Seedlings: week 5 - zucchini trouble

So last week, I documented some strange discolouration on a few of two zucchini plants' leaves.  It hasn't gotten better; in fact, more leaves now have this.  So I decided to cut the affected leaves off--but not before taking copious amounts of photos.

 

Below:  You can see an eerie discolouration all around the edges of the these leaves: yellow on the left picture and a sickly grey on the right picture. These may be from different causes, but it seems a bit too coincidental that the only plants who were potted in the new soil suffered...

  

Below:  The tip of this leaf is rotting away!

Below:  Some unsightly mouldy-looking grey-white spots...

Below: the poor plants after their trimming.

I've put the trimmings into a metal coffee can, and hope to burn them at my earliest convenience.  But I've never burned yard waste before...  I wonder if I can just drop a match into the coffee can once the leaves are dry enough...

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