Drought-Stressed Trees: Infections, Defenses and How to Help

Jess Divin, district manager of Davey's East San Antonio office, talks about how Texas' long-term drought is impacting trees, as well as the infections that can result from drought stress and the natural defenses trees use to survive. 

In this episode we cover: 

• Oak wilt fungal infection (00:42)
• How can homeowners take care of oak wilt? (3:40)
• Years-long Texas drought is highly affecting trees (7:37)
• Tree defense mechanisms during drought stress (8:20)
• Tree watering during a drought (11:28)
• Biscogniauxia fungal infection (14:13)
• Invest in your trees to ensure prolonged health (18:47)
• Root systems under drought stress (20:07)
• How did Jess become an arborist? (21:00)

To find your local Davey office, check out our find a local office page to search by zip code.

To read our blogs about how droughts affect trees and how to spot the signs of stress, visit our drought blogs at Blog.Davey.com by clicking here - Davey blogs about drought.

Connect with Davey Tree on social media:
Twitter: @DaveyTree
Facebook: @DaveyTree
Instagram: @daveytree
YouTube: The Davey Tree Expert Company
LinkedIn: The Davey Tree Expert Company 

Connect with Doug Oster at www.dougoster.com. 

Have topics you'd like us to cover on the podcast? Email us at podcasts@davey.com. We want to hear from you!

Transcript

Doug Oster: Welcome to the Davey Tree Expert Company's podcast, Talking Trees. I'm your host, Doug Oster. Each week, our expert arborists share advice on seasonal tree care, how to make your trees thrive, arborists' favorite trees, and much more. Tune in every Thursday to learn more because here at the Talking Trees podcast, we know trees are the answer.

This week, I'm joined by Jess Divin. He's a district manager in the East San Antonio, Texas office of the Davey Tree Expert Company. Jess, welcome to the show. Good to see you.

Jess Divin: Absolutely. Thank you for having me.

Doug: Summer stress. I'll tell you some summer stress. I had guys from Davey here today, and I got three more oak trees that got to be cut down, two of them because of oak wilt.

Jess: That's a crying shame. Oak wilt is a vascular disease where the tree actually senses the disease and shuts off its own vascular system trying to stop it.

Doug: Well, this is probably tree number 10 in the last five years, but that's the way it goes. When you're dealing with oak wilt, and you live in an oak forest, I don't know what I'm going to do. I can't afford really to treat all the trees and continue the treatment, so I pay to cut them when they go and then plant something else. We'll talk a little bit about what else to plant. You've dealt with oak wilt before, right?

Jess: Yes, sir. I am an oak wilt, Texas oak wilt certified arborist.

Doug: Let's just start there. If we're seeing signs of stress from an oak tree, and it's dropping these leaves that are weirdly colored, is that an indication of oak wilt?

Jess: Not always. If you're in drought conditions, which here in Texas, we've been in a pretty bad drought for the last five to six years, it's not typically my go-to. Really, with oak wilt, one of the main ways that it spreads is through the root system. It can spread pretty quickly, 75, 100 feet a year in the root system. With oak wilt, if you have an individual tree in your yard, and you're pruning at the right times of year, spring is the no-go. You want to be the heat of the summer, the cool of the winter, and fall.

If you're not pruning your tree in the spring, and you don't have any other trees around it that are showing signs of oak wilt, more than likely, it's not oak wilt. Typically, it's something-- life feeds on life. There's all kinds of bugs, insects, diseases that can attack the tree. It's not necessarily oak wilt.

Doug: It's an interesting point you made there. While we were looking at the trees, I told the arborist, "While you're here, why don't you just trim this one over here because it's got dead wood?" He goes, "I'm not trimming that because of oak wilt." I was like, "Oh, yes, I forgot all about it." We had a real bad storm in this part of the country-- I'm in Pittsburgh-- in April. It caused all sorts of problems. Of course, oaks were getting cracked off and such, and so opening up these trees to the pathogen.

For me, I have to live with the oak wilt. For your clients, what do you tell them to do when you're dealing with oak wilt?

Jess: Really, again, sanitizing tools, painting wounds. If they use Davey or anybody else, make sure that those two things are paramount. Sanitizing before and after you prune, and then again, painting wounds. It doesn't have to be any special paint. It doesn't have to be the wound sealing paint. Really, all we're doing is putting a temporary Band-Aid over that vascular tissue, because when we make that cut, it will literally start sucking air into the xylem and phloem. The tree immediately senses that those individual cells sense that siphon leak when they get exposed to air, so they start walling off that wound.

Really, we're trying to treat right after we cut in order to, one, mask the scent of that sweet sugar sap that the Nitidulid beetle is attracted to, and put a temporary barrier over that wound so if a Nitidulid beetle does land on it, and it has oak wilt spores on its body, it's not transferring them to the tree. Keeping trees happy and healthy, again, it's really pruning at the right time and the proper pruning.

Making sure that we're not pruning in that springtime window, which is typically the active time for both the beetle and the oak wilt fungal spore itself, becomes more viable or is able to get into the tree easier in the springtime via the Nitidulid beetle, or the root-to-root transfer. Again, oak wilt only gets in two ways; root-to-root, or the Nitidulid beetle.

Doug: I think I'm seeing a lot of root-to-root.

Jess: Yes, sir. These root systems on these trees, 70%, give or take, is underneath the drip line, but that other 30% is going far and wide looking for water and nutrients. Any time roots cross one another, they can graft together and spread that oak wilt via the root system. If oak wilt is within that 100 to 500-foot radius around our clients, picking the trees that you cannot live without, that you're identifying or your special trees on the property, and treating those, preventative treatment with propiconazole via the injections into the trunk.

Then basically just monitoring, really keeping an eye, having an arborist, certified arborist, better yet, an oak wilt certified arborist, come out, and if oak wilt is in your area, regularly checking your trees. If you suspect oak wilt, there are lab tests that can be done that can identify the genome, gene testing of oak wilt. That is a good indicator, because there are some people out there that'll see a tree that's weakened or stressed and say, "Oh, that's oak wilt." Well, it's not necessarily oak wilt. You need to really have it lab tested to make sure that it's a positive ID on oak wilt.

Doug: That's why I brought it up because that was my first indication how many years ago it was, maybe 10 years ago, where mid-summer I saw these weird-looking bronze oak leaves defoliating. That's when it started, and exactly what you said, they suspected but then did the lab test and it's here. Five-year drought, is that what you're talking about, where you're at?

Jess: Really, the last three or four have been really bad. In 2022, we were negative 20 inches behind our annual rainfall. In '23, we were negative 15 inches. '24, we were negative 8 to 10. Annual rainfall is a moving target. It's nature is nature. Those three years in particular, we have seen a lot of stress in trees, hypoxylon, drought stress. When trees are highly stressed, they are fairly intelligent creatures, there's a number of different defense mechanisms that they do to maintain the moisture in their tree.

First and foremost that we typically see is the loss of turgor pressure. Turgor pressure is what keeps the leaves upright, that allows sunlight to hit the full face of the leaf and actually do really well as far as photosynthesis. When that turgor pressure drops, the leaf itself will actually drop down. It's a defensive mechanism because now you don't have the full leaf getting exposed. You only have sideways or not full exposure to sunlight. That's one of the ways that they help retain some moisture is not having that leaf out there that's just transpiring water all over the place.

Then there's the lenticels on the leaf that allow for air exchange. They do have guard cells around those lenticels that can actually close up when the tree is losing too much moisture. It's not like putting a lid on a jar. It's like putting a paper towel over your picnic plate. There are some gaps, water still does escape. If that goes on for too long, leaves will start dying back at the tips because they're not receiving enough moisture. You'll start seeing little tip burn.

Then if that continues to go on without sufficient rainfall or irrigation, trees will actually start dropping leaves in order to conserve moisture. They'll start cutting leaves prematurely because every leaf is a spot of water loss. Like I said, even though the guard cells do close, it's not airtight, so it still is losing water. Trees will actually cut leaves in order to conserve moisture.

There's a number of different signs. If you come out first thing in the morning, and you look at your trees and all the leaves are drooping, that's a really bad sign. There are parts of the country where even though the tree may have sufficient water, come noon, one o'clock, two o'clock, three o'clock, in the hottest part of the days, even trees with enough water will actually droop their leaves. Again, the self-defense mechanism, get that full leaf exposure out of the full force of the sunlight and drop it, or it's not able to pull up enough water to have that turgor pressure full and keep that leaf nice and upright and erect.

If you see that in the morning that turgor pressure, that's a bad sign because that means the tree isn't having enough water to replace that turgor pressure overnight in order for it to do what it does the next day as far as generating sugars via photosynthesis.

Doug: If you do see that on a tree in the morning, is watering the key? Is that what we should be doing?

Jess: Typically, yes, sir. Like I said, if you see that first thing in the morning, typically that's a bad sign, one of the best. For tree watering, it's not automatically turn on sprinklers because there's a lot of parts of the country that are experiencing water deficits. One of the easiest and best ways that I've found to water trees is turning on just a simple water hose, an eighth or a quarter of the way on, just so it's dribbling out.

Everybody wants to water the trunk of the tree. You actually want to pull that out away from the trunk; halfway between edge of canopy or drip line and the trunk, right in the middle of those two is where you want to water. Let the water hose sit for 20 to 30 minutes on that really low trickle, or to the point of, I say, 20 to 30 minutes, or to the point of runoff. We don't want to waste water. Then move it around that trunk at that same distance all the way around.

Trying not to water the same place over and over and over again because trees don't do anything for no good reason. If you constantly, I mean, I love bubblers on trees and drip systems and things like that, but over time, if you continually water one tree in one place, it never has to go look for its own water. It will put a mass of its roots right at that one point because why exert energy that you don't have to growing roots that you don't have to when you're getting water right there?

Moving that water around, including the bubblers, it's a little bit harder with drip lines if they're below the surface, but moving that watering around does help quite a bit. Early morning, you see those leaves drooping, it would be a good idea to supplemental water. Our irrigation water doesn't hold a candle to rainwater. Rainwater is, I mean, we can water 10 times and not do the same thing as a quarter or a half-inch rain. Rainwater is phenomenal. Really what we're doing with irrigation, we're limping the tree along until we get that, my daughter calls it sky juice, until we get that sky juice to really help those trees out.

The irrigation, again, is just to keep it moving, keep that basic metabolism going until Mother Nature actually provides the good stuff.

Doug: Sky juice, I love that.

Jess: Sky juice, yes.

Doug: That's got to be a new term for us to use. Now, you did use a word, though, that I'm not familiar with. Is it hypox? What is that?

Jess: Hypoxylon, I'm sorry, I'm using the old name. It has been renamed Biscogniauxia. Hypoxylon is a fungal infection, or I should say the correct name, Biscogniauxia. Biscogniauxia is a fungal infection that is, at least in our area, is on our trees all the time. Even healthy trees have fungal spores of Biscogniauxia. It's when the tree gets really weak and stressed, and the actual water content of the wood gets low enough that it actually allows the fungus in to the vascular tissue via wounds, or stress, or openings.

Unlike oak wilt, Biscogniauxia will actually clog the vascular tissue, the xylem and phloem. It actually grows in high enough numbers to where it basically will completely wall off that vascular tissue. You see it typically in strips where the bark will start falling off, and you see there's different colors for different species, and there's a number of different varieties of Biscogniauxia. It'll look like a grayish or it can be a tan color. It affects oaks, and elms, and hackberries, and pecans.

There's quite a few species that are susceptible to it, but again, it's because the tree is weakened and stressed that it allows it in. It's not a very virulent fungus. It's not actually getting into the vascular tissue on its own. Again, trees are just like us. If we're unhealthy, unhappy, stressed, we're much more likely to get an injury, or a disease, or a sickness. Same thing with trees. When they're weakened and stressed, they tend to get more or have more issues, and continue it down that mortality spiral.

Doug: This drought can't be fun for a plant person. I find nothing worse than a drought when we have them, and we don't have them like you're having it. That has had to cause a lot of stress for your trees in your area, I have to assume.

Jess: Oh, yes, sir. I've been the bearer of a lot of bad news for a lot of folks for too long, because a lot of times the Biscogniauxia is a fairly rapid fungal disease. Once it gets in, it does a lot of damage really quickly. It's not always evident until it's to a point where the tree is severely damaged. Even if it does survive, even if we can get it to climb back up that mortality spiral by providing nutrients, and humates, and phosphonates, and pruning out of the dead limbs, a lot of times it's not that big, beautiful, majestic tree that it was before it.

Again, the stresses, people seeing the same tree over and over and over day to day, they don't notice the small subtle changes over time, until an arborist comes up and sees it and is like, "Whoa, you've got a problem." They saw it at the beginning, and the tree got a little bit thinner, and they didn't really notice, and the tree got a little thinner, and they didn't really notice. They may have had to rake some extra leaves off their driveway or something like, "Wow, this tree's losing leaves." They don't really associate that with stress or an issue that they have to resolve.

Again, getting an arborist out to look at it, because I see at least 100 trees a day because I'm looking at trees all the time, even driving to other appointments and other properties, I'm constantly looking at trees across the landscape. Our landscape is definitely changing because of the lack of moisture. There's a lot of dead trees around, unfortunately.

Doug: That's brutal. Nothing worse than a long-term drought, that's for sure. When you think summer stress on trees, what else comes to mind? Is there anything else besides that type of water stress or maybe disease stress?

Jess: Again, Biscogniauxia is one of them. Again, really, I say it a lot, trees are a lot like people are. If we can be happy, healthy, and strong before something happens, before a sickness, before an injury, we're typically much better off on the backside. That's the biggest thing that I try to convince people, trying to get your trees as happy and healthy as possible before all of this stuff starts. It's really hard, again, to climb up that mortality spiral. It's really easy to go down it because one thing leads to another, leads to another.

Trees are like big energy savings accounts. It takes an investment in the spring to put out the new leaves. Those leaves have to make that return of the initial investment plus next year's leaves. Again, getting a tree as happy and healthy as possible is not going to prevent a drought, or prevent insects, or even a disease. Typically, they do much better off on the backside of it if they're happy and healthy and strong going into it.

Keeping a well-developed root system, again, with the drought that we've seen, most of the really important roots that do all the heavy lifting, the water and nutrient uptake, are the hair-like root structures off of the main feeder roots and things like that. If the soil dries out enough, those little hair-like root structures will die off very quickly under prolonged drought stress. That means the root system is somewhat shrinking. Even when we do get rain, they're not able to use all of the water that they could if they had a healthy, happy root system.

Everything starts with your soil and the roots because the roots are the hearts and lungs of trees. Typically, if you can keep the roots happy, 9 times out of 10, the canopy's happy.

Doug: Well, Jess, that's good stuff. Before I let you go, I have to find out how you got into this and why this job is right for you, because you obviously have a passion for it.

Jess: I grew up in South Texas on a cattle ranch and peanut farm climbing in these massive live oaks. Our native live oaks can live 300 to 600 years depending on the soil type, and how mean Texas is being in a given set of years. I just loved climbing trees. I moved to Colorado after graduating high school. Got my degree from Colorado State University and forestry was it for me. Once I started learning about trees, it was all over.

My first job out of college was with Boulder Open Space and Mountain Parks where we were doing forestry, wildland firefighting and forestry health improvement. After the chainsaws and running around in the woods, and then I found Davey. I'm totally biased because I work for Davey, but it is the greatest company on the planet when it comes to trees. It's evolved since then.

I've done a lot of other things, but this is what-- I've been a park ranger, a wildland firefighter, urban forester. Everything that I did after that initial first job in the forest has always led me back to trees. Finally, I gave in and decided, "Hey, this is what makes me happy. This is what floats my boat." Trees is where it's at for me.

Doug: Well, Jess, great stuff. Glad to meet you and talk to you. Really appreciate your time and wonderful information. Let's get some rain down there in Texas. It would make life so much easier, wouldn't it?

Jess: As long as it's not all at once. Kerrville is not very far from us. It's only about an hour drive for us. We've been trying to help out. That is Texas in a nutshell. It's either feast or famine. Thank you for the opportunity to talk about trees. I truly enjoy sharing knowledge. What I've learned over the last 20 years means nothing if I don't share it with folks. Really appreciate the opportunity, and would love to come back on and talk about anything tree-related ever. I really enjoy it.

Doug: Well, I got a feeling that's going to happen. We need to do a deep dive on oak wilt, that's for sure, because I talk about it a lot on the podcast, but people really don't know a lot about it and what's going on. Again, hey, thanks so much, Jess, and we will talk again.

Jess: Thank you, sir. Appreciate your time.

[music]

Doug: As you can tell, I really enjoyed talking with Jess. Can't wait to have another discussion with him. All right, you tree lovers. Tune in every Thursday to the Talking Trees podcast from the Davey Tree Expert Company. I'm your host, Doug Oster, and I need you to do me a favor. As always, subscribe to the podcast so you'll never miss a show. What should we be covering on this podcast? Let us know.

There's a couple different ways to reach us. Send us an email to podcasts@davey.com. That's P-O-D-C-A-S-T-S @ D-A-V-E-Y.com. Or you can just simply click the link at the end of our show notes to text us a fan mail message. Your ideas could be on a future show. Love to hear from you. As always, we'd like to remind you on the Talking Trees podcast, trees are the answer.

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