This shows typical leaf damage done by biting insects such as greenfly and
blackfly. They particularly attack the growing tips and the damage becomes more
exaggerated as the leaves grow. They suck the sap causing the plant to become
weaker and can also introduce viruses which they can
transfer from plant to plant.
I rarely try to prevent this type of damage as I have too many plants. Vigorous hosing will knock the insects off but there are just too many to bother with. As the plants get bigger and the season gets later the problems reduce.