Question
I received a dwarf lemon tree for Christmas 2 years ago. It had grown and filled out well. I have increased the pot size 2x. Now the leaves collect dirt from somewhere, and it is "stuck" to the leaves. I live in San Diego and my tree is on a very sunny 2nd story porch. I water when dry, but I am still going to make the drainage holes larger. I also put some little plant food pellets in the top-of-the-line soil. Will this problem go away on its own, or is there something I need to buy? The lemons are stunted as is any new growth. My brother is going to buy me a little lime tree this year. Is this contagious? Any tips? Thanks for your help.

Answer
Hello Gina,

When you say that the "dirt" is "stuck", do you mean that there is something sticky on the leaves?  If so, this could be the honeydew excreted by a number of insects such as aphids, mealybugs or scale insects.

Scale insects and mealybugs are closely related, yet they have quite different life cycles.  The scale insects have a mobile juvenile phase at which time they look like very small cotton bits.  Adult scale insects attach to the leaves and establish feeding sites. They appear as small (1/8" diameter) circular to oval disks.  Mealybugs, on the other hand, usually are found near the stems where the leaf petioles meet.  Mealybugs appear as small cotton-like white/grey bits, and they form aggregate colonies which are often quite large.  Both can be controlled by insecticidal soap, but the scale insects are tenacious.  If the plant is small and the infestation isn't too advanced, I have used insecticidal soap and an old toothbrush to dislodge the adult scale insects.  Alcohol swabs are also effective to control mealybugs.  

The "dirt" is most likely a fungus that feeds on the honeydew from the insects.  This produces "sooty mold" which appears as dark grey to black spots on the leaves.  The fungus does not feed directly on the plants, so if you wipe the soot off the leaf, the leaf below appears green.  When you control the insect infestation, the fungus will also disappear.

Your brother is very thoughtful to give you a lime tree, but ask him to wait until you get this infestation under control; both insects and fungal spores are easily transmitted from plant to plant.

Good luck!

John