Hello Penelope,

Thank you for contacting the UVM Master Gardener Help Line and for sending the quality photos.  You certainly have a bacterial ailment going on and it's not surprising given the hot humid weather we are having.  I am including a few articles that may help you to get to the bottom of it.  While it is difficult to tell exactly which bacterial issue you are having, despite your quality photos, the yellowing of the tips of the leaves might point to Black Rot.  Following is a description that lets you know how it is spread and some guidelines on control.

https://extension.umn.edu/disease-management/organic-management-black-rot
   
For your further knowledge on the problems that brassicas can have I am including a couple other articles

https://extension.umn.edu/disease-management/alternaria-leaf-blight

This last one gives you access to Cornell's comprehensive list of fact sheets.

https://plantpathology.ces.ncsu.edu/pp-vegetables/pp-vegetables-brassicas/