There is not enough information is your query to answer it. I would need to know what garden zone you are in, and what it is where the tree sets, is it in a pot or the ground, how old is it, what is the care routine you have been giving it, what to you feed it and how often, etc, etc, etc?

Lemon Trees are subjected to a host of pests. Check for bug damage, the list is big: Phytophthora foot rot, gummosis, sooty mold (associated with scale insects), scab, greasy spot, mushroom root rot, heart rot, canker, and anthracnose can effect many Citrus species. Pests (which I would look for first, the above list is more for orchards) scale, varity of nematodes, plant bugs, weevils, aphids, mealybugs, spider mites and whiteflies.

Having said all that, if the Lemon tree is young, small, and in a pot it probably needs some good fertilizer. Start with a balanced liquid ferilizer every 2-3 weeks and see if that doesn't help the yellowing, but do keep eye out for white fly, aphids, and spider mites, they love weakened plants. And protect the tree from sever cold - maybe wrap in blanket at night or plastic, just be sure to remove durring day so it can enjoy the sun and not sufficate.

Hope the fertilizer helps, also in winter months cut back on the watering a little, water constant during spring and summer.

When life gives you lemons, make lemonade...

Ok, but when life doesn't give you lemons.. make pesticide?!?

Say Violet does see some of those bugs, if you dont want to use pesticide on a lemon tree that you might potentially eat lemons from, is there anything else to do to get rid of those pests besides asking politely if they could please find another tree to destroy?

I know that gardeners have been at war with bugs for tens of thousands of years, but Im new to this.

Yes, there are plenty of choices for pest control that won't harm you or the plant or fruit. If the lemon tree is outdoor, use a sprayer that attaches to the garden hose, filled with a heavy solution of liquid dish soap and spray the hell out of the tree (best done on a sunny calm day with grubby clothes on). Sometimes just a high pressure spray of water will remove pests. If you are concerned about powdery mildew, add baking soda to the sprayer solution. There are products available that are natural pest controls, like 'oil' sprays, copper sulfate, etc.
In the end, the very best thing you can do for that tree if it is dying, if it is potted, take it out of the pot, look at the roots for any tell-tale signs of disease or pests, repot into all fresh soil made up of at least 50% worm castings, with the other 50% being made up of compost, soil lighteners, like coir, vermiculite, peat, etc, and maybe a little bone meal. If you do this, it will have a good chance of returning to you full of vigor and prevent further weakness that makes it suseptable to infestation or disease. Worm castings have been proven to remove toxins and disease from the soil it is added to. If you have a more specific problem that you can discover and need help with, let us know.

Have it treated. Some pest may have lurked inside of it. Save it before it's too late.
_____________
phoenix pest control

put in a big pot and add lot of soil. I do not guarantee that it will keep up though since the lemon tree should be on the ground and not on the pots inside the house.

-------------------------------
Pest Control Fairfax