Drippy nut disease

 Fresh goo drops on a tool handle.

"You ate aphid poop?!" exclaimed Richard.

"I only tasted it." said I, sounding somewhat Clintonian.

"Go ahead and try it", it's not bad." I urged.

"No way", chuckled Richard.

What prompted this interchange were drops of syrup-like goo, the fallout from the canyon live oaks that covers cars, driveways, patios, outdoor furniture and everything else at this time of year.

It looks just like honey when fresh, and a few days ago Fred was licking it up off the asphalt like there was no tomorrow.

That's when I decided to sample it -- hmmm, mum-mum-mum. . . . acceptably sweet with a hint of blackberry and a subtle oaky aftertaste.

The stuff is sticky like pitch, but you don't need to gargle with turpentine to remove it. Water does the job. (Just kidding.)

I googled up some facts about the Woolly Asian hackberry aphid, and decided the goo must be honeydoo -- which is actually the doodoo of aphids -- the nutritious seductive power drink consumed by ants.

That was my thinking when Richard and I were pouring cement and the sticky shovel handle prompted my gustatory disclosure.

Well, I was wrong. Aphids weren't making this stuff, and I wasn't guilty of coprophagy.

Unlike real honey and honeydew, this goo doesn't crystallize.

It darkens, evaporates and looks just like the mellowed doodoo from fruit bats (which is semi-digested fruit pulp, in case you have ever wondered).

The sooty mold converts the honeydew to black granular spots.  

After consulting with Mr Smiley of Bunyipco and more googling I learned that the cause of the goo and its agent of change from amber condiment to nasty smudge is the bacterium Erwinia quercina, also known as sooty mold.

The bacterium is the culprit.

Any number of hole-boring acorn parasites -- including acorn weevils and cynipid wasps may pave the way for sooty mold to infect the oak, and this causes dripping nut disease. (Makes you cringe a little, doesn't it?)

I decided to examine the trees more closely, and found that many leaves showed damage from leaf mining insects like moth larvae. Some had drops of the oak goo, but closer inspection showed that intact leaves had the goo too.

Leaf miner damage -- but not the cause of the goo.

The nuts told the story. Big ones were green and looked robust and healthy.

Healthy acorns of a canyon live oak (Quercus chrysolepis)

Little nuts were misshapen and discolored and had a small hole with a residue of goo -- apparently victims of dripping nut disease.

The shriveled victims of dripping nut disease.

I found no drops from the holes in a few woody wasp galls, so conclude that the acorn weevil is Erwinia's main accomplice.

Feel free, dear entomologists and arborists to kindly set me straight on any deficiencies or errors in this post.



References

An article on Erwinia (Brennaria) quercina: http://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO.2003.93.4.485

Erwinia factoids (there are many species): http://www.tgw1916.net/Enterobacteria/Erwinia.html

On interaction of boring insects and Erwinia: http://www.forestencyclopedia.net/p/p2208