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Research is being done on resistant varieties , but at this point , information on less susceptible plants is mostly anecdotal . Research results using nematodes have been inconsistent , although they can potentially be effective in reducing iris borer populations . An average apple contains about 90 calories , and small amounts of vitamin A , vitamin C , thiamine , riboflavin , calcium , phosphorus , and potassium . Currants and gooseberries are self-fertile , but research suggests that planting more than one cultivar results in better yields . If environmental conditions are favorable for disease development , the fungus can spread through most of the root system within 7-10 days , causing death of the plant . If you prune off the wilted shoot near the base and look at it in cross-section , you will see that the water conducting system inside the stem has turned brown . Although there is more than one way to prune a fruit tree , home gardeners in Minnesota commonly prune and train using a " central leader " system , where a single central trunk runs the entire height of the tree and supports the fruiting branches . Some research indicates that the use of oat straw mulch may reduce leafhopper numbers . Position each tree so that the graft union ( identifiable as a swelling several inches above the root system ) is 2 to 4 inches above the soil line . A planting of pear trees on " Old Home X Farmingdale " ( OH X F ) semi-dwarfing rootstocks at the Horticultural Research Center in Chanhassen has performed quite well for the last decade . If lower canes are covered with soil to a depth of two to three buds , this will encourage a larger root system and the development of numerous renewal canes , a strategy that will maximize the useful lifespan of the plant . Pectin and fiber aid intestinal activity .