The Q&A Archives: Diseases Of Cosmos

Question: Year after year I try to grow cosmos, and year after year they succumb, mid to late season, to some sort of disease problem. I don't know if it's a mildew, a fungus, or a stem rot. They also get leggy. This happens whether I buy new plants in the spring or let plants grow from seed that has fallen the previous year. I am an experienced gardener and know that cosmos need no fertilizer and full sun. And I try to minimize getting water on the stems (I know these things but don't always follow them). My question is: If I discipline myself not to plant cosmos next year and pull out any seedlings that sprout from seed that has fallen from the previous year, will I lessen the disease problem in one season? Do I have to wait longer than one season? Or won't skipping a year kill whatever is in the soil because it will happen anyway if the above requirements are not met? (I also have tried mulching so that the spores, if that's what is causing the problem, do not splash back up.) Thank you.

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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