Flowering dogwoods, Cornus florida, whose white blossoms were once a welcome and common sign of spring's arrival in the United States east of the Mississippi, are rapidly disappearing.

The loss is mainly the result of an exotic fungus called anthracnose (Discula destructiva), which causes lesions on leaves and trunks, and eventually kills the plant. The fungus was probably introduced into the United States decades ago on nursery trees from Asia, and was first spotted on the east coast in around 1976 before spreading rapidly throughout eastern states.

Blooming lovely: but the flowering dogwood is in decline on the Great Smoky Mountains. Credit: CORBIS

In a recent study in the Great Smoky Mountains National Park in Tennessee and North Carolina, for example, National Park Service forest ecologist Michael Jenkins and colleagues have found evidence of severe declines in the flowering dogwood population since the 1970s in a variety of forest habitats. Similarly dismal results have also been found in other states.

Peter White, a plant ecologist at the University of North Carolina in Chapel Hill and an author of the Great Smoky Mountains study, says that, although the fungus seems to be the cause of the population decline, other factors may have contributed to its spread. For instance, the end of older Native American practices such as intentional burning means that fire is now far less prevalent. This results in thicker forest canopy and increased moisture, which helps the fungus to thrive.

The dogwood population has also declined in Indiana's Ross Biological Reserve, but not as severely as in the Smoky Mountains. Kerry Rabenold, a bird ecologist at Purdue University in West Lafayette, Indiana, says the reserve has also experienced changes that would help the fungus to spread, such as less fire. But anthracnose has not been found there.

Changes in the make-up of the forest appear to be affecting the reserve's dogwood population. Less fire helps maple trees to thrive, forming a denser canopy that stops dogwoods getting the amount of sunlight they need. Rabenold says that other understorey trees not susceptible to the fungus have also been dying off.

Dogwoods are an important part of the forest ecosystem. They are a source of calcium for animals and plants, for example, drawing it from deep soil and concentrating it in their leaves.

The trees also produce berries, which Rabenold says are “probably the best food in North America for songbirds”. He believes that declines in dogwood have adversely affected bird populations.

White says effects similar to those cited in the Indiana work are probably at play in other areas with dwindling dogwood populations, but that, in his study and many others, anthracnose is the clear culprit. “You might compare it to being run over by truck versus a chronic disease,” he says.