What is going on with my Satsuma and what to do about it

Black sooty mold. It’s a fungus that grows on honeydew. Looks like scale insects are the culprit. The sooty mold should wipe off of the leaves. Neem oil should take care of the scale.

The other 2 responders nailed it, but keep an eye out for citrus rust mites, especially when your tree has fruit on it getting ready to ripen---which may be a while by the size of your tree.

They won't ruin the fruit, but do make the peel dry and brown in color and harder to peel.

Thanks guys, got some of that BioAdvance to knock it out

Scale. Spray neem oil and you probably have to spray multiple times

Fighting whiteflies with my viburnums by the 100s. Cannot believe how many of them there are and I hit them with neem oil frequently

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I treated the plants about 2 weeks ago with the Bioadvance. Any clue how long before I start seeing the insects gone? I still notice some of the scale insects and white flies on the leaves

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You used the Bayer imidacloprid drench that you pour at the base of the tree? If so it can be a few weeks as it takes time for the insecticide to be translocated throughout the plant and into the leaves. Scale exoskeletons may remain on the leaves even though the insects are dead. Proactively apply it next spring, April or so, and it should prevent, or at least minimize, the issue.

I have the same shite on one of my magnolias. It has stunted the growth over the last 10 years.