Answer: Your asparagus ferns were killed by rust disease, says Tom Kalb, Extension horticulturist at the University of Wisconsin in Bristol. The cool, wet weather we had last spring and into the early summer was perfect for the development of asparagus rust, Kalb explains. The fungus overwinters on asparagus ferns, reinfecting young spears in spring whenever moisture is present. The ferns continue to develop, but eventually yellow and die. A telltale sign of rust is the reddish brown blisters on the stems. Repeated infection weakens the crowns and reduces yields. The best controls are to plant rust resistant varieties such as Jersey Knight and Jersey Centennial or to remove and destroy infected spears before next year's crop emerges in spring, says Kalb. In Wisconsin, we recommend waiting until late winter to remove the ferns so they can collect snow and help insulate the crowns through winter, he adds. Removing any wild asparagus plants in the vicinity will also reduce the amount of infection. If sever infections recur and cleaning up the old ferns doesn't slow the damage, you may have to use a fungicide. Kalb recommends spraying maneb every two weeks after harvesting is complete through the end of August.
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