Common Tree Diseases in Wisconsin: Identification & Treatment

A group of fungal diseases causing leaf spots, cankers, and shoot blight on many shade trees. Sycamore, dogwood, maple, and oak are commonly affected. Cool, wet spring weather promotes infection of newly expanding leaves.

The most economically important disease of apples worldwide. The fungus overwinters in fallen leaf litter and releases spores during spring rains. Severe infections reduce fruit quality and yield and can defoliate trees by mid-summer.

One of the most widespread and destructive forest diseases in the world, caused by a complex of honey mushroom species. It attacks stressed trees through root contact and can persist in dead stumps for decades, infecting nearby healthy trees.

Asian Longhorned Beetle Damages hardwood trees by tunneling deep into trunks and branches - identified by USDA APHIS as a pest threat.

A native North American wood-boring beetle that attacks birch trees, especially paper birch and European white birch in landscape settings. Larvae feed under the bark in a serpentine pattern, disrupting nutrient and water transport. Dieback starts at the top of the crown and progresses downward, the opposite pattern of many other pests.

A fungal disease requiring two hosts to complete its life cycle: eastern red cedar (juniper) and apple or crabapple trees. It causes economic losses in apple orchards and aesthetic damage to landscape cedars.

A fungal disease that functionally eliminated the American chestnut as a canopy tree species in eastern forests by the mid-20th century. Once numbering in the billions, mature American chestnuts are now exceedingly rare. Root sprouts survive but are re-infected.

A common fungal disease of spruce and other conifers that causes branch dieback, typically starting from the lowest branches and progressing upward. Colorado blue spruce and Norway spruce are especially susceptible. The fungus enters through wounds or stressed tissue. White resin bleeding on bark is a hallmark sign. Stressed trees are most vulnerable.

A common fungal disease of pines, particularly Austrian and Scots pines in landscape settings. Spores infect new shoots as buds open in spring. Repeated infections progressively kill branches from the bottom of the tree upward.

A fungal disease of pines that causes premature needle loss and can significantly reduce growth and vigor. It affects many pine species but is especially damaging to Austrian, ponderosa, and Monterey pines. It has increased in severity with climate change.

A devastating vascular wilt disease spread by elm bark beetles that has killed millions of American elms since the 1930s. The fungus blocks water-conducting vessels, causing rapid wilting. It fundamentally changed the American urban landscape.

An invasive wood-boring beetle native to Asia that has killed hundreds of millions of ash trees across North America since its discovery in Michigan in 2002. Larvae feed on the inner bark, disrupting nutrient and water transport.

A bacterial disease that primarily affects members of the rose family, including apple and pear trees. It can destroy entire orchards in a single season. Bacteria spread rapidly during warm, wet spring weather through blossoms and wounds.

An invasive beetle introduced to New Jersey in 1916. Adults skeletonize leaves of over 300 plant species while larvae (grubs) damage turf grass roots. They are particularly damaging to linden, birch, and fruit trees.

A broad group of fungal diseases that cause discrete spots on the leaves of deciduous trees. Most are primarily cosmetic and do not seriously harm established trees. They are most common during wet spring and summer weather. Many different fungi cause leaf spots on different tree species.

A lethal fungal disease that affects all oak species, with red oaks dying within weeks of infection. It spreads through root grafts between neighboring oaks and via sap-feeding beetles. White oaks are more resistant but not immune.

A group of water-mold diseases that attack the roots and root collar of many tree species. Phytophthora thrives in wet, poorly drained soils. It destroys fine roots and can girdle the trunk at the soil line. Affected trees show a general decline that is often misdiagnosed as drought stress. Oaks, dogwoods, rhododendrons, and many other species are susceptible.

A lethal disease of pines caused by the pinewood nematode, which is transmitted by pine sawyer beetles (Monochamus spp.). It causes rapid wilting and death of susceptible pine species, particularly non-native Scots pine and Austrian pine. The nematode clogs resin canals and destroys cells lining them, leading to cessation of resin flow and rapid desiccation. Trees typically die within weeks to months of symptom onset.

A widespread group of fungal diseases that produce a distinctive white powdery coating on leaves and shoots. It affects hundreds of tree, shrub, and plant species. Unlike most fungi, it thrives in warm, dry conditions with high humidity.

A common fungal disease of spruce trees, especially Colorado blue spruce, throughout the Midwest and Northeast. The fungus infects current-year needles through stomata, causing them to turn purple-brown and drop prematurely. Inner and lower branches are affected first because humidity is higher there, and the disease progresses upward and outward over successive years. Severely affected trees may retain only the current year's needles at branch tips.