Answer: There are various stem diseases that can cause the symptoms you describe--generally the stem damage is near soil level and the plant wilts above that point. It could also be slugs--I have a particular problem with them this year, with all the wet weather. Inspect the plants in early morning, looking for the critters as well as their slimy, silvery trails. Since slugs love damp, shady conditions, it could be that now that your plants have filled out and are shading the ground, the slugs have found them particularly attractive. If you think slugs are the problem, try removing mulch to help speed drying. There are also several slug traps and barriers available.
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