Bushy petunias in trouble with dried stem, help!

Lima, OH(Zone 5b)

I have been fighting slugs but are not seeing too many now; yet these petunias continue to die off due to the lower stem drying up and breaking off from the weight of the bushy live growth above. I have always had slugs eat the lower leaves of petunias, but never did they destroy the stem like this. The plant always grew back bushy. I am not seeing worms of any sort or earwigs, etc. and I am inspecting during the day and late at night and early in the am. Any ideas?...any help is much appreciated...and is it too late to save them...dig them up and plant them deeper? Petunias have always been so easy until this year.
THANKS for any advise.
Previously posted in "Garden Pests & Diseases" with no response.

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