Asked January 14, 2021, 11:38 AM EST
Howard County Maryland
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It's possible that harlequin bugs killed the top growth if the infestation was significant. Here is our page on Harlequin Bugs, which will offer help with monitoring and management of the pest for the future:https://extension.umd.edu/hgic/topics/harlequin-bug-vegetables
Rotting of the crown could have been due to secondary disease pathogens as the plants declined.
It's also quite possible that a primary infection occurred to the roots and/or crown. We had above average rainfall in many areas during late summer. Wet soil under the mulch may have also reduced gas exchange and oxygen levels in the soil.