Answer: Cosmos seem to thrive on neglect - in my garden, at least! It must be very disappointing to you to have failure year after year. There are several diseases that befall cosmos. Bacterial wilt (a sudden collapse of the plants) is common, as are powdery mildew and root and stem rots. Based upon your description I suspect your plants have been infected with one of the pathogens that causes root and stem rot, which is often brought about by overwatering and/or poor soil drainage. Unfortunately these fungi are soil-borne and very difficult to eradicate without soil fumigation or solarization. Just to test this theory, try growing cosmos in a container or in another spot in the garden to see if the plants remain healthy. Remove all volunteer cosmos seedlings in the affected bed next spring and plant something else there instead. If the soil is poorly drained, build a raised bed or choose plants that are tolerante to the extra moisture. Root rot pathogens can remain in the soil for several years so you may have to find another sunny site for your cosmos.
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