Ellen,

Thank you for your inquiry about your lemon tree. In order for me to respond, please fill in a number of details, as requested below. (Frankly, this will be rather like a game of 20 Questions!)

The easiest way to respond is to copy and paste my questions into your response, then insert the answers where needed.

Please send several more images, namely these:
1. The tree and its surroundings.
2. The tree, alone, including where it enters the planting media.

In the images you included, the leaves appear to have small droplets on them.
- Did you wash the leaves before taking the picture?
- If you didn't wash them, do they feel sticky or tacky?

Also, tell me a bit about the lemon tree's care and growing conditions, including the following:
a. It sounds like the tree is in a pot. Is that correct?

b. How was the tree planted? -- What did you use for the growing mix?

c. At planting time, what fertilizer, or other additives such as compost, sand, etc.; drainage material, were added?

d. Explain how the tree is watered:
-- How often?
-- The method: hose; drip system; or?
-- How do you decide to water again? (e.g., It's Saturday; the leaves wilt; the leaves look grayish; or?)

e. Fertilizer:
-- What kind (if any) do you use? (Brand name, please.)
-- How often do you add fertilizer?

f. What treatment(s) or remedies have you tried? And what was the result?

Thank you for your quick response! see my answers below in red

Ellen,

Thank you for the additional images and information.

First, let's discuss the scale you found earlier.

To be truthful, one treatment seldom gets rid of all scale insects on a plant. So, be alert and closely check for their return. The reason: A treatment of whatever kind typically gets rid of the easily visible large adults, but often misses the recent hatchlings because the latter hide in the smallest of cracks and crevices. Then, one day, you suddenly realize, "Uh, oh; they're back!" 

Keep the tree outdoors as much as possible, as doing so will help limit a return of the scales. The reason: Naturally occurring beneficial insects will be able to manage the scale population for you.
In fact, the tree can remain outdoors year-round. Only take it into a shed or garage if temperatures will drop to freezing or below.

Here's another task for you: please double-check the undersides of the affected leaves. If the brownish areas and somewhat raised, the underlying problem is probably an overly wet rootball. 

The reasons for excess rootball moisture for this tree are several:
A. The so-called drainage layer of chips. Even though it seems counterintuitive, a layer of coarse material below the potting mix does not improve. Instead, the effect is to inhibit drainage.
See "Adding gravel to your planting container does not improve drainage" - click here.

B. The pot is sitting in water. The evidence is the white ring at the bottom of the pot. It's true that water should drain out of the pot each time the plant is watered. But, that excess water should be removed after 20 minutes.

If not, two things occur.
(1) The remaining water means that the potting mix will be saturated; thus, the roots will slowly lose function because they lack sufficient air exchange.
(2) The soluble salts (dissolved from the fertilizer) in the drainage water will gradually increase in the potting mix and will, eventually, damage the roots.

C. The tree needs to be moved into a larger container. (With a vigorous plant, repeat every 12 to 18 months.) To do so, work gently so that the rootball remains intact. (Citrus have a tough time recovering from a ruptured rootball; follow-up care will be similar to tending a patient in Intensive Care. Tedious attention, often with a poor outcome.)

To remove the plant from the pot, tip the pot on its side, then gradually wiggle the rootball out of the pot. It may be difficult to do so without tugging hard because the root tips are most likely firmly embedded in the terracotta. Healthy root tips will be cream-colored; trim off any that are brown or black with a sharp tool.

Note: If you need an opinion about the condition of the root system, send me an image or two when it's out of the pot.

Before you re-pot, scratch the potting mix away from the stem because to determine if the tree is planted too deep. (Container-grown citrus typically have one stem, then branch just above the potting mix.) Whenever the trunk is buried, the bark becomes excessively wet and dies, in effect, girdling the trunk. A very bad thing.

In addition to the above, realize that a container-grown plant must be fertilized throughout its lifetime. It's fine to continue adding the blue fertilizer, as directed, until they are gone. (It's a slow release formulation that gradually "feeds" the plant.) After your supply is gone, consider obtaining another slow-release fertilizer, perhaps a "citrus food" or a product called Osmocote. Both are available at local garden centers; use according to label directions.

But forget about the liquid formulation; it's essentially water, and has little value for your tree.

If you have more questions, please ask.