strange plant problem.

Hey all im new around here, use to use riu alot.

Anyway....grown oudoors for few years but....(sigh) had so much drama this year, from dodgy seeds, to wind snapping sprouts etc lol.

anyway got few plants going (from australia so its right time to start em)

well up until yesterday all was going good, 2 of my plants, both similar type size only youngens. well on tip of 1 leaf it looks black, well closest color would be black, then on another leaf its more gold/brownish ( with a few brownish circle spots) , its only on a small zone of it but im worried when i wake up tomorrow or next few days i will find a dead plant.. which would be disapointing these seeds cost a fair bit.

ok so,

medium : good compost soil
Water : destilled
Lights : good old sun..
Nuets : Have not used any seeing as they only like 3weeks old.

so the day i noticed this it was really sunny 23c, but that shouldnt be the issue in past i grow em in like 34c aussie heat and they fine. tried to skim through other posts cant really find it. / Could it maybe be some type of bug? i had my pots in the grass, but once again in all the years ive grown it has never happened.

(here a pics sorry they not really clear just a dodgy phone, but you can basicly see the problem just not in HD)


(edit) The two bottom fan leaves, are lighter in color, the top part of plant looks really nice and lush apart from brown spot on 1 leaf)
Thanks in advance guys/girls