Just a guess but I would say that the fertilizer is allowing the plants to grow leaves faster than the blight can infect them. I don't think that the plant is resistant, it just has more volume of leaves.
Excess fertilizer runoff (mostly from farms) into streams is a big problem because it causes the algae to grow out of control which in turn blocks the light and lessens the amount of oxygen available to fish and other aquatic life.
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