Pennsylvania

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.

Affecting 45 regions

Oak Wilt

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.

Affecting 18 regions

Chestnut Blight

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.

Affecting 25 regions

Pine Wilt Disease

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.

Affecting 19 regions

Fire Blight

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.

Affecting 32 regions

Beech Leaf Disease

An emerging disease caused by a foliar nematode first identified in Ohio in 2012. It has spread rapidly through the native range of American beech. Affected trees can decline and die within 6-10 years. No effective treatment exists at scale.

Affecting 12 regions

Thousand Cankers Disease

A disease complex involving the walnut twig beetle and an associated fungus that has caused significant mortality of black walnut, particularly in western states. Thousands of small cankers coalesce to girdle branches and the trunk.

Affecting 15 regions

Armillaria Root Rot

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.

Affecting 36 regions

Rhizosphaera Needlecast

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.

Affecting 25 regions

Stigmina/SNEED Needlecast

A fungal disease of spruce very similar in appearance to Rhizosphaera needlecast but caused by a different fungus. Often occurs alongside Rhizosphaera. It has become increasingly recognized as a major cause of spruce needle loss, particularly on Colorado blue spruce. Diagnosis requires microscopic examination to distinguish from Rhizosphaera.

Affecting 20 regions

Cytospora Canker

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.

Affecting 25 regions

Bacterial Leaf Scorch

A chronic bacterial disease that affects many shade tree species, particularly oaks, elms, sycamores, and maples. The bacterium is transmitted by xylem-feeding leafhoppers and spittlebugs and colonizes the water-conducting vessels, restricting water flow. Symptoms worsen each year as more of the canopy is affected. There is no cure.

Affecting 19 regions

Verticillium Wilt

A soilborne fungal disease that affects hundreds of plant species including maples, elms, redbuds, and many other trees. The fungus invades through roots and colonizes the vascular system, blocking water transport. Symptoms often appear suddenly on individual branches. Maples are particularly susceptible. Olive-green to brown streaking in the sapwood is diagnostic.

Affecting 27 regions

Phytophthora Root Rot

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.

Affecting 31 regions

Bronze Birch Borer