The fungus among us live and breed in the soil. It's spores go airborne to expand its range. The rain hitting dry ground sends them air born as do birds and bugs etc. WSU told me only steaming the soil in the spring up to two feet deep and an area 2x the height of the tomato plant in circumference at maturity is the only guaranteed way to kill the blight. You can replace the soil with sterilized soil or treat with fungicides also. If you have never had blight I would think being less aggressive in treatment makes some sense. I have it so I steam the soil with a big ok Bon fire every spring .