well.. whatever it is.. its not a pest.. so dont contemplate such ... [at least i dont think so.. since its washed out, but could be]

it might be powdery mildew ...

did you take this pic at night with a flash ... because i cant really tell what it is.. since its washed out in light ...

i agree with no chems on babes... and do understand.. it will be fall soon ... and the leaves will fall off.. so who really cares whats going on between now and then ... leaf wise ...

lets also see a pic of the whole ... taken in evening or early morn ...

be nice to know where you are.. and what the ambient weather has been this year ... zone and city ...

ken

California.. 100 degree weather.. Yes at night.. Here's another picture

White area is bleached out in picture and not close enough. Need an extreme close up of object.

Could be some kind of scale? Insect eggs? Fungus? Bird droppings? Too hard to tell from picture. Have you smashed it with your finger to see if it is 'alive'?

does not resemble mildew. More like insect egg sacks/nests. Is it like cotton and can you scrape them off?

Definitely not powdery mildew. Looks almost like bird poop (hard to manage on the underside of a leaf!) but if not that, then some sort of insect egg mass, which I would remove if at all possible.

Mealybugs (a type of scale).

If it were mealy bugs, smashing it would yield some internal fluid i would think.

I don't think its mealybugs... Just from looking at pictures of mealybugs this is different

How so? The pictures you provided look remarkably like mealybugs to me.

Don't mealybugs leave more of a cottony residue? This stuff is hard and crusty

re: "internal fluid"

Maybe some types of mealybugs (the actual mealybugs, not the hairy/cottony/waxy residue they leave behind) would be large enough to squish and produce a liquid, but I haven't really noticed that before (and I have rubbed them off of plants many times).

Bruin27, yes, most of the time the residue is more cottony, but I have seen it a little crusty (not the entire area, though). There are many types of mealybugs, so maybe that could account for some differences in texture.

I wouldn't worry about it unless it became widespread. If it did (or has) become so, I would consider treating with a systemic soil drench. Some Bayer Advanced (there are various formulations) works well and is widely available.

Other than the fact that these things are white, I see no resemblance to mealybugs of any kind that I've ever seen, heard of, or read about.

I wonder if it might be old egg sacs from one of the cottony scale insects....with the scale dead and gone and the remainder of dead eggs in the residue?

Can we see examples of damage caused by whatever is "eating" your tree?

Rhizo, a number of Google Image mealybug residue photos look very similar to the photos above. The mealybug problems I've seen also looked like the photos, especially that last one. The only two things that give me doubt are that 1) the areas seems a little more isolated/concentrated in just one spot than typical with mealybugs and that 2) Bruin reported the areas to be hard and crusty. They may indeed be egg sacs, but they also do visually resemble what I've seen with the presence of mealybugs.

I'm thinking at this point maybe it's sap that has whitened and hardened. Probably a sucking insect has fed there and the holes in the leaves are oozing sap which whitens and hardens? Bird poop wouldn't be on the undersides.

When you remove it with spray, is there any indication of leaf damage beneath it? Holes of any kind?

No holes... Just residue from the white stuff.. If I scrub it off it's as if it was never there...

I disagree that bird poop could never be found on leaf undersides. Leaves flex and twist all day, everyday. The simply do not hold their leaves perfectly stationary all the time. And while I still don't know-from my remote location-what that material is, I wouldn't get too worked up about it. It's not mealy bug, it's not one of the cottony scales, it's not powdery or downy mildew.

+oM

I've never seen bird poop that looked anything like what I'm seeing in the pictures.

It really does look like bird poop in person... I don't think its bird poop though, don't think its mealybug either