What is on my lemon tree?

My lemon tree has mealy bugs on it but it also has weird brown scabs on it. They come off easily and seem hollow. Any idea what they are. I ended up wiping and spraying it with water to remove it all but it has come back in a couple of days. I also thinned it to improve ventilation. It is in a large pot in my porch and seems to grow well with lots of new leaves and flowers well but the fruit forms fairly small and then drops off. It is a few years old I think as I rescued it from college as they were going to shred it. I've had it at home for a couple years and haven't had any fruit ripen yet. Any advice on caring for it would also be appreciated eg what feed etc.
Many thanks Joe

Sorry the picture isn't very clear

 

Scale insects

Needs a systemic pesticide

 

I agree scale insects are your problem. Lemons should be put outside in the summer months, not on a porch which I presume from your description is glassed in. I have two lemon plants which spend the winter inside against a SE facing window. I place them outside as early as I can which is usually early May after taking hardening off precautions. It's late October now and they are still outside. I will bring them in as soon as frost is forecast.

Lemons are the hardiest of the citrus fruits and can stand a little frost, not too much. They require high light levels even in the winter. They need lots of feeding during the growing season,: I use a seaweed liquid feed. The worst thing you can do is to overwater them, both in the summer and the winter.

 

Thanks