Some years back, my Arabian mare Mia spooked hard going down a trail we had been down a couple hundred times. The only thing I could figure triggered it was a Palo Verde tree had blossomed, covering it in yellow flowers that hadn't been there before. Internet picture of a Palo Verde in blossom
Sure enough, two weeks later, she spooked again. The yellow blossoms had fallen off!
Aliens! What else could explain it?
Now...how does one desensitize a horse to trees that blossom? Particularly when there are trees just like it next to her corral, and had been for years! And a year later, she did it again. Same type of tree. Different location.
"what you really want is for him to look to YOU for a way out
The only way I know to do it is to ride them out, face some tough spots, hang on, try to listen to the horse BEFORE the explosion - and eventually teach them your ideas on what to do when something scary happens are ideas that work well.
A few years ago, I was riding Cowboy when some idiots started testing their pistols without bothering to use a backstop or even looking to see if there was a trail going away just behind their target. Bullets started whipping around us. The other guy said he could see the bullets cutting vegetation next to Cowboy's legs. With no practical way to turn around and get out of the field of fire, we decided to race TOWARD the guns. Trooper and Cowboy, both experienced horses, were afraid. They had no idea what to do, but they were willing to listen to OUR idea. Which worked.
There is no sane way to prepare a horse for that specific scenario. But if they have learned you have good ideas, then they may startle hard...but then they will listen. Because they WANT to know what you think, and they WANT to obey you!