Pests

Please accept our apologies for the delay in responding to your request! The leaf damage on your plant could one of the following: On some leaves it looks very much like a fungs called Sooty Mold, but its not actually a plant disease. And on other leaves it looks like it could be damage cause by spider mites. We’ve included links and info about both. Sooty Mold is a secondary issue caused by the excretion of sucking insects that are feeding on the leaves above those that have the black mold on them. These insect excretions (which contain sugars) are a perfect place for the sooty mold to grow, but it does not infect the plant. You can wash off the mold with a high pressure spray of water (and maybe a little rubbing of the affected leaves). But, in order to get rid of the mold permanently you’ll need to control the sucking insects on the undersides of the leaves. If it is an insect, you can send us a picture of what you see on the underside of the leaves and we can try again to help. If its spider mites causing the damage, you will want to take action as the damage appears to be heavy. Please refer to these references for more info on both possiblities.
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74108.html
http://msucares.com/pubs/infosheets/is1938.pdf
http://www.ipm.ucdavis.edu/PMG/invertebrates/links.mites.html