Question
Hi John,
I have a lemon tree which has got brown dots similar to an other question I saw you answering here, however the dots appear on both the upside and downside of the leaves as well as the branches. Just like for the other person the leaves turn yellow on the top where the dots are on the underside, but the yellow spreads along the 'spine' of the leaf as well. I can not see any insects or similar either.
I can feel the dots and scrape them off with my nails and underneath they leave a yellow/light green mark both on the branches and leaves.
They collect especially at the top part of the branches, but are also spread all over the stem and branches. Branches do dry and die constantly as the leaves falls off. The leaves are very sensitive, so if I only lift it on my finger to see the underside they 'plop' off. The tree is 2.5 years and has not grown much, and it now looks like a stem with three sticks with leaves on growing out. New leaves grow, as well as flowers, however the flowers die if I do not scrape of the dots close to their attachment constantly. The leaves then get sensitive and fall etc, the circus goes on. the fruit I have got only grows to the size of a thumb nail or two and then falls off. The peel goes very lumpy too.
I have used a fungus spray which I was recommended, but it does not seem to work.
After two weeks away the branches and larger leaves are all affected almost covered in a thick layer of dots, however new leaves are also sprouting together with flowers. The same happened last year and I think 'he' is very keen to survive, however I wish it could work without my scraping every second day. And the scraping really does not seem to work in the long run anyway since the fruit and leaves fall off sooner or later.
I live in Sydney, Australia, and I have re-potted him twice with soil from my worm-farm.

I would be so very appreciative if you could advice what may be the problem and/or if I can give him something that will make the dots go away.

Thank you very much in advance!

Louise

Answer
Hi Louise,

Thanks for all the information regarding your lemon tree.  What you describe are classic symptoms of scale infestation. As you can remove the small "dots", that also suggests a scale infestation.   However, there are two things usually apparent in scale insect infestations which you have not noticed: 1) one will see juvenile insects that appear as small white "crawlers", the size of a white fly, or tiny speck, and 2) one will usually feel a sticky, sugary residue (honeydew) that is secreted from the scale insects.  As the scales suck the sap from the plant, the leaf will exhibit chlorosis (yellowing), and eventually the whole leaf will drop.  

Juveniles will often be seen at the node where the leaf petiole attaches to the stem.  So, examine your tree very, very closely for juvenile insects.  If you see any small, white "flecks", you probably have a scale insect infestation.

There are many treatments for scale infestation, but one of the most effective is to physically remove the scales (as you are already doing), and apply an insecticidal soap solution over all of the plant surfaces.  Also, there are many horticultural oils that can be used to smother the adult (attached) insects.  The oil has no effect on the juveniles, but isopropyl (rubbing) alcohol can be used to kill the juveniles.

I am unsure of your climate in Sydney, but in my climate, when lemons are grown indoors, they are prone to scale infestation because there is an absence of natural predators indoors.  When I place my plants outdoors for the warm months, the infestation usually goes away as the predator populations increase.  If your climate is agreeable to lemon cultivation outdoors (i.e, not below 0 C), I would place the plants outdoors after applying the oils and soaps.  Follow the instructions carefully when applying horticultural oils as they can "burn" the plants when applied outside a fairly narrow temperature range.  So, timing is important.  Your local garden centre can give you advice about which to use.

I found the following website that describes Black scale insect that is common in Queensland and NSW.  

Check it out, and see if your "dots" look like those in the photos.

http://www.sgaonline.org.au/info_blackscale.html

Good Luck.