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Unfortunately , insecticidal soap is generally not very effective against mobile insects such as fourlined plant bugs . Trees planted on M.26 generally require staking for the first few years of growth or , on windy sites , for the life of the tree . For example , pear trees produced by a local wholesaler are grafted to the semi-dwarfing rootstock OH X F 333 , one of the reportedly hardiest semi-dwarfing rootstocks . Powdery mildew generally does not warrant chemical control in the home landscape . Hosts commonly include , but are not limited to : begonia , carnation , chrysanthemum , cyclamen , geranium , impatiens , marigold , petunia and zinnia . However , only heavy infections are generally life threatening to a plant . 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 . Dimethoate ( often sold as Cygon 2E ) was a commonly used product for iris borer management , however it was canceled in 2002 and is no longer available . Commonly observed symptoms include : bud blast , leaf spots , flower blight , stem canker , and / or crown rot . Because infections of this disease generally occur in the roots and lower portions of the stem , spraying with fungicides is of little value . The good news , however , is that the main differences between grocery store pears and those we can grow here are size and fresh fruit keeping ability . Although its eggs can survive Minnesota winters , most adults migrate or are blown here from the south in the spring . Fresh fruit quality is excellent , being notably sweeter than other cultivars commonly grown in the north-central United States . When temperatures are warm ( 75 F ) and humidity high ( 85 % ) , the fungus will readily produce spores that spread the disease . Since the beetles are strong flyers and can readily move into the garden ; continual vigilance throughout July may be necessary .