What is wrong with this lemon tree?

I need some help please! Does anyone know what's wrong with this lemon tree?

Too much sunburn, not enough water, lack of fertilizer,/ feed, insect attach, sap sucking aphids, I could go on but without more info then all these problems are just guess work.

How long have you had the plant, is it in the ground or in a pot, how often do you water it, do you test the soil with a finger stuck way down into the soil to feel if the soil is wet AFTER you have watered,
How much strong sun does the plant get each day.
All that info will help you get a quicker answer or help once we know more about the environment and care the plant gets and how old is the plant.
Best regards, WeeNel.

Thank you WeeNel. I cannot answer all your questions because the tree belongs to a family member. What I do know is she lives in TN and keeps it in a green house in the winter and brings it outside in the summer months. It's in a pot. She said that she waters regularly, just sprayed insecticide and fertilized it. What she sprayed it with is unknown at this moment. My quess was like yours. I thank you WeeNel for replying.

LOL, you sound just like myself, always worrying about other's plants and trying to fix their problems,
I have had a close look at the picture you sent first time and not too sure, but right in middle I thought I noticed webs, as you said the plant is in a greenhouse then could be an infestation of RED SPIDER MITE'S, these tiny little mites (NOT really red unless female is breading stage) and to be honest, you need a magnifying glass to search the underside of the leaves to be able to spot these tiny pinhead size mites, once you focus the glass onto the leaf, watch for a few min's and you will see the tiniest litte dot's moving about, IF you see all I'm describing, the problem with the lemon tree is the dreaded red spider mites and they are not easy to get rid off, the problem being, IF the relative put's the plant back into the greenhouse with other plants and they do have these mites, the mites will rapidly spread to every area of the greenhouse so my advice is, get rid before it goes near the greenhouse.
untill they get something specially for RED SPIDER mites, then mist the plant every morning and night, do top and underside of all leaves and the wood stems, pot too.
Sorry to upset you but there is also the chance it might NOT be the spider mites, there fore the search still goes on if I'm proved wrong LOL,
Hope this will give more clue's as to searching for a solution to the problem.

Best Regards. WeeNel.

Haha....I love all plants!
Thank you for all your help. I will forward this info along, maybe it will help. I hope it helps, I hate to know a plant is declining and there's nothing you can do.