Hello I have two lemon trees, over the summer they got a lot of new leaves. As winter came to our region, the current temperature is 15 ° C during the day and -1 ° C at night. With one of the lemons, the new green leaves started to turn black, so I removed them so that today I could see how its few leaves had fallen off, literally a small contact with the leaf and it was falling off. I don't know what is the problem , the other lemon is quite ok. I fertilize them every 2 to 2.5 weeks with citrus fertilizer. The last two weeks there has been little sun in our region, can the cause of the falling leaves be weak sunlight? If I put them by the window they will be close to the radiator. The window is tilted during the day, we are at home around 5 pm. Is it possible that the apartment is too cold for them? I post a picture before, during the appearance of blackness on the leaves and the current situation Any help welcome.