Insects and diseases affecting trees- Year in review

Our 2016 growing season is drawing to a close. Cool, wet weather in the spring during leaf emergence caused an explosion of fungal spores. This caused fungal disease to be widespread. Many of our common leaf diseases were more severe than average this year. Extended dry, humid periods in the summer caused moisture stress in many plants and provided fungal spores the ability to grow and multiply. Here is a recap of some of the most common problems we saw in 2016.


Tar spot on maplestar spot

Tar spot disease is a fungal disease typically found on Norway maples and sometimes on silver maples.  The disease starts becoming noticeable in mid-summer with pale spots on the leaves.  Through the course of the summer the spots grow and darken, eventually causing premature leaf drop. Tar spot is often considered only a cosmetic problem, but in some cases with smaller trees, or trees heavily infected in consecutive years, foliar spray applications may be warranted.



Apple scab disease in crab apple and apple trees 

This disease is often a yearly occurrence, especially on older varieties.  Most newer varieties have been breed to have resistant to apple scab disease.  Resistance varies between different cultivars and resistance to disease can lessen over time.  Apple scab starts out as black spots on leaves that multiply and grow, eventually causing early leaf drop often in mid-season.  Effective control includes gathering up and disposing of fallen leaves in fall to prevent reinfection in spring.  Pruning to thin out the canopy and allow for more rapid drying of foliage can slow spread of scab disease.   Chemical treatments are available and are applied in spring and summer as a spray application to the foliage to prevent fungal spores from establishing on the leaf surfaces


Rhizosphaera needle cast


This disease slowly attacks spruce trees. Largely a problem of Colorado blue spruce trees, it also on occasion can affect black hills spruce. Symptoms include needle loss on lower branches from the inside out. Older needles drop off, leaving only growth on the very tips. (which eventually drop off). Treatments include pruning out of infected limbs, and thinning out competing trees to reduce amount to bring more air flow and sunlight to allow for faster foliage. If you don’t prune, the rhizospharea needle cast disease will! Chemical treatments are available and can protect new growth from fungal establishment. Spruce trees grow slowly so recovery is typically a slow process as well. Fertilization can help create faster recovery.


Cedar-Hawthorn rust disease

Largely affecting hawthorn trees, this disease causes yellow spots on leaves that merge, multiply and eventually the leaves drop off early. This disease has an alternate host in juniper plants. The fungal spores start out in junipers and then spread to hawthorn trees. In order for this disease to flourish, typically juniper plants must be present in the vicinity. Chemical treatments can help control the disease and a series of spray applications starting in spring and reduce the effects of the disease.


Black knot fungusblack knot fungus

Conspicuous black knobs growing around branches and twigs are characteristic of this disease. Black knot affects trees in the Prunus family. (plums and cherries). We most often see it in the commonly planted Schubert cherry variety. Over time the black knots increase throughout the canopy and slowly girdle branches, leading to die back. Chemical treatments are not very effective. If the diseases is caught early pruning out of infected branches is possible to slow it’s spread. Often the disease continues to spread and tree removal becomes the only realistic option.



Yellowing leaves is a symptom of chlorosis in trees and shrubs. This is a function of a stressed root system, often brought out by drought, poor soil quality, and/or alkaline soil. In cases of chlorosis soil micronutrients such as iron and manganese are not available for the tree to uptake. Without treatment, this problem can worsen with leaf scorch, decline and eventually death. Chlorosis is best treated when caught early. There are numerous treatment methods that can be utilized depending upon the tree species, site characteristics and condition of the tree. Treatments include soil injection fertilization, soil amendments, root aerations and trunk injections.

Every year we encounter insects that feed on to trees and shrubs. Some insects are only cause minor damage while others can be quite destructive.  This year there were a few insects that spiked in their populations can have caused widespread damage. Watch out for the following insects!

Scale insect on magnolia- Look for white bumps on the newer growth twigs, and blackish soot like substances raining down insectsfrom the scale insects.  Treatment timing is important.  The most effective treatments are foliar/branch spray applications in early spring and late summer.  





Emerald Ash Borer- This insect continues to get a lot of publicity as it marches thru WI.  Over half of our state is currently in the EAB quarantine zone.  It is expected that over 90% of untreated ash trees will die of this pest.  The most effective treatment for most trees is a 2 year dosage of a trunk injected insecticide.  Studies have shown this treatment to be very effective. 

Japanese Beetles:  These insidious insects feed on over 200 varieties of trees and shrubs.  They love lindens, birch and roses.  Damage often starts in upper canopies and then spreads to lower portions of the tree.  In the larvae stage the insect feeds on roots.  Heavily irrigated lawns with thick root systems can become a breeding ground for the larvae “grub” stage.  Soil systemic treatments can be performed to control the grub larvae stage.  Foliar applications can be used to control the insect once they become adult beetles.

Insects, diseases and environmental stresses on trees can be complicated.  Hoppe Tree Service has considerable experience in understanding the complex nature of these problems and we perform expert diagnostic services.  Our arborists can help you understand which problems are true threats and which are only minor issues. 


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