Question
Hello,

I sprouted a lemon tree from seed approximately 18 months ago.  It's been growing well and has been under a grow light as I'm in Winnipeg and we don't get a lot of sun during the winter.  The older leaves have been gradually developing yellow spots that eventually turn brown and cover the entire leaf, which then drops off.  The leaves are generally quite large, very dark green with the newer growth being a lighter green.  It's not old enough to flower and hasn't been transplanted in at least a year, since I moved it from the pot I sprouted it in.  I had sprouted one other lemon tree at the time, which I gave to a friend, and it isn't having the same issues.  I did have spider mites on some hot pepper plants that were under the same grow light about 5 months ago, but got rid of those plants and haven't had any bugs since.  I really want it to stop losing leaves.  

Thank you,
-Raylene

Answer
Dear Raylene, You probably have a fungus on your lemon.  First be sure when watering to water when dry to about an inch or so into the soil, water thoroughly, and since it is an indoor plant, be sure to drain the water in the saucer underneath the plant.  You will also have to spray; I would suggest a product called Triple Action by Fertilome (I prefer the NEEM formula of that product) or a horticultural oil like SunSpray.  Be sure to first remove all leaves with yellow spots as those will not get better, and spray the plant thoroughly top and bottom of leaves.  You may have to spray several times about a week apart to completely remove the fungus.  Now in case you have residual spider mites, you can find this out by removing one of the leaves (one you would remove anyway because of spots), hold it over a white sheet of paper and tap sharply.  If you see anything fall onto the paper, watch it and if it moves around it is a spider mite.  This problem can also be controlled by Triple Action because it controls mites, insects and fungus.  Try these measures and I'm sure you will save your tree.  Good luck, Melissa