I am dealing with that right now, too. I have been reading a shitton of stuff trying to get rid of it. "Leaf Spot" is a broad category of fungal issue, but it is definitely not yellowing cuz of nute lockout, etc. I have a ton of different things that I am using to try to fight it back because now it's in my indoor as well. (Mine starts to attack during veg in some strains, but not until flower in others.) I haven't tried the copper yet, thanks for that tip.
They say some strains are more resistant to it, but it is not a "strain issue". It is way more complicated than that. In order for a condition to develop, there has to be a set of environmental conditions favorable to that condition, something to get it started, and in the case of leaf spot, a host plant that is unable to defend itself from the fungus.
Any strain will be unable to defend itself from the fungus if it is sufficiently stressed. Some strains are very easily stressed, some are not. Some strains are more able to defend themselves from a given pathogen than others, generally speaking.
If the environmental conditions favor the pathogen, and the pathogen is introduced (in the case of an outdoor grow, lots of pathogens are always there in some quantity). If the plant is not one that is good at fighting off a particular pathogen, and the rest of the environmental conditions favor the pathogen, then you get an outbreak. Once you have an outbreak, it is tough to get rid of it.
Some of the environmental factors for leaf spot are high humidity, pollution, and low air flow.