Autumn in upstate New York is flush with brilliant lights: reds up against yellows, oranges against greens. It’s spectacular. The roadways can feel like hallways through a palace of flickering torch lights hung in the trees. People come from all over the country to witness it. I myself am from the west coast—from evergreen forests. Where I am from, every fall season the greens remain green. So when I moved to upstate New York during the fall season a couple years ago, you can imagine my amazement. It’s an experience that I will never forget.
That’s why I was so confused this year. As fall approached, the leaves didn’t seem to care. Many of the greens were still green. And if not green, they were scarred with blotches of black or brown, covered with holes, or completely overtaken with a black or white rot. It wasn’t until dead, brown, leaves had already started falling off of the branches that yellows and oranges started appearing in some of the leaves that remained attached still. What’s going on? I asked. It was pathogens—leaf pathogens. Leaf pathogens are either a fungus or bacteria that can infect and cause disease in a plant’s leaves. Usually we see fall colors because photosynthesis stops happening in the leaves. Photosynthesis is what allows leaves to turn sunlight into food, and is also what gives leaves their green color. So once photosynthesis stops happening, the green color stops happening also. This year, there were pathogens in the leaves that forced photosynthesis to keep happening while the leaves continued to sustain various infections. So the leaves would look crinkled and destroyed, but would be green in the uninfected parts of the leaf. Eventually the leaves just died and fell off the tree brown.
Knowing that leaf pathogens have taken a toll on one of the most beautiful displays of autumn in the country might make some people come to despise leaf pathogens overall. But not all leaf pathogens work the same. In fact, there are over 19,000 different pathogens known to infect plants, and some of them are necessary for maintaining the diversity of a forest. For that reason, some leaf pathogens can play valuable roles in our lives. Describe to yourself the most beautiful forest that you can possibly imagine. What do you see? You may have imagined a forest constructed by an array of plant species, like a rainforest is. If so, you might have to thank plant pathogens. The presence and absence of certain kinds of pathogens often dictate the diversity of a forest. One popular theory to explain this (called the Janzen-Connell, or JC, effect) suggests that if a pathogen can kill one particular plant species alone, then the pathogen may restrict the growth, and thus the spread, of that one particular plant species. By restricting the spread of certain kinds of plants, other kinds of plants can take root instead, and thus diversity is achievable. So not all leaf pathogens are bad. For a diverse forest to stay diverse, some leaf pathogens are necessary.
Leaf pathogens not only live in growing leaves, but also in the leaves that fall off and land on the forest floor. Indeed, what alarmed me the most about returning to the same plot of land week after week was the sheer amount of leaf litter that accumulated on the forest floor. By the time winter came, the forest floor was completely shrouded by leaves: maple leaves, birch leaves, beech leaves, oak leaves, tulip leaves. They will all mix and fall apart together on the forest floor. The seedlings and saplings that popped up during that year often die—like the tall rattlesnake root and the white aster I saw in my plot. Everything on the forest floor is covered in dead leaves. But that’s not all. The pathogens are there too. They never left. As the winter progresses, as rain falls and the snow comes through the east coast, the leaf litter will fall apart. Meanwhile, many of the pathogens lie dormant. To lie dormant means the pathogens do not die, but rather can no longer actively infect anything because the conditions are not optimal. Once the conditions are optimal again, the pathogens may infect plants again. During their dormancy though, as the leaf litter gets worked into the forest’s soil, so too will the pathogens. Not enough research has been done to fully account for the importance of this phenomenon with regard to maintaining diversity in a forest, but I speculate that that it is an impactful process. This cycle repeats every year, and there are several pathogens in each individual leaf. In forests where the JC effect relies on pathogens, it is difficult for me to imagine that the soil pathogens and leaf pathogens don’t influence each other in ways that maintain that forest’s diversity.
While I am frustrated by the delayed bloom of the incredible autumn colors here on the east coast, I cannot help but appreciate leaf pathogens as well. Perhaps we have a lot to learn from them. In the meantime, take a walk through the winter forest and look down. What do you see in the leaves?