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However , chemicals are only effective if root rot is not extensive and environmental conditions are improved . If root rot is not extensive , repotting in pasteurized soil may rejuvenate some plants . Iris borers spend the winter as eggs on old iris leaves and plant debris at the base of iris stalks . Spots may appear as small elongate reddish areas that enlarge to lesions with gray centers and reddish borders and may develop into extensive zonate cankers . Since this fungus is able to infect many different garden plants , it is important to verify a plant 's immunity . Currants and gooseberries do not require extensive spraying to control pests in the Upper Midwest ; careful site selection and good cultural practices such as mulching , pruning , and sanitation will minimize pest problems . Leaf and stem spots may be controlled best by removing old top material in the fall , cutting the stems at ground level and destroying those parts . If your goal is to have a supply of fruit to eat for snacks and desserts , you probably will want to choose an early variety and a later one , so that you are not overwhelmed by too many apples all at once . A fungicide can be applied to protect new shoots , leaves and buds from infection . Diagnosis and control of this disease may be difficult since the fungus infects stems near the soil line and survives on old stem debris . Insects and Diseases Apple Pests Apples are hosts to many insects and diseases . Infected roots become brown and mushy due to extensive tissue decay , whereas healthy roots are cream to white colored and firm . burning or burying ) old iris leaves , stems , and any nearby plant debris . Like apples , pears are insect pollinated . Try to have a good balance between berry production and growth of vigorous new shoots . Pruning new bushes is needed only to remove any dead or dying parts of branches . Scab resistant varieties , however , are not resistant to insect damage and may still require insecticide sprays to manage insect pests . Keep the bush fairly open by cutting out any weak , old stems that no longer produce strong young wood at ground level . Apples are insect pollinated , with bees and flies transferring pollen from flowers on one tree to those on another . Iris borer caterpillars ( Macronoctua onusta ) are the most destructive insect pests of iris . Powdery Mildew shows as a white powdery dusting on leaves , stems and buds of many flowers and shrubs in Minnesota . If you have any doubts , consider putting a new plant in a quarantinearea of your garden , where , if it is diseased , the disease can not spread to other plants . Do not replant with another peony or other plant susceptible to Verticillium sp. Sclerotinia sclerotiorum can cause stem rot on peony as well as on many different herbaceous plants found in the garden .