Identification: Giganteus Sunflower plants leaf damage

Hi All,

I was hoping somebody could tell me what's wrong with my Sunflower leaves. I've attached pics.

My initial thought was that the plants were suffering from a Potassium deficiency so I went out & bought some Pro-Mix soluble Organic fertilizer that was higher in Potassium but I'm not certain this is the issue.

Or is the leaf damage from over watering or disease?

Any help would be greatly appreciated.

Cheers