Tree Diseases of Minnesota
Apple Scab
Description: Fungal disease of leaves and fruit causing early defoliation and branch tip dieback.
Trees Affected: Crabapple, apple
What to look for: Yellowing leaves, black lesions, early leaf drop, reduced growth, thinning crown
Treatment Options: Preventive fungicide sprays. Rake up diseased leaves. Watering and
mulching to restore health. Install disease-resistant varieties. A suggestion from The BIG Oak = Install Royal Raindrops crabapple, a variety with cut-leaf purple foliage and pink / red flowers (resistant to apple scab).
Anthracnose
Description: Fungal leaf disease causing browning, curling, and early leaf drop.
Trees Affected: Oak, ash, maple
What to look for: Browning leaves, curling leaves, early leaf drop, reduced growth
Treatment Options: Preventive fungicide applications, but not usually needed. Rake up infected leaves. Watering, mulching, and fertilization to maintain tree health.
Dutch Elm Disease
Description: Vascular fungal disease resulting in dieback, decline and death of elm trees.
Trees Affected: American, red, and slippery elm
What to look for: Yellowing leaves, wilting leaves, dropping leaves, thinning crowns,
crown dieback, dead trees, staining of cambium just under bark
Treatment Options: Preventive fungicide trunk injections. Removal and disposal of diseased trees (sanitation). Install disease-resistant elm varieties like Patriot, New Harmony and Valley Forge.
Oak Wilt
Description: Vascular disease causing dieback, decline, and death of oak trees.
Trees Affected: All oak varieties, but red oak family (red, pin) is most susceptible, white oak family more resistant (bur, white, swamp white)
What to look for: Sudden browning and falling leaves, dieback, staining of cambium just under bark. Decline much slower for white oak family.
Treatment Options: Preventive fungicide trunk injections for red and pin oak. Therapeutic fungicide trunk injections are possible for bur and white oak only. A note on pruning from The BIG Oak = prune out diseased limbs for white and bur oaks only (red and pin oaks die too rapidly), but DO NOT prune oaks during the active growing season if possible. Remove and dispose diseased trees. Do not leave diseased wood anywhere it can spread infections. Root trenching to disrupt root grafted oaks.
Diplodia Tip Blight (Sphaeropis)
Description: Fungal disease of pines causing stunting and dieback of entire limbs (stem disease)
Trees Affected: Austrian pine most susceptible
What to look for: Browning needles, dieback of limbs, stunted limbs.
Infections can be spread through trunk wounds caused by physical damage,
improper pruning cuts and damage from hail.
Treatment Options: Fungicide applications. Rake up infected needles.
Keep tree healthy as possible with watering and mulching. Minimize stress on tree.
Dothistroma Needle Blight
Description: Fungal disease of pines causing browning of needles, loss of needles,
and some dieback of limbs (needle disease)
Trees Affected: Pines, Austrian pine very susceptible
What to look for: Browning needles, falling needles, tufted appearance due to needle loss.
Disease is usually worse during periods of stress or drought.
Treatment Options: Preventive fungicide applications, rake up infected leaves.
Keep tree as healthy as possible with watering and mulching.
Rhizosphaera Needle Cast
Description: Fungal disease of spruce trees causing dieback, thinning and needle loss starting on the lower and inner crown.
Trees Affected: Colorado blue spruce is most susceptible, white spruce intermediate. And now a rant from The BIG Oak = STOP planting excessive numbers of blue spruce. Yes, they look cute and blue, until they don’t. Plant something else. Colorado spruce cannot tolerate the humidity in Minnesota and will always be susceptible to this disease.
What to look for: Needles turning purplish to brown on older needles, primarily in the lower and inside portion of the spruce and progressing outward and upward.
Treatment Options: Promote evergreen diversity with fir, larch and pine.
Stop planting Colorado spruce. Fungicide spray applications are available,
but results can vary and can get expensive over time.
Minimize urban stress factors as much as possible.
Trunk and Stem Cankers
Description: Cankers are localized dead areas in stems and trunk wood. Most cankers are caused by fungi, which can create further invasion from other fungi, bacteria and insects.
Trees Affected: Spruce (Cytospora), maple (Utypela), Honeylocust (Nectria)
What to look for: Sunken areas along limbs or trunk wood, dieback of limbs, oozing around sunken area, wilting leaves.
Treatment Options: Prune trees at proper time to minimize the chance of spreading cankers. Prevent physical damage to trunk wood or branches. Keep trees as healthy as possible with proper watering, mulch, fertilization. Application of Tree Growth Regulators may help minimize impact of the canker.
Pruning hint from The BIG Oak = Do NOT prune spruce, maple or honeylocust trees during periods of wet weather to reduce the risk of spreading dieback from cankers. I have seen trees with minimal canker issues killed by pruning at the wrong time. Pruning did not kill these trees, but ignorance did. No excuse.
