Citrus trees--black crud on leaves?

I recently noticed a great deal of black crud on the leaves of my citrus (and avocado) trees. These are small trees, barely giving any fruit yet.

I use neem oil to keep down the pests, which works fairly well, but I’m wondering if it’s just building up on the leaves, and dirt is sticking to it.

There seems to be a sort of gradient of crud, with the lime tree having the worst buildup, then the avocado, then the orange, then the lemon. This is strange to me, if it is the neem oil, because I use it liberally on all the trees.

I don’t think the crud is fungus, because if I wash it off the leaves are green and healthy underneath–and I need to use a soap solution to get it off, water alone won’t do it.

So:

Is it the neem + dust causing this?

If so, is it bad for the plants? Or should I live with dingy leaves?

Should I cut back or replace the neem for pest control?

Thank you.

PS, I also use neem on my bougainvillea to stop whatever chomps on that, and have not noticed any crud on those leaves.

Did you look closely to make sure there are no aphids on the trees?

“Aphids suck the juices from leaves and often deposit a sticky substance called “honey-dew” on the leaf surface. Later, the surface of the leaves may turn black in response to a fungus that grows on the honey-dew. This condition, known as “sooty-mold”, may prevent light from reaching the leaf surface and thus reduce photosynthesis.”

Also check for:
Red Spiders
Quick Decline

With small trees, periodically rinsing the leaves (using a hose spray attachment to hit upper and lower leaf surfaces) is a good way of removing pests as well as cleaning off Neem residues and whatever other crap may be sticking to them.

I’d try using insecticidal soap for a while and see if that does better than the neem is doing.

(don’t worry, insecticidal soap is just regular soap with a fancy name)

I would bet it’s Sooty mold, caused by whiteflies.

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7401.html

I use Bronners Peppermint oil soap as a spray, but they are tough to get rid of. You have to spray the undersides of the leaves, where these rat bastards live.

If you shake or spray and see a cloud of tiny white flies, you know it is them. The black is not the problem, it’s what is causing it. Whiteflies, and I have been battling them all summer.

This stuff is awesome, but it’s also kind of the nuclear option, complete with fallout.

Thanks all.

It isn’t whitefly, I’m well aware of what that looks like. And I don’t see symptoms that look like red spiders (I assume you mean spider mites?)

It could be aphids I suppose. But wouldn’t the neem control either the aphids or the fungus or both? Maybe not. The more I think about it the more I’m convinced it was a mold, so maybe it is aphids I’m not seeing.

I’ve been trying to control pests in a way that’s as least deadly as possible…but now I’m worried that my orange tree has Quick Decline. Oy…

There might be a county agriculture center you could ask.