A pair of chestnut trees in Wissahickon Valley Park are mysteriously unscathed by pathogenic fungus

Reposted from Grid Philly article written by Andrew Conboy on 1 September 2025

In June 2023, I followed my friend Josh past old, arching oaks and tall tulip poplars along a path in Carpenter’s Woods in Wissahickon Valley Park in search of a rare and intriguing pair of trees he had heard about. We eventually arrived at two initially normal-looking trees: a 45-foot tree that was growing towards a gap in the forest canopy, and a smaller 20-footer nearby. The taller tree’s blooms were on the wane, and long, fragrant stalks of creamy white flowers littered the forest floor. I took a closer look. We had found what we were after — two healthy, apparently blight-resistant American chestnut trees. These were the largest ones I’d ever seen.

Just over a century ago, the American chestnut (Castanea dentata) dominated much of the woodlands east of the Mississippi River. The trees provided an abundance of nuts to wildlife and humans alike and supported pollinators with their plentiful summer flowers. The species was essentially wiped out in the early 1900s by an introduced pathogenic fungus called chestnut blight (Cryphonectria parasitica).

Andrew Conboy examines the blight-free chestnut trees of Carpenter’s Woods. Photo by Chris Baker Evens.

The Carpenter’s Woods chestnuts are puzzling for a few reasons. First, it’s unclear if they are resistant to the fungal pathogen and thus considered “survivor trees,” or if they have somehow avoided being infected in the first place. Chestnuts that have been infected usually bear scars. Even resistant trees have cankers (patches of dead tissue) caused by the fungus. Josh and I didn’t see any cankers, though, which may suggest that the trees have never contracted the fungal infection. But this is quite unlikely, since the pathogen not only is spread far and wide by birds, insects, other animals, wind and rain but also can persist on oak trees, which are abundant in Carpenter’s Woods. American chestnuts found elsewhere in Wissahickon Valley Park exhibit the growth patterns typical of trees susceptible to the blight; after dying back from the fungal canker, an infected chestnut uses any stored energy to resprout at its base. And if the Carpenter’s Woods specimens are indeed resistant survivors, questions about their origin arise. In the middle of the woods with no trees known to be resistant nearby, how did they get there?

The Carpenter’s Woods chestnuts are puzzling for a few reasons.

In late 2023, I sent leaf and twig samples from both trees to the Pennsylvania-New Jersey chapter of the American Chestnut Foundation for testing, and both were confirmed to be pure American chestnuts. This exciting result ruled out the possibility that the trees could be resistant hybrids of Castanea dentata and Castanea mollissima (Chinese chestnut).

I revisited the trees in the fall of 2023 to see if the larger tree had yielded any nuts that could be grown, shared or perhaps used for research plots. The findings were a bit disappointing but not unexpected: chestnuts develop inside husks, and most of the tree’s husks contained only shriveled, unfertilized nuts. An American chestnut needs to cross-pollinate with another chestnut tree to produce fruit. Although mostly unable to self-fertilize — the male and female flowers on a given tree mature at different times — a few fertilized nuts will appear on a standalone tree on occasion. I found one apparently viable nut — it appeared plump and healthy — but my attempts to germinate it failed.

I’ve been checking on the trees each year to see if the younger one will begin to flower. If it does, that could allow the two trees to cross-pollinate. Another option is to manually pollinate the flowers of the larger tree with the pollen of a known survivor tree from another location. This would require coordination, some tree climbing and careful hand-pollination skills. If anyone is interested in helping to pollinate the tree next spring, send me a message on Instagram @andrew_the_arborist.

Andrew Conboy examines the blight-free chestnut trees of Carpenter’s Woods. Photo by Chris Baker Evens.