Bleeding Cankers of Deciduous Trees Caused by
For the past 5 years, we have been studying a lethal disease of European beech caused by either one of two clades of Phytophthora citricola or P. cactorum. The most conspicuous symptom of the disease is the presence of one or more “bleeding cankers”, usually at or near the root collar. When the symptomatic bark is removed, dead
vascular cambium (surrounded by a margin of pink-colored wood on purple-leaved trees) is apparent.The disease and each of the three pathogens occurs throughout the northeast U.S. on about 40% of the beech population. Most of the affected trees are not showing any symptoms in their leaves or branches, and for them the disease may not be life-threatening for many years.
A similar disease occurs on European beech in forests in Europe. There, at least eight different species of Phytophthora, including P. citricola and
P. cactorum, have been found to cause the same symptoms. However, there is no evidence to suggest that the pathogens in Europe were introduced to North America to cause the current problem here. Instead, we suspect that prolonged summer drought, especially as it affects large, old trees that are already competing for scarce water with surrounding turf, may be a major predisposing factor.
Tar Spot of Norway Maple (Acer platanoides) Caused by Rhytisma acerinum.
In the mid-1980s, we discovered a “new” species of a fungus in the genus
Rhytisma causing extensive defoliation on Norway maples in several central New York communities. Affected trees had crispy brown leaves by early August and residents in the affected communities fully expected the trees to die. They didn’t, and since this first episode, we have watched the disease move into one community after another – first along the Mohawk River Valley and eventually throughout much of New England.
The disease is called “tar spot” and it is caused by a fungus that causes large (up to 1 inch diameter) black, tar-like spots on the upper sides of host leaves.(Fig. 1) In years when the disease is severe, individual leaves may have as many as 6-8 spots and be literally covered with the black fungus growth. Tar spot is common on Norway maple in Europe but it doesn’t seem to cause such conspicuous damage there as it does in North America. It was apparently introduced to this continent on nursery stock sometime in the early 1940s.
One thing we are now certain of after nearly 25 years of observation is that in the occasional year when tar spot incidence “explodes” to cause massive premature defoliation, the health of the host trees is not in jeopardy, and a second significant outbreak will not occur the following year. There is no need to treat trees with fungicides to protect their health.
Two other species of Rhytisma are endemic to North America. One is R.americanum (Fig. 2) and it commonly causes smaller, less damaging spots on leaves of silver (Acer saccharinum) and red (A. rubrum) maples. The other species is R. punctatum (Fig. 3) and it causes clusters of smaller, tarry spots on leaves of striped (A. pensylvanicum) and bigleaf (A. macrophylum) leaves. Neither of these other species causes significant damage to their hosts.