Many thanks to everyone for your precious advices and points of view.
I tend to believe it's leaf streak and not simply sun scorching. It started to happen to a couple of seedling I planted in the ground past week, They were in pots and no signs of disease but since I moved them in the garden the small leaves are starting to show the same signs. The pots got even more sun because I always moved to be always in the sun, while plants in the ground can't move, and get less hours of sun.
The fungicide I have doesn't have the ingredients mentioned by Daniel. Here the weather is really dry except for a once a week watering by hand, and yes I watered overhead to clean some plants from spider mites. These can be the pest that opened the way to leaf streak.
