I've seen something similar happen with leaves, where the green tissue quickly turns brown after accidentally dripping fert mix on them. I'm guessing that's what is going on there. Spraying leaves like this with a water/fert mix is a bad idea. The purpose to spraying water/fert mix is to aerate the mix, putting more oxygen in the mix. I thought you were going to drip, not spray. This plant doesn't respond well to fert mix being put on the leaves directly. I don't know why, but time after time I've seen foliar feeding go bad. So I'd say, use the pebble to build a wall around the plants or somehow block the spray from the plant directly, or stop the spray. If you stop the spray, you'll need to get the fert mix to the plants by pouring it over the pebbles or bury the sprayer below the pebbles. About all I can suggest. let us know what you do and if it works.