I checked my sprouts this morning while counting my containers. I have 53 varieties that have sprouted - a lot of people contributed to these!! They are: Bupleurum Green Gold 5 varieties of Lupines Delphinium Cherry Blossom Lobelia Crystal Palace Lobelia Great Blue Queen Ann's Lace Marsh Mallow Blue Larkspur Larkspur 7 Dwarfs Pink Silene New England Aster Chives Angelica Giga Ice Plants Lavender Sancho Panza Bells of Ireland Variegated KMOTGG Lemon Mint Potentilla Melton Fire Shasta Daisy Becky Wild Daisy Excelsior Foxglove Alyssum Gold Dust Dame's Rocket Verbena Boanrensis on a stick Campanula Canterbury Bells Campanula Peach Leaved Painted Daisies Nepeta Nervosa Blue Moon Agastache Golden Jubilee Agastache Rupestris Agastache Korean Mint Agastache Pink Pop Maltese Cross Maltese Cross Dusty Salmon Gaillardia Oranges & Lemons Gaillardia Razzle Dazzle Gaillardia Tokajer Gaillardia Summer's Kiss Gaillardia Arizona Sun Gaillardia Red Plume Gaillardia Double Mixed Gaillardia Burgundy Yellow Ratibida Rudbeckia Rustic Dwarf Rudbeckia Cherokee Sunset Rudbeckia Green Wizard Rudbeckia Prairie Sun Rudbeckia Becky Mix Becky ramazz Newport News, VA

Just cover it with 2-3 inches of mulch and that´s it, it will come back next year. You see, you don´t need to lift the tuber unless you have to. I usually lift mine every 3-4 years to divide and give away, they can get huge.

Did they actually sprout then disappear? Did you actually see slugs? How much more dirt and mulch did you pile on top? If there's no sprouting, you probably have nothing to lose by digging up the tubers. They may well be mushy and rotten from a wet spring and the ground staying wet under an extra layer of soil and mulching. If so, they cannot be saved. If you love dahlias (and who doesn't), get your gardening experience with them next season by buying a box of tubers from Costco, Sam's Club, hardware stores, or any of the other big box stores for not too much of an $ investment. I've had good luck with those the years I buy them. They might not be the exotic mail order types, but they grow and bloom. I'm not interested in digging and storing so get fresh ones and discard at season's end.

Looks like a seasonal fungus I see sometimes on my bark mulch. It looks like slime at first, then expands into a foam-looking nastiness, then dries into puffs of spores. If it's the same thing, I'd scoop off as much as possible and THROW AWAY sealed up in the trash. I'd cut off the plant parts touching it to avoid it's spread.