im alsoe concerned about the leaf color patterns ....

@ken_adrian Adrian MI cold Z5 what do you mean?

Ca(lcium) is required for o/a plant vitality, cell elongation and division, protein synthesis, and normal development of all cells/ tissue, including those of roots and leaves. It also facilitates influences uptake of other nutrients and as might be expected, increases Ca content of plants. It is essential as a structural component of cell wall structure and acts as an enzyme activator.

Deficiencies are fairly uncommon in mineral soils, but VERY common in plants grown under conventional container culture. A Ca deficiency often presents as: hooked new leaves; new bud inhibition; die-back of terminal buds; chlorosis (yellowing) - more prominent in newer leaves while older leaves remain a darker shade of green; mature leaves often display cupping. While you might not see or notice it, symptoms first occur in roots with the most notable being dieback of root tips which severely limits growth.

Causes of a Ca deficiency in containerized plants include: over-watering; soil compaction; a genuine lack of a sufficient amount of Ca in the grow medium; too much phosphate potassium, and particularly - magnesium in the grow medium. Excess some micronutrients can decrease availability of Ca as well, but are generally not an issue unless you have done something well beyond what is appropriate, from the perspective of the plant.

By the looks of things, you're chronically over-watering and the lack of oxygen inherent that practice is preventing uptake of an adequate amount of Ca. This is not difficult to diagnose because of it's commonality and the evidence presented in the images you provided pretty much makes it a slam-dunk.

Did you make or buy the grow medium the plants are in, or did they come in that medium? If you made it, what is in it and did you add dolomitic lime as a liming agent (provides Ca and Mg in an appropriate ratio)? What are you using as a fertilizer?

The short term remediation options would include using a "tell" to monitor moisture levels deep in the pot (see below), then water only when the soil is nearly dry at the bottom of the pot. It would be best if you were to work toward increasing the vitality level of the plant to ensure it will tolerate a repotting, then repot it into a more appropriate medium ..... at least the plant in the last image above. I can't be sure about how appropriate the other medium is, though I do see what looks like Seramis in the medium, but it's not represented in a volume sufficient to do the job its capable of doing (increasing aeration and drainage).

Al

@tapla (mid-Michigan, USDA z5b-6a), " Did you make or buy the grow medium the plants are in, or did they come in that medium? If you made it, what is in it and did you add dolomitic lime as a liming agent (provides Ca and Mg in an appropriate ratio)? What are you using as a fertilizer?"

this plant is shipped from thailand to uk. it tooks about 2 weeks to deliver to my home. it arrived with moss wrap tightly around its root. when it arrived 90% of roots had turn black and mushy. there is only a small section of taproot alive. I coat the root with rooting powder and keep it in moist sphagnum moss. I water it by misting the moss. I did that every few days for about 1.5 month. then it finally grow some root again. I pot it up with around 60% compost with 20% Seramis Clay Granules and 20% horticulture sand. I didn't use any lime, because it's an acid loving plant. I give it RO water only because my tap water is about 230ppm. I had only feed it 4 time in 6 months because it had go through bad root rot twice in 6 months. I use 20-20-20 balance fertiliser twice, iron sulphate once and ericaceous fertiliser once. do you think I should use garden lime or bottle water to water my m.alba. but those thing will increase soil Ph isnt it? it seems to have bad iron deficiency. if I increase ph, the iron deficiency problem may get worse?

there is one more thing I should mention. the soil mix I use ( 60% compost with 20% Seramis Clay Granules and 20% horticulture sand) seems to helps my m.alba grows a lot root. even when I notice the brown leaves problem. all the root seems strong and healthy. BUT around a week after I found the root stop taking up water. the soil stay WET for more than 4 days. I take it up from pot again and found 90% of the root rotten. so, I cut off all black root and coat it with rooting powder again just like what I did before. then I pot it with your 511 mix. then the brown leaf problem is gone and the root start growing although it still has very serious iron defiency and grow very slowly. recently, I found one if its new leaf dry up. I am worry the brown leaf problem happen again. that's how it looks now and the soil it's in.

If you look at the contents of your fertilizer, I wager you would find that it contains no Ca and perhaps no Mg, either. If true, you'll need to find a fertilizer that contains these nutrients or a supplement that supplies them individually or collectively if appropriate. You could use gypsum (CaSO4) as a Ca source and/or Epsom salts (MgSO4) as a source of Mg w/o changing pH notably.

I'm not sure what you've settled on as the medium you'll use, but if it includes the ingredients:

Seramis, compost, hort sand, I'd change the %s to 60% Seramis, and 20% each of the compost and hort sand. Even then, I'd use a wick and some form of ballast in the pot's bottom.

Compare D to A to see how much excess water an overturned pot or a couple of hockey pucks (in smaller pots) can make. A wick through the bottom at a/the pot sidewall can be very helpful as well, as can simply tipping your pots to a 45* angle after a thorough watering (compare B to A).

Al

@tapla (mid-Michigan, USDA z5b-6a) if I use gypsum and epsom salt. how much should I use and how often should I apply? the gypsum I can find is food grade. so it doesn't say how much I can use on plant.

You have mites, a severe case..That is what killed mines three times. Now my recent one has lasted two years and thus far ok. No mites. Got my eyes on that.

Your top pics says it all. Look at the underneath of the leaves. and the infestation on the new growth in your third picture. Notorious for mites, and the same goes for jasmine. I have those exact ones too.

Your choice of mix and the nutrients you choose will make all the difference in keeping your plants very healthy so that they don't become so easily susceptible to mites.

At late winter, “all young leaves start to turn brown as soon as it uncurl. before they grow big. they just turn brown totally and die. all the old leaves/big leaves stay green. just all the young leaves turn brown”:

this could be because the cold temperature at that time, too cold temperature could cause the young tender leaves & buds to die while more seasoned old leaves survived. Once the cold periods pass the temperature increases the michelia alba will grow again. Here in Sydney we have much milder winter than UK but some michelia alba trees still shed lots of leaves at the end of winter then gradually grow normally again in late spring, so if your tree comes back growing again when temperature is higher then this late winter new leaves/buds dieing problem might not be a problem or just move the pot to a warmer spot if you wish to avoid this