It's possible that the wet weather provided an ideal habitat for a bacterial disease to attack your shasta daisies (Leucanthemum x superbum). They require well-drained soil, and if the soil was constantly soggy, their roots may have rotted in the ground. If that's what happened, all you can do is amend the soil with a lot of organic matter (compost, shredded leaves, etc.) to improve drainage before you replant. The black stems and skimpy root system you describe doesn't sound like the kind of damage slugs leave behind, and if it were grubs, you'd not only see them in the ground, but you'd also notice other dying plants in your garden and turf in your yard.