Answer: Based on your description I would suspect any one of a number of bacterial or fungal problems, all of which would be encouraged by wet foliage due to either excess rain or frequent overhead watering and worsened by overcrowding or lack of air circulation. They can be spread by working on the plants when they are wet and may winter over on old plant debris, too. The zinnia problem sounds like it could be stem cankers caused by alternaria leaf spot in particular. The marigolds might be suffering from a stem or root rot, possiby caused by botrytis. You might want to check with your county extension (461-1000) for a more specific diagnosis and suggested remedies, if any. I'm sorry about your plants.
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