Looks like it's been munched on by a type of insect, most probably aphids or some larva of some sort.
I would try to tackle this by spraying a neem oil solution.
Good luck!
Neem oil wont kill the larva
Not instantly no but I've had very successful results when using neem oil to drench the top 1 - 2 inches of soil against larva.
Another method i've used is to use Hydrogen Peroxide mixed with water.
