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I have been growing in my flat an Avocado plant sprouted from a seed in December. It developed well, up until about a month ago, but then brown dots started forming on the leaves, the oldest leaves first and spreading to the newest. The dots start small on the edges and very quickly spread over the leaf, until the leaf dries out and the plant discards it.

Below I put an example. The first picture was taken last Sunday, when the leaf started showing the spots. The second picture was taken today, three days later, showing the progression of the spots. Usually it takes about two weeks for the leaf to be dropped by the plant.

At first I blamed this on dry air caused by the central heating (In the picture you see a radiator, but that's just where I put the plant now that the radiators are off. During winter it was in a different position.). The plant now is next to a window open most of the day, so the air shouldn't be as dry, but still the spots returned.

A second hypothesis I had was high salinity of the water. Tap water here is very hard and reading other articles it seems that there could be a stagnation of salt. To counter this I replaced most of the earth in the vase, but this still didn't stop the leaves from browning.

The plant itself is now down to three leaves, one of which is the one in the pictures above, and the top gem is not showing signs of development, so I'm running out of ideas to save the plant.

Any hints on what might be causing this and how to prevent it from extending to the rest of the plant?

Brown spots starting to form at the base of the leaf Spots extending to the leaf's border

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This leaf is telling me that this guy isn't getting enough nitrogen and that you are using tap water. Have you fertilized? Are you using tap water (very high in salts that cause browning of the tips and margins, very common)? Get a little bottle of OSMOCOTE 14-14-14 it is extended release and the only fertilizer I recommend for newbies and for simplicity. Go by the directions to the letter. It looks like he is ready for a bit larger pot. Use only potting soil and get another clay pot that is 6" versus the 4" I am looking at (is that correct)? No rock or gravel below the soil, get the bottom of your pot off the surface and do not allow the bottom of the pot to sit in the excess water such as in a saucer. Put thin tiles or chunks or flat stones beneath to put a layer of air beneath the pot and the surface that will enhance drainage. Definitely needs some fertilizer first and foremost. Not a lot. Osmocote will probably only be needed twice a year...like a tsp at first a tablespoon in 8 months in the newer larger pot. Allow the soil to dry out a bit before watering. I'd get bottled water and not use the tap water (same for you btw, don't drink the tap water, where do you live)?

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    I love getting to know a little bit about other countries and talking with people all over the world. I think that if you could find a simple NPK of even numbers that is EXTENDED release perhaps some micronutrients you'll be fine. As for the tap water a simple Britta filter that you could use for your own drinking water will be just fine. When this plant gets back in balance the salts (not calcium) in your filtered water won't be a problem. Later, if you see tips and margins browning I'd repot in fresh soil. Is this soil potting soil? If it had added 'fertilizer' then you did well to not – stormy Apr 7 '17 at 16:14
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    ...your plant loses leaves because the plant is dumping the leaves that are doing less work making food than what that leaf requires for maintenance. The leaves that are absiscing are the lower leaves, yes? When low on chemicals that plant needs to photosynthesize what little Nitrogen is available is sent to the top or new leaves. The spots have nothing to do with the loss of leaves. This guy needs some fertilizer, extended release. You aren't able to get OSMOCOTE 14-14-14 the little yellow beads over the internet? I love it because it only needs one or two applications per year and... – stormy Apr 7 '17 at 16:24
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    ...it does the thinking for you delivering just bits of nutrient/chemicals to the soil, nice and slow. I am glad you didn't add any fertilizer at first if this is potting soil that said, 'with added fertilizer or nutrients or food or enriched'. Many plants have died because people fertilized this kind of soil and killed their plant. Over fertilization will also cause the browning of tips and margins...not your problem here. He should green up quite quickly, follow the directions to the letter. Erring by adding too little is far better than over fertilizing which could be a death sentence – stormy Apr 7 '17 at 16:31
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    ...and I am very detailed and verbose, but I would also repot this plant gently into fresh soil after you've soaked and scrubbed this pot to get the salts out of the clay. I love clay because you can easily see when there is too much salt and it breathes, allowing air and moisture to move through the clay. Leaving behind salt rime, white crusty stuff on the outside of the clay. In plastic, you'll see the white rime on top of the soil but far later with more salt concentration. Growing a plant like this from seed is just like having another kid or a pet, you get attached! – stormy Apr 7 '17 at 16:41
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    Oh my! I did not see that but am quite thrilled and proud of you for not allowing your plant to sit in the water!! You DO know a thing or two about plants!! Oh I hope we were able to add enough to help save your baby, I have to once again say how amazed I am that you knew so much! Rare. Let me tell you! Stay in touch, please. If you think of it a picture later would be so very nice to see... – stormy Apr 7 '17 at 20:43

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