Photographs of plants that are perhaps suffering from some disease that is affecting them and preventing a large harvest

For example, to start, we have this avocado plant, which is producing fruit for the third time. The first time, we had an incredible amount, but the second time, only a few survived, maybe 10 or less, but most of them fell off when they were small. Something very similar is happening right now, where the plant doesn't seem to have the strength to support the fruit, and they're falling off. I picked these up so you could see them better, and then I took another picture of the ones on the ground.

Well, even so, looking towards the top of the plant, I noticed that there are still quite a few fruits up there, so we have a chance of getting a good harvest. At the same time, anyone who can give me a little advice on how to prevent them from falling off or how to give the plant a bit more vitality would be welcome, since we're here to learn and make our plants much more productive thanks to the dedication my dad has given them in recent years. One detail I want to share is how almost all the leaves of the plant are dry up to about halfway down and then normal, which makes me think it might be some kind of disease affecting it.

There's a lemon tree very close by, which we've also harvested quite a lot from in recent years, but lately the tree has been drying out to the point of losing its leaves and hardly producing any lemons anymore. Something similar is happening; something white is growing on the main trunk of the plant and is covering it almost completely, which makes me think it's a disease, but I don't know if any of these plants can be cured so we can see them green again and full of fruit for our food.