bit sticky as well.