The fact that the dust is lighting on top of the hood panel or even when piles of it are exposed on the ground leads me to believe it may have more to do with it's reactivity to oxygen, than to static discharge.

I never put much faith in self starting fires until last year. Our water supply here is heavily laced with naturally occurring dissolved iron & last year my house well pump quit turning because the iron had built up so much on the inside that the rotor could no longer spin. Pump quit in the evening, but because I use kind of a cistern arrangement for the house supply, I left it until the next afternoon to investigate. I unhooked the pump from the power and well and set it on a workbench to disassemble. I found the housing was heavily built up with a black, hard, iron deposit, so I took it outside on the grass and proceeded to scrape the deposit off with a screwdriver. It came off in the form of a fine black powder and flakes. At the time, the neighbor was cutting field peas, and as I sat there, I could smell smoke, like burning grass makes. I continued to scrape, looking over at the neighbors field and searching for signs of a fire. I couldn't see anything, but the smell got stronger and stronger. Finally, the smoke from the fire begin to sting my eyes and I looked down to see many small fires burning right where the black powder landed on the green grass in the yard. The pump housing was completely cool to the touch and the iron deposit was wet when I was scraping it, but when exposed to the air, it ignited itself and dried the lawn grass enough to burn that too. A classic example of; not believing one's own eyes, as everyone knows you can't start a fire with wet powder????

Dad used to talk about cutting smutty wheat and the problem with fires caused by dust heated enough by the exhaust pipe, to alight. I'm guessing that the top of the engine hood gets hot enough to light your mildew dust also. It is well to remember that different substances will have a different combustion temp. Here, we often find that the cause of a fire in an old building is steam pipes passing through wood joists. With the passage of time, the wood becomes dry enough and the composition changes enough, to where the temperature of a steam pipe touching it, will ignite it and burn the building down.