Leaf infections are appearing

Small trees like the crabapple, pear, or hawthorn, show strange discolorations and growths on their leaves this time of year. What’s up with that?

The cause of the above symptoms is from a fungus called Pear Trellis Rust (Gymnosporangium sabinae) colonizing the leaves of a callery pear.

For the infection to occur, first, the right environment needs to be present at the right time. Cool and wet! Just like every other day in spring here. The combination of cool and wet weather while the leaves begin to form is the perfect recipe for these types of infections to take place. The incubation period—the time from when infection occurs to the time symptoms occur—is a few months.

Once you see the symptoms, there is no meaningful treatment can do to minimize the infection and its symptoms. The leaves are colonized by the fungus, and the symptoms will run their course. Preventatives for this type of infection are given to these plants as the leaves are beginning to emerge in spring, when they’re most vulnerable.

Interestingly, these are technically not chronic diseases, even if we might see them each year. Repeated infections can chronically affect a tree. Once the leaves fall off, the fungal colony falls off with it. And once spring rolls back around and presents the same conditions at the same time the tree is leafing out: a new infection takes place.

This is a fascinating element to the caretaking of trees. The existence of these types of fungi depends on the very specific alignment of weather and plant physiology. Pretty cool if you think about it.

Below is another infection that occurred in spring, but on an oak in July. In fact, I was called out to this tree to check for oak wilt! Luckily, it totally was not.

This fungal leaf infection is also, like the pear, associated with moisture while a leaf is underdeveloped—oak leaf blister (Taphrina caerulescens). Properly named, given the symptoms it causes.

Similar to the pear again, the conditions have to align for these infections to occur. And this one is a bit less common than the pear trellis rust, because the oaks leaf out in late spring. The weather tends to be less damp during that time, and so you don’t often get the perfect conditions for these infections in our climate.

Most of these types of diseases are fairly benign to larger trees, as the ratio of live-to-damaged leaf area is sort of negligible. Especially when there is just a one-off year where symptoms are bad.

As a general notion, these types of infections are pertinent when a large percentage of the leaf area is dysfunctional, like with smaller or younger trees. This can stress a tree out, and that can be sometimes a reason for treatment. But in the one off year, I wouldn’t bother treating.

Though in other cases, such as in the case of the pear, repeated infections can cause drastic and noticeable dying back of their crowns. These types of infections also can negatively impact the fruit of these trees.

These fungal organisms aren’t inherently evil, they’re just doing their thing in the ecosystem. They’re deeply involved in the world of trees and forests. Go out and have a look around, try to find some. Send them to me. Let’s talk about ‘em.

That’s all I got for you for now, I’m working on a longer article. But I didn’t forget you. I also didn’t forget that I told you to water your trees in the last email. Did you?

Tree First