They most certainly are scales. You can search here and find many ways to deal with them. Fortunately they usually don't affect the fruit and your lemons are quite safe to eat. Deal with your scale and you will be rewarded with more lemons next year.

Spray with neem oil.

It's amazing how many different types of scale there are. Truly disgusting. I routinely beat back 2 different types myself.

There are quite a few recipes for spraying with plain ole vegetable oil if you want to go a cheaper route. I used canola oil and a biodegradable dish detergent on my in-ground trees last fall and it wiped out my scale. I still have lots of scale "bones" stuck to the under sides of the leaves but the scale is dead and no longer sucking juices from my plants. Nothing against neem oil by the way...I know it's a great product but I had lots of trees to do and I had read that a mixture of water, vegetable oil, and a bio-degradable detergent would work so I tried it.

Yes, you can use vegetable oil but the neem and horticultural oil is finer and will not clog up the plants pores as easily. You have to be careful about that if you use vegetable oil you buy in the grocery. Also, use a soap and not detergent as detergent is too strong and can injure the leaves. Evenustum soap can strip the plant leaves protective coating if used too frequently. I have used vegetable oil and mild dish soap or shampoo in a pinch though. Be careful if you do.

Cory

Thanks Cory, I know just enough to be dangerous :>)

I did notice the leaves had a sheen or oily look after treatments. I forget the brand dish soap I used...still have it and can look it up. Seems like it was one that was recommended by some.