By Cbassett
United Kingdom 
We have a lemon tree which lives in the conservatory most of the year and is put outside one safe to do so. Last year it blossomed well and gave us 15 good sized and tasty lemons.
This year it has a lot of blossom but is losing leaves at a rate and some of them have a sticky, sugary substane on them. It is formeing new leaves and seems to be forming good fruits too. I feed it once a week with citrus 'stuff' and water it with tepid water (just like I did last year). I'm worried that it has some sort of pest.
We have a similar sized (6 foot)orange tree in the same area and this seems unaffected.
Your advice will be welcome.
Regards
C Bassett
10 Apr, 2014
Answers
The aphids that attack citrus are often black, and citrus are also susceptible to several different kinds of scale, which will also leave sticky deposits.
10 Apr, 2014
i think the plant has aphids(green fly) check if they are on the new leaves as well as they love fresh young leaves as they suck the sap in the leaves and the leaves feel sticky!! if you see small green fly then spray the plant with soapy water or introduce a predator eg:ladybirds.if you use the soapy water tecnique you may have to spray again over a few days as the adults will be effected but the eggs won't!! if you use the predetor tecnipue the predator wont kill the aphids completely but keep them under controll.. also the predator may fly off but you can always catch more.(Ps:the ladybirds wont eat the lemons so u got nothing to worry about :) )
happy to help!
10 Apr, 2014