It's normal for more than half the buds to not set fruit on a healthy citrus.

Is your tree in a pot or the ground?

Whee do you live?

Thanks for a quick answer!

The problem is that no (not 1!) fruit has reached more than 3 cm in length. ALL of them eventually fall off.

Tree is in a pot.

I live in Slovenia (central/southern Europe). Stats here are for our capital:

http://www.weather-and-climate.com/average-monthly-Rainfall-Temperature-Sunshine,Ljubljana,Slovenia

But I have more mediterranean climate which is quite good for citruses (my neighbours have lots of fruit bearing trees).

Actually, cow manure is an almost perfect fertilizer for lemons planted in the ground; but not for potted lemons. Your tree is probably just experiencing transplant shock multiple times. Keep doing what you are doing and it should recover; it is flowering and setting fruit, only the current nutrition provided by the shocked root system cannot support the fruit and it is being aborted. When a lemon aborts a fruit, it will turn yellow and if you leave it, it will turn black. Take them off when they turn yellow.

So I should leave it now as it is. I mean, no anti fungicide or anti bacterial compound is needed?

Here are 2 more photos from today (yellowish colouring of leaves):

I have removed all black / yellow fruits and left only dark green.

Maybe I should mention that when I re-potted the plant yesterday I mixed citrus fertilizer with rainwater and water it + also small amount I sprayed on leaves - foliar fertilization (quantities as described on box) .

I don't see any fungal or bacterial issues; only nutrient deficiencies partly caused by root disturbance. I think if you leave it alone and continue with regular feeding, it will be fine.

I live in S.W.Fla. and grow lemon (Bearas) , lime (Persian) and Blood orange. The dark green wedge running down the mid-rib of the leaves indicates magnesium deficiency. Get some magnesium sulfate (Epson salts) and scatter lightly (about 1-2 Tbl spoons) out as far as the drip-ling of the canopy but stay 3-4 inches away from trunk. It may take several weeks to a couple of months to correct. Most citrus fertilizers have a good % of Mg but sometimes a little extra every 2-3 months really helps a lot, especially on a fast-growing young tree. Also, spraying every 2-3 months with a trace element mixture, commonly called a citrus nutritional mix, will improve the immunity system in the plant & prevent several deficiencies. Hope it takes off for you!! P.S., citrus of all types don't like "wet foot" which is too much water around the roots constantly.
Since its in a pot, it really needs to drain easily to prevent "foot rot" ( a disease of the crown roots & can kill a tree quickly). It also will leach out nutrients before roots can take them up.

I re-potted tree in specialized citrus soil (30%C, 0,8%N, C/N 37,5, 150mg/kg Cu, 500mg/kg Zn, 70% peat, 1,2 dS/m salinity) and been using once per week specialized citrus fertilizer NPK 30-10-10+ Mn(0,25%)+Mo(0,01%)+Zn(0,07%).

I apply 2/3 of fertilizer mixed with water, and 1/3 as a foliar fertilizer.

The soil seems to be light as I saw vermiculite/perlite inside.

Hey Jure, sounds like you are using a good citrus fertilizer mix except for no magnesium (Mg). Try getting a small bag at a nursery or box store. If they don't have it in stock, you can buy it at a drug store as Epsom salts; it's a little more expensive in the pharmaceutical grade vs. the ag grade but isn't expensive for the small quantity you need. A small 3-5 lb. bag will last you at least a year. The element is so important for the complete metabolism of the plant, especially citrus, I'm a bit surprised it's not listed on the citrus fertilizer tag. It should be listed as a secondary element along with calcium and sulfur. As I suggested earlier, a citrus nutritional spray should be used several times a year and will also contain magnesium as well as the other minor (trace) elements such as iron, manganese, zinc etc. Just be sure when you spray that you don't get the spray on anything you don't want stained because it will not come off easily. It would probably be wise to move the pot onto an area like some grass. By the way, citrus absorbs trace elements way more efficiently through the leaves than through the root system (make sure the undersides of the leaves receive the spray since the biggest absorption occurs there). Can't stress enough how important a complete nutritional fertilizer and/or spray is and the mix you are using is lacking several besides the Mg, such as iron and boron. Hope this has not been too wordy but consulting with citrus growers plus my own growing experience here in SW Fla. has brought this home to me many times. Good luck and let us know how well it goes.

Thanks a lot for all the additional info!

I've asked for a dedicated fertilizer for citruses and that is what they gave me...

I've been searching on the web on what we have available here. I can buy Epsom salts (to solve magnesium deficiency), but we also have fertilizer called FERTILON COMBI. It's quite expensive (roughly 6 USD for 100g or 30 USD for 1kg) and has this listed on the box:

0,5% B
1,5% Cu*
4,0% Fe*
4,0% Mn*
0,1% Mo
1,5% Zn*
9,0% MgO
3,0% S

It doesn't say that it can used for citruses?

So what should I buy?

P.S: I've been already spraying on top and bottom of leaves as I imagined it absorbs better from below.

Where you are at in southern Europe, you may not have not have the choices we have in a citrus-growing area like ours. The FERTILON COMBI looks very good for citrus. Is it a sprayable? and if so, a low dilution rate may make it economically feasible as a spray. By adding the Epson salts along with the dry mix you are already applying to the pot will do an excellent job for you. If you get the FERTILON, COMBI, you don't need to spray with the the mix you're using on the soil; that may be overdoing it and may be injurious to the plant.

What about liquid magnesium fertilizer (as Epsom salts replacement?)

http://www.unichem.si/vrt_in_trava/sadovnjak/specialna_in_varovalna_sredstva/izdelek?prid=88

Sorry it's in slovenian, google translation:

Benefits

liquid form for foliar activity or operation via the roots,

use of a preventive or curative during the growing plants,

fertilizer can be mixed with plant protection products.

Description

The fertilizer is used on vegetables, ornamentals, fruit trees and crops. Among the vegetables it is especially recommended to use magnesium for cabbage, lettuce, spinach, carrots, parsley, celery, beans, peas, cucumbers and tomatoes.

Application

Plantella liquid magnesium can be used for watering or spraying (foliar application). Watering 100 m2 surface, mix 100 ml of fertilizer with 30 l of water. For foliar application need 10 ml of fertilizer, which is mixed with 10 liters of water. Fertilizer use preventative or curative during the growing plants. Especially recommended for fruit trees, vines and sugar beets.

Yes, that will be good. You could add that to the NPK 30-10-10 when you spray the foliage and add to the soil as well. Bottom line, just use what you've got along with the magnesium and you should be fine. The FERTILON mix could be fine as an every 3-4 month spray at a very diluted rate: don"t want to see your tree get toxicity since Cu, Mo and Boron can be toxic if overused. Magnesium is ok at moderate rates since it is considered by some experts to be more of a primary rather than secondary element due to it's overall importance in the plant"s metabolism. Just remember, the fact that the tree is in a pot means too much fertilizer of any kind can be overdone very easily. Best to you and your lemon tree! Keep me posted.