Pecan tree black goo - sap or aphids? challenges...

[Allegre Nee]

I have a pecan tree in my yard that is approximately 100 years old. In the late summer and early fall, something black and sticky and shiny and utterly icky comes off of it. The stuff coats everything, all of the leaves on the ground are covered as is my car, the sidewalk, everything is "shellacked" with it. It pretty much ruined my garden - where it hit leaves and stalk of my tomato plants they looked really gross and almost like the stuff corroded the plants. Needless to say....I didn't want to eat anything from the garden after that!
I've looked online and have read that it's sap, but other sources indicate that it's aphid honeydew.
Does anyone know what this REALLY is? Also how do I deal with it?? If it's aphids I will spray the tree in early or mid summer, and then buy some ladybugs to see if that helps.
But if it's sap....well, I can't do much about it. I don't want it to ruin my plants every summer but how do I keep it from getting all over my garden? An idea I had was to make some kind of "roof" for the garden....something that lets in light and water (not that there's much in S TX in late summer!) but keeps this goo from destroying my plants. The garden is set up between a shed and a 7' wood privacy fence so it IS feasible that I could have some kind of cover extending from the shed roof to the fence (and as I'm only 5'tall, it doesn't need to be super-high). I suggested unbleached muslin, my husband liked the idea but suggested sunshade screening instead. My other idea, which was less well-received by my husband, was to make, umm, like....mini "chuppahs" over each square foot garden bed using narrow poles at each corner and fabric or something like that. My setup is 5 square foot garden boxes, the garden area is roughly 25' square. (I am sorry, I have a pic but don't know how to post it).

Ideas? Advice? Has this happened to any of you? Is my idea utterly ridiculous (because it sounded ridiculous to me once I started typing it out, LOL).

[WildcatNurseryman]

I have come across this on several occations and it could be aphids, but it is most likely Scale. If you look at the bark closely there are probably oval to roundish, slightly raised disks on the bark. Ants are probably tending their Scale/Aphid herd. The Scale is usually easier to see and more plentiful on smaller pencil-sized to thumb-sized branches. There are many different types as well. Nasty little creatures.
Don't expect to see them moving around either. They have a crawler stage but I have yet to see one physically move. They can be killed with insecticidal soap/oil but it has to be applied during their crawler stage.

[Allegre Nee]

WildcatNurseryman......would I have to treat the entire tree? I know that sounds like a dumb question but to get to a pencil or thumb sized branch I would have to get up on the roof of my two story house and still use a ladder. This tree is seriously huge.

[WildcatNurseryman]

The easiest way to treat it is with a soil drench like Bayer Systemic Insecticide or the one from Bonide. Both work for a year and can be simply poured onto the root-zone. Work GREAT too.

[Allegre Nee]

OK, that I can handle. Thanks!