Septoria, should I cut the leaf?

[boomerfan]

Hi all. Need some help. Posting with a problem here. I noticed a small brown spot on one of the oldest and biggest leafs on the planta few days ago. Did not bother, but today I saw that the spots are multiplying on the leaf and getting bigger (maybe because of the high humidity as it is raining since last night). Now I did research and saw it is most probbably the fungal disease Septoria. Today I will go and get a wide-spectre fungicide to spray and prevent spread, however what sould I do with the infected leaf as it is one of the biggest and I know it is not a good idea to dedoliate autoflower plants?
Today is day 24 since sprouting. EasyBid as it is my first grow.
The plant is in a 12l pot, sitting outside
the soil is : Canna Terra

 

[Bill.de.Cat]

Looks like a nute drop or water drop burn to me.....overall your plant looks good.

 

[boomerfan]

Thank you. I have sprayed it with a blue powdered copper phosphate based fungicide which I disolved in water, just in case to protect the leaves. The last time I have watered the plant was on the 15.05 with around 1l of water, because I saw a slight run-off at the botom and decided it is enough. Today I sprinkled equaly on the top of the soil 80g of Bat Guano diffuse classic fertilizer https://www.guano-diffusion.com/en/boutique/bat-guano-diff-poudre-en-sachet/ and watered slightly (around 200ml of water). This is the first feed I have ever done to this plant ever. Just before the flowering starts I plan to add 80g of Bat guano diffuse Bloom https://www.guano-diffusion.com/en/boutique/guanodiff-bloom/.

My question is should I water the plant tomorrow morning so the fertilizer powder soaks deeper into the soil or wait?
I really don't want to overwater. Still can't figure out the watering schedule/ammount completely.