Question
I have seen several questions regarding white spots on Meyer lemon leaves...but not an answer to get me going.  My tree is in a large pot and moved into the house for the winter. I just removed a very large crop of lemons this week. Now I see that nearly all of the leaves have  white  specs on them (somewhat powdery-topside of leaves) ...also at some of the joints there is some white frothy foamy powdery (best description) substance.  Please help with treatment idea, also..it sits next to a jasmine tree in my patio window, is there any risk of spread to it?  thanks!  holly

Answer
Hi Holly,

It is really hard to help without more information. Did the specks appear suddenly?  Do they appear to grow from a central point?  (Do they form concentric circles?) Is there any leaf discoloration around the specks?  Have you noticed any insects (scale insects, mealy bugs, etc.)? The "foamy" description sounds very much like insect infestation. There are many insects whose juvenile stages appear to be white flecks. Sometimes they will aggregate at the crotches of the plants.

If you could provide a close-up photo, that might help.  You can send it to [email protected].

Fungal infestations sometimes begin as small lesions, but as they grow, the form nearly perfect circles (sometimes with multiple colors in rings).  One of the more common fungal diseases is powdery mildew which can produce some of the same signs that you describe.
Powdery mildew likes humid conditions and cool nights.  Powdery mildew will appear as a light, white dusting, sometimes in blotches, and found on leaf surfaces, sometimes both.  In serious infestations, the leaves may become deformed, and the fungus will begin to produce dark brown/grey spores.