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Signs of nitrogen deficiency include yellowing leaves ( older leaves yellow first ) and poor growth . The symptoms of blight caused by this fungus , Phytophthora cactorum , may be confused with the symptoms produced by Botrytis sp. Narrow fastenings such as wire or cord may cut into the thin bark . White mold , caused by the fungus Sclerotium , causes a crown rot in daylilies . Because some types fit tightly to the trunk , they can cause the bark to stay moist , leading to disease . Disease Cycle : The fungus overwinters in infected canes and spores can be spread to healthy canes by splashing water , wind , and pruning tools . One method of control is to hang a very strongly scented bar of soap in each tree in summer . As the affected areas enlarge , the margins remain reddish and the bark in the central part turns gray and then brown . Hollyhock rust , caused by the fungus Puccinia malvacearum , is a common and destructive disease affecting hollyhocks . These exclude voles ( “ meadow mice ” ) and rabbits , preventing them from feeding on the bark . If the bark temperature gets above freezing , water in the conductive tissue under the bark becomes liquid and begins to flow through the cells . Northern gardeners should be aware that the soil in our climate will not reach temperatures high enough to kill the fungus .