I too can think of arguments against
@Parsnip 's scenario.
I can't imagine an illegal grow on slopes so steep, nor so near to popular (except for July/Aug heat) trails. I think there would be plenty of places more hidden, both from the public and from land management activities such as trail maintenance etc, where illegal operations could go undetected or at least more so.
Second, an illegal grow would not put a plane in the air to aerially spray anything, as the plane would be visible and trackable. They would spray by hand or atv, or apply "whatever-cides" using their irrigation system, IMO.
Lastly, while I'm a staunch environmentalist opposed to all sorts of toxic stuff that others use on the regular in their landscaping and lawn care, I'm not aware of any "whatever-cide" that would kill instantly like that. I think more of increased cancer rates etc.
Someone earlier in the thread talked about trip wires that spray poison -- I suppose that's possible and I know nothing about what poisons might be used, but again I would fall back on a well-used road and trail system not being a likely location for illicit activity, combined with the idea that the poison would have to have been undetectable in the autopsy (or on the ground around the victims, assuming LE tested nearby soil and plants for poisons, which I think we read they did, given the river algae toxin focus at the beginning of the case).
I'm trying to come up with a scenario other than heat that makes sense to me, but so far have drawn a blank.