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Daylilies ( Hemerocallis spp. ) are considered one of the more disease-free flowering perennials . If symptoms are severe , and growth and flowering are greatly reduced , the plant can be removed and discarded . This type of pruning can be delayed until the extent of flowering can be determined . It is unlikely for geranium rust to survive winter outside in Minnesota , but removing infected debris may help prevent infections the following spring . Adult fourlined plant bugs overwinter as eggs laid in clusters into a slit near the top of canes of currants , brambles and other woody plants . Surface mulch helps maintain uniform soil moisture and good soil structure and reduces soil temperature in the summer . Black Spot is caused by the fungus Diplocarpon rosae . Apply nematodes when iris leaves are almost fully expanded but before flowering . A key diagnostic feature is the overwintering structure , called sclerotia . In general , these insects do not kill the plant , but may stunt or kill parts , affect flowering , or cause aesthetic damage .