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Helping trees grow in Minnesota

Spring recommendations

Here are some recommendations for shade tree, woodlot and plantation owners who want to keep their trees healthy during May and June by controlling pests and other unfavorable conditions/ events.

Pest or ConditionTreesRecommendations
Winter dessicationAllPrune only those branches whose buds are dead. Previous years' needles may be brown, but their buds may survive and grow. Always cut outside the branch collar when pruning main branches.
Branch breakageAllPrune broken branches back to a main fork or the trunk. If breakage is near the trunk, prune outside the branch collar.
Pine bark beetlesPinesPrune and destroy infested branches, or remove and destroy the entire tree before the new generation of beetles emerges.
Birch and elm leaf minersBirches and elmsSpray foliage when mines first appear as yellow or white leaf discolorations. Use acephate or other labeled insecticides.
SawfliesAllHand pick and destroy larvae or spray these leaf and needle-feeding pests when larvae are observed.
Bronze birch borerBirchesPrune dead branches. Remove and destroy seriously declining trees before the next growing season.
CankerwormsMany hard-woodsSpray these leaf feeders when observed with labeled insecticides. Bt is preferred.
Spruce gall adelgidsSprucesAs buds swell spray with acephate, dimethoate or diazinon.
Leaf-feeding caterpillars, lacebugs, leaf beetles, aphids, and externally-feeding weevilsAllSpray with a labeled insecticide when damage is unacceptable.

Bt is preferred for caterpillar control.

ScalesAllSpray with acephate or other labeled insecticides when crawler stage is observed. Repeat as directed.
Mite gallsMaple, elm, ash and more.Spray dicofol (Kelthane) or other labeled insecticides at budbreak and 7-l0 days later.
Shoot boring weevilsPinesPrune and destroy infested shoots and leaders before mid-July.
Root collar weevilsPinesRemove duff around base of tree out about 1 foot and prune lowest branches at least l foot up from ground. As a last resort, drench root collars with acephate, diazinon or other labeled insecticides.
White pine blister rustWhite pinePrune branches with dead needles or blistered bark.

Prune all branches up to 9 feet on stem in southern MN and up to 17 feet in northern MN.

FireblightApples, crab-apples, pears, mountain ashRemove infected branches 4-6 inches below visible infection. Apply copper compounds when buds are showing 1/4 inch green tissue. During bloom spray streptomycin at 3 to 4 day intervals to inhibit bacterial growth and reduce blossom infection. Disinfect pruning tools with diluted bleach after each cut.
Anthracnose and other leafspot fungiHard-woodsRake and dispose of/or burn fallen leaves and twigs. Prune to increase air flow and leaf drying. At budbreak apply chlorothalonil, mancozeb, Bordeaux mixture, benomyl, dimethyl phenylenebis or other labeled fungicide.
Dutch elm diseaseElmsConfirm disease by sending 6-8 inch sections from freshly-wilted branches to Lab Services Div., Dept. of Ag., 90 West Plate Blvd., St. Paul, MN 55107-2094. Remove and destroy infected branches or entire tree.
Oak wilt diseaseOaksConfirm as directed for Dutch elm disease. Install root graft barrier first and then the remove entire tree.
Needlecast diseasesConifersApply chlorothalonil or other labeled fungicide for the particular disease and host in order to protect young needles.
Pine needle rustRed pineEradicate goldenrod and aster in area as they are alternate hosts for this disease.

Mention of common or trade names of pesticides does not constitute an endorsement by the DNR.