177: The Role of Arbuscular Mycorrhizal Fungi with Paul Schreiner

Arbuscular Mycorrhizal Fungi, commonly abbreviated to AMF, coevolved with plants from 500 million to one billion years ago. Fossil evidence shows AMF in existence back when dinosaurs roamed the earth.

Paul Schreiner, Research Plant Pathologist at USDA-ARS in Corvallis Oregon explains that grapes are a very receptive host for AMF and their symbiotic relationship benefits both organisms. AMF helps plants obtain nutrients like potassium and phosphorous. The plant provides AMF with sugars and fatty acids. AMF lives both inside and outside of the plant. Inside the plant, they form arbuscules inside the root cell. These structures look like little trees and increase surface contact dramatically. Outside the plant, AMF mines for nutrients, likely releases carbon, and prevents soil erosion with its root hair-like structure.

Listen in to learn the practices you want to use, and not use to increase AMF populations.


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Craig Macmillan  0:00 

Our guest today is Paul Schreiner. He is a research plant physiologist with USDA ARS in Corvallis, Oregon. And today we're going to talk about our arbuscular mycorrhizal fungi. Thanks for being on the program.


Paul Schreiner  0:11 

Thank you for having me, Craig, happy to be here and talk about one of my favorite subjects.


Craig Macmillan  0:17 

Obviously, can we just go with AMF, we'll just jump right to that. Yes. Because if I have to arbuscular too many times today, I'm gonna, I'm gonna crash.


Paul Schreiner  0:27 

It's a tough one. And it's back in the old days, it used to be called vesicular, arbuscular mycorrhiza. So it was even harder.


Craig Macmillan  0:34 

Yeah, it was even hard. We're not in the old days. Whenever a bunch of scientists were sitting around and going, like, you know, what the V this just too much. Can we get into three words, you know, was that big national meeting of mycologist. All right. So let's just go for you studied EMF for a long time. And you have stayed in the field. So you've studied vineyards? Let's start the very beginning. So what are AMF? And what kinds of roles do they play in the soil and interacting with plants?


Speaker 2  1:00 

AMF are, as you said, arbuscular mycorrhizal fungi. So there are a group of fungi that evolved a really long time ago, estimates are from at least 500 million to a billion years ago.


Craig Macmillan  1:15 



Speaker 2  1:16 

Yeah, that the billion number comes from some molecular clock kind of work, which, you know, is based on mutations in DNA that might be slightly overestimated, but there's fossil evidence that shows them at 450 million years ago. That's a really long time. You know, that's, that's what dinosaurs were around.


Craig Macmillan  1:35 

Early dinosaurs if we had time later. I want to know how paleo Micology where I How do you find fungi, but we don't have time for that right now. But if we can come back to that, that's mind blowing.


Unknown Speaker  1:48 

Yeah, we can.


Craig Macmillan  1:49 

Go ahead.


Paul Schreiner  1:50 

Yeah. So one thing I should say I think that's helpful is there's more than one kind of mycorrhizal fungi group, basically, right? The arbuscular mycorrhizal fungi are this older group, or they evolved a very long time ago, and there's been a long period of coevolution with plants. And what's happened is the arbuscular mycorrhiza, which is the kind that grapevines form. Most of our crop plants also form arbuscular mycorrhizal that mycorrhizal group can no longer grow or complete their lifecycle by themselves on their own, they require a host plant to get carbon to survive, there's a whole bunch of other kinds of mycorrhizal fungi. The most important besides the arbuscular would be what we typically call Ecto mycorrhizal fungi and the Ecto are typically on forest trees, especially in temperate and boreal regions. Interestingly enough, a lot of the trees in the tropics are am or arbuscular mycorrhizal fungi. So, it is important to just think about that, because some people, when they hear Mycorrhizal, they automatically assume one or the other Ecto or am or arbuscular. It's even far more complicated than that, because there's like five or six other types of mycorrhizal fungi that different plant groups associate with. However, the mycorrhizal fungi that I studied the arbuscular type are the most prevalent, they're the oldest. And you know, it's kind of an interesting thing, science wise that, based on our best information, they evolved, the whole world was one big continent, right? That's when they really radiated and evolved rapidly, I can get a sequence out of grapefruits. That's a mycorrhizal arbuscular mycorrhizal fungus. And it'll match 100% to a sequence in Europe somewhere, or in South America somewhere on any other crops. And, you know, so that's kind of an interesting thing.


Craig Macmillan  3:39 

That's a question. And so if we have that kind of similarity in different parts of the world, does that make the research that's done either in on vines in another continent? Or on a different crop? Does that is that useful for understanding how these things work in advance?


Paul Schreiner  3:56 

Certainly, yeah. Especially in the last, I'd say 20 years, because we've developed molecular tools to really delve into evolutionary questions and DNA, those kinds of things. Were starting to separate that group of fungi with a finer tooth comb, if you will, right. In a very broad sense, there was this massive radiation during Pangea, and then all the continents separated, a lot of that genetic material is very, very similar. However, there is still evolution going on. It's just you have to look harder for it. And you have to do whole genomes. That's not something that I do. I'm much more on the practical side of agriculture. But I tried to stay in tune with all that. This group of fungi traditionally were thought to be asexual. Now, we think there might be some sort of sexual phase, but it's unclear and not clearly demonstrated yet. We're learning more all the time. But it's also a slow, kind of a slow process. Their interest intractable to study this group of fungi a little bit because they do rely on a host plant to complete their lifecycle. So like, we can't culture them and put them in a lab. So were grown on petri dishes, you know, that kind of thing. They have to be grown with a plant. So we've developed ways to do that. But it does present challenges.


Craig Macmillan  5:10 

Yeah, how do you do that.


Paul Schreiner  5:11 

And this is really important for this group of fungi. From a practical sense, you have to grow them on a plant companies that produce mycorrhizal inoculum have this kind of fungi, the arbuscular type, they are growing them on plants. Typically, they're doing that in a some kind of either soil or soilless mix, and producing that in a greenhouse. And what like when I grow cultures of these fungi, we grow them on plants. One of the challenges with this group of fungi that relates to all this is that they're also ubiquitous around the globe, pretty much anywhere you have plants, these fungi are are there, the diversity is different in different places, of course, and there probably are some specialists, you know, groups, for example, that might be in more tropical climates versus more boreal climates. But I mean, we're still just beginning to understand that kind of information with this group of fungi. You asked also, what role do they do in terms of soil ecology or plant ecology? There's no question. The biggest role that this fungi plays is in helping plants obtain phosphorus. There is evidence of uptake of other nutrients, particularly those nutrients that are more immobile in soil. And that's why phosphorus is one in particular, but Potassium is another nutrient that's not super immobile in soil, they help take up potassium, they also help take up copper, zinc, I'd say those four are probably the top. However, they also play a role in nitrogen uptake in some plants. You know, we have addressed this in grapes with my former student, Tian Tian, who's now a farm advisor in Southern California working on table grapes, part of her thesis work was looking at the nitrogen impact on mycorrhizae, and how they help with nitrogen uptake. And we're continuing that work to some degree now with my new student. So far, we've not been able to show that the arbuscular mycorrhizal fungi are helping grapevines take up nitrogen. However, lack of evidence doesn't mean it can't happen. Other groups have shown in other plant systems that they do help the plants take up nitrogen. But still even even though that's true, without question, Phosphorus is the main thing that this group of fungi helps plants obtain from soil, you know, and phosphorus is a really critical thing, because yeah, phosphorus reserves are running out, you know, we've been mining basically guano, right? I mean, that's our main source of phosphorus all around the world. I just read an article it was in the New Yorker the other day, about phosphorus. I'm glad it's, you know, being highlighted again, because, you know, some people think 20 years from now, we're going to be out of phosphorus.


Craig Macmillan  7:40 

I think so.


Paul Schreiner  7:41 

And we're going to be in a world of hurt. Yeah. So it's hard to know for sure. Other people estimate we've got hundreds of years, but I don't know how good these estimates are. But helping plants get phosphorus means we don't need as much phosphorus to apply as a fertilizer. This is becoming an issue for basically the whole globe and human production. Yeah, their biggest role is Phosphorus, they also probably help do other things in plants, which I can kind of go down a list if you want.


Craig Macmillan  8:07 

Well, before we do that, I do want to do that. Because I think that that's crucial because a number of ideas have come up and some I think are probably accurate. Some I think are not, but I don't know, AMF, it's a parasite or it's a symbiotic organism?


Paul Schreiner  8:21 

Yeah there symbionts.


Craig Macmillan  8:23 

Okay, there's symbionts. Now, how does the plant and the fungi interact? Are there things that go into the root or the root is coded by something? Or how does that work?


Paul Schreiner  8:34 

Yeah, so that's, it's a pretty interesting process. If you start at the very beginning of a naked root, let's say, you know, a root does not colonize this starting to grow in soil. It sends out signals that the fungus consents, or the fungi, you know, there's more than one fungus in this group. Some of those signals, we already know what those are, like Striga lactones are one for example, you know, complicated term, but it's just a particular group of compounds plants make, the fungi can sense that. And they can grow towards the root, you know, the hyphy of the fungus, when it touches the root or makes contact with the root, it forms what's called an apex thorium, and then it makes a penetration peg and can basically punches through the wall of the root. This is the same way that fungal pathogens, you know, who are bad fungi, let's say right, they also use the same kind of mechanism. And typically when that's happening, you know, there's enzymes involved, bits of the cell wall of the plant and or some of its cuticle get kind of chewed up by enzymes and it releases certain compounds and then the plant can say, Oh, I know this one's a bad guy, or sometimes a plant is producing enzymes, for example, kinase that's trying to degrade the fungus itself and then you release certain other signal compounds, so the plant can sense that and in certain pathogens. We know that a very specific metabolite can be sensed by the plant and it stimulates the plant to respond in a defense response with the mycorrhizal fungi. The defense response is repressed.


Craig Macmillan  10:00 

Okay, oh.


Paul Schreiner  10:01 

Yeah, and so part of that is probably the kinds of chemicals that the and fungi have that are being released, you know, through these various enzymatic interactions have yet to be recognized by the plant as as the bad guy. Anyway, that's, that's maybe getting a little too into the weeds. But yeah, it all starts in the same way like a pathogen trying to get in or even, you know, there's a group of parasitic plants that form these things called hostaria, that attack roots of other plants. Same kind of process.


Craig Macmillan  10:31 

It sounds to me like there's an enzymatic reaction, and then also a signaling reaction, which would probably be some kind of a protein, I would guess it's complicated, or it's a feedback thing in that, oh, I've been poked, oh, I'm gonna do this, Hey, wait a minute, this is okay. And then they kind of settles into a balance, I guess. Is that fair to say?


Paul Schreiner  10:49 

Yeah, I think that's a good way to look at it. It's really complicated. And I mean, we only are beginning to understand the way they communicate. You know, there's a whole new class of compounds called effector proteins, which are secreted by different organisms and soil plant can recognize a lot of those, it crosses many things like even goes to nematodes, right? Like this is all kind of newer stuff that we're learning. But the bottom line is, the am fungi get in because they don't stimulate a defense response in the plant. And that's because there's been at least 500 million years of coevolution the plant knows these guys are okay, these are the good guys. Once they're inside, they grow throughout the cortex of fine roots. And then they form these things called arbuscules. And that's where they get their name. So the arbuscular mycorrhizal fungi form arbuscles, our bus skills are basically like a little tree, if you can see a picture of it, it looks like a little tree inside a root cortical cell. It's just basically a way that both the fungus and the plant increase their surface area contact by like, a huge amount, you know, like, like, imagine what a tree looks like above ground, you know, like, especially without the leaves on the wintertime is a great time. That's exactly what it arbuscular Looks like in miniature inside a root cortical cell, the plant membrane grows all the way around that there's all kinds of activity that arbuscule cell is super active, because there's a lot of metabolic things happening. And that's where the plant and the fungus are exchanging nutrients,


Craig Macmillan  12:17 

then then exchange is cell wall to cell wall. It's not puncturing into a cell, or is it punching into? Oh, heck, what's the word I'm looking for, a pipeline?


Paul Schreiner  12:28 

No, the and fungi don't get into the vascular tissues of the plant, they actually colonize these cortical cells, they're sort of like, if you think about leaves, leaves the cells, we think about most of the mesophyll, or the spongy mesophyll. They're the ones that are doing photosynthesis, right? In the root cells that are most active in the fine roots are the cortical cells. That's where most of the activity is happening. So the fungi colonize there, they form these arbusculs which is, I mean, they're, they're amazing structures, they're very cool. They're short lived, like, a lot of times in arbuscular, will form, develop, and then degrade within, say, a week to 10 day period, you know, so it's like a fairly rapid turnover during that you increase the surface contact between the two organisms, but there's always still a membrane on the fungal side and a membrane on the plant side that keeps those two organisms separate, you know, their cytoplasm doesn't mix. You know, that would be weird, kind of, you know, would be weird. Yeah, we just don't see that in biology, you know, they really have much greater metabolic activity in those arbuscular cells. So what happens is, the fungus is giving phosphorus to the plant in this process, and other nutrients. And then in exchange, the plant is giving the fungus sugars. And we now know also fatty acids. Yeah, that's been a recent discovery in the last two decades. Anyway, I can't remember exactly when it came out. We now know because of genome sequencing efforts, that this group of fungi lack the ability to make fatty acids, they actually get those from the plant as well.


Craig Macmillan  14:01 

So we've got the arbuscules on the roots. That's kind of the structure, we've got the peg in there. Now what's going on away from the roots? Are these big long, multi celled single identifiable organisms, or is it kind of a community or what what's going on?


Paul Schreiner  14:19 

There are different fungi. Okay, so there's multiple species, a single root can have many species of fungi inside it. However, those species also probably mark out some territory. This part is still a little bit unclear because it's really hard to pinpoint this stuff. Just as an example, our research vineyard here at Oregon State University that I work on, even though I'm ARS I also work in in part of OSU, our research vineyard. I think we found 19 Different mycorrhizal fungi. colonizing the groups, the roots of the grape vines, you know, there's a fair number, how they actually interact on a very tiny scale like within an individual single individual root is it's hard to know for sure, that's again Getting off into the weeds a bit.


Craig Macmillan  15:01 

Not so much because I'm going somewhere with this.


Paul Schreiner  15:04 

I want to get back to your question though about what's happening outside because that's really critical. Yeah, what's happening inside is we have these aruscules and hyphae growing inside the root, and it can be, it can be very intense, especially in Grapes. Grapes are a super host, in my view, they really love mycorrhiza they get heavily colonized. But then on the outside out in the soil, the naked hyphae, if you will, of the mycorrhizal fungi are exploring the soil. And that external phase we call extra radical hyphae, it actually is physiologically different than what's on the inside of the root. I'm trying to think of a good analogy, but basically, the inside part has a different function than the outside part. And so the inside part is trying to get carbon from the plant give the plant phosphorus and other nutrients, the outside part is mining the soil for that phosphorus, exploring the soil, it also probably exudes a significant amount of its carbon into the soil and helps the soil microbial community get a carbon source as well. And these fungi seem to play a critical role in helping soil aggregate and or resist erosion, basically, I mean, the evidence of this is, is pretty clear. But we also know that roots do the same function, especially root hairs, you know, so one way to think about these fungi is they're, they're sort of like root hairs. Except they're even finer, you know, their job is to connect root to the soil and to the soil, water and nutrient supply


Craig Macmillan  16:30 

Two spatial questions. One, when they say they explore space, how deep do we find an organism? Do we find a fungus that's connected to a to a vine or plant, right? So how far down is it going? And then how far out in lateral space is it going?


Paul Schreiner  16:44 

People have studied this by using artificial system where we can put a screen for example, like we can grow a plant in a greenhouse in soil, have it be colonized by microbes and fungi, and then put a screen in place that the roots cannot cross. But the fungi can, you know, something below like, or I don't know, somewhere in the ballpark of 40 micron diameter screen, so very fine screen like a silk screen, the hyphae can grow in there. So like people have shown they can grow 15 or 20 centimetres away, no problem, you know, significant distance when you're talking about soil as far as how deep they go in soil, that varies a lot with the rooting depth of whatever the plant of interest is, or, you know, the ecosystem, we tend to see greater colonization in the, in the surface soil, which just fits everything else that happens in the surface soil, right? I mean, that's where more of the water and nutrients are being turned over. It's also you know, where the soil environment is more favorable to life, right, because of this whole soil structure, idea and porosity, allowing oxygen to get into the soil in a vineyard. Particularly, you know, we always talk about vineyards and how the roots go all the way to China kind of thing, right?


Craig Macmillan  17:52 

Until you hit clay or limestone.


Paul Schreiner  17:54 

Sometimes roots can go really far. I mean, 30 meters I've seen reported and get into, you know, basically rock, right? There's not too much mycorrhizal fungi down where we're there in rock, we did the study, again, it was at the research vineyard. And when you get into what is known as the sea horizon, in the soil, which we would typically think of as the subsoil, it's where it's more compact, there's less porosity, and it's pretty dense, right? And it's more like clay, colonization drops off a lot, you know, we might have 90% of roots are colonized in the topsoil. But in the subsoil might be 30%. That's because many things, one, the environment is just not suitable for life in general, at that depth, because it's compacted, there's less oxygen, it's a different environment.


Craig Macmillan  18:37 

And again, you've mentioned AMF need more than just a plant root, they need to be out in the environment, there needs to be oxygen, there needs to be water, there needs to be other, there needs to be a favorable environment for life period. Right now, one thing we've talked about water holding. In other interviews, we've talked about water holding capacity improves in fields that have a higher or more successful AMF population, and that you talked about aggregates, it's part of that picture. We've talked about nutrient movement particular phosphorus, one of the things that I've heard people just kind of say colloquially is that if you have a meaningful mix, whatever the popular population, ecosystem involving AMF, it's going to lead to greater stability in the vine, and give the vine an ability to tolerate drought stress a little bit better. Are you finding those things? Are those things true? Even if it's kind of anecdotal? I mean, you're scientist, so you don't like anecdotal probably. But


Paul Schreiner  19:33 

Yeah, so that's, you know, that's the interesting world of science in my world. I need to have evidence for what I say, especially, especially when it comes to publishing scientific papers, right?


Craig Macmillan  19:43 

Well, of course, yeah.


Paul Schreiner  19:44 

But then there's also opinion, you know, sometimes you can't show things in science. I mean, science isn't perfect, right? Mistakes happen, and some things are just more intractable and difficult to show, however, okay, on a broad scale, there's pretty good evidence that mycorrhizal fungi helped Plants tolerate drought stress better than non mycorrhizal plants when you know when they've been compared. So that has certainly a long term consequence that you might think would eventually relate to stability in some way.


Craig Macmillan  20:16 

So one of the reasons that I asked that is science, Applied Science, especially applied Agricultural Science often is moved by growers noticing something or having an image in their head about how something works. And then folks like you come in and say, Okay, well, let's find out. Another thing that I've heard people mentioned that I don't know is true or not, is do AMF actually move water into the plant? We know that they transport minerals, or they actually move water into the plant?


Paul Schreiner  20:42 

Yeah, that's a great question. The answer to that question at this point in time is, they don't move water in a way that we would like to think of it, they're not acting like a pipe, because their own cytoplasm is a, you know, it's a vital part of them, it's just like us, you know, like the inside of ourselves, we're not just gonna give that away, people used to think of them like, Oh, they're just pipes out there, and the water just flows right through them into the plant. Well, that's impossible, that just can't happen. What does happen potentially, is water moves on the external surface of the hyphae. Because similar to a plant root, they exude some carbon, they have some structure makes connection to soil water in the pores of soil. And so in theory, because these fungi are much finer diameter, let's say 50 to 100 times smaller in diameter than a root is, you know, fine root of a plant so they can get into smaller pores inside the soil and get access to soil water that the root may not be able to get access to. On top of that, there's potential especially because they help aggregate soil and help improve soil structure, they may actually in the long run, improve soil water holding capacity, because they're adding to that long term carbon storage of the soil. It's really well known that as you add organic matter to soil, you improve the water holding capacity of soil. The am fungi do do that. I mean, partly it's this bit of carbon that they exude into the soil rhizosphere itself or the we call it the micro rhizosphere. Even their turnover. So when they die, or when they're eaten by something else, they're also then contributing to that pool of soil carbon, and the more old and complex that carbon is, probably the more it's tends to be tied to soil, water and small pores. Yes, they do help, we can show that they help plants take up a little bit more water, but it's not a big deal. You know, it's kind of like let's say the plant on a given day use 10 liters of water and you let the plant go to the wilt point. Maybe the mycorrhizal plant got another 10 mils of water out of 10 liters, you know, it's not a huge amount.


Craig Macmillan  22:55 

They're not the pipeline, but they are changing the soil environment such that the water holding capacity is changing. And that makes it more water for the mines to pick up. So it's not that there is a pipeline through the mycorrhizal fungi but that it's changing the environment in a way that makes it more likely that the water will be held and that the mind then has it available.


Paul Schreiner  23:12 

Right and that that effect is small, it's hard to show because it's very small. The other thing that they probably help with the plants is that as soil dries, nutrients are harder to get. And particularly those nutrients that are more immobile and soil like phosphorus, a big part of why we see improved drought tolerance in a mycorrhizal plant is because they are accessing soil phosphorus better than a non mycorrhizal plant can and that's contributing to the overall drought tolerance of that plant. So some of our effects that we see are an indirect effect of improved phosphorus nutrition that goes across to any of the other functions that AMS might help plants do. Like another big category that I feel I should mention is there's good evidence that mycorrhizal fungi help plants resist or become more tolerant to other pathogens in the soil. So the bad guys or even nematodes, a lot of work has been done on this, you know, the experiments run the gamut, like they're there all over the place, because, you know, we're talking about really complex things. One of the things that we know, is that just improving the overall phosphorus nutrition of the plant and or other nutrients, sometimes it's, it might be another new nutrient that's limiting that gets you added tolerance to to any of these other effects, right? Whether it's drought, whether it's a root pathogen, even like insect feeding on above ground parts of the plant, you know, I mean, if you're in a better nutritional state, you're going to be better able to tolerate a lot of things. A lot of what happens with AMF is linked to their role in phosphorus, you know, so going back to this phosphorus story, some of my colleagues get mad at me because I they think I'm too opinionated about phosphorus. But I mean


Craig Macmillan  24:59 

You You're having beers with people. And they're like, Paul, when you get off the phosphorous thing?


Paul Schreiner  25:05 

Yeah, they're like, come on, Paul, you know, they play a role in nitrogen too.


Craig Macmillan  25:10 

Okay, so we're in at a time with a couple of things I just absolutely, positively have to hit on if we draw the big old box around this topic, we would say, AMF are beneficial for vineyards. Okay, so what kinds of things can I do as a grower to encourage a AMF and what kinds of things should I not do that might dink the AMF community?


Paul Schreiner  25:29 

Very good question. The most important thing probably is to think about AMF, before you plant a vineyard. And so like in some of the materials that I've I've, I've written about and published on, especially for like trade journals and trying to help growers, it's really important in my mind to separate pre plant versus post plant, and at the pre plant stage is really a time you should think about mycorrhizal fungi because that's the time. If they're not there, you've got a problem. But chances are, they're already there. It's also the pretty much the time that you can add mycorrhiza and they're going to do something, you have an opportunity to inoculate vines if you want when they go into the ground. The biggest thing about pre plant is what is the past history of that land, especially the recent couple of years if you've had plants on it, especially if their host plants for mycorrhizal fungi, which almost all of our crop plants are, even if it came out of say, forest land, and then was converted to vineyard. Typically, there's a AMF there because even in the forest, which are dominated by Ecto mycorrhizal trees, for example, here, and in the north, west, for example, there are still understory plants that rely on AMF. And so the AMF are there, normally, you don't have to inoculate. But knowing what the land history is, is very helpful. The worst thing you can do is of course, apply a fumigant, which we are doing much, much less now than we used to. Not that that will will stop entirely. But if you fumigate especially with like in the old days, methyl bromide was the main fumigant used, you'll kill the mycorrhizal fungi. And so you would want to inoculate if you did that. The other thing is if you have a really, really long period of fallow land, and when I say fallow, I mean fallow no weeds, nothing, most of our weed species also support AMF. So I mean, even having weeds on the ground before you plant a vineyard is going to keep the population up. And again, that goes back to the biology, this group of fungi that they can't grow on their own. And so eventually they'll be depleted in soil if there's no plants to keep feeding them. So that kind of relates back to the very beginning of our conversation, which is why this group of fungi is different. So like, basically avoid long fallow plant a cover crop of clover, for example, that's a good one, because Clover is very heavily colonized. It also provides nitrogen, which is good for vines, you know, especially at establishment and avoid fumigants. Once you get to the post plant side of things, I think the most critical things to think about are tillage, and then fertilizer use.


Craig Macmillan  27:56 

What happens there?


Paul Schreiner  27:59 

With tillage, you breakup the mycorrhizal network that's in the soil. And so like we talked about that external phase, or what we call the extra radical hyphae of this group of fungi, that phase is out in the soil and it actually survives and overwinters even for example, like you know, some of it dies back, but some of it remains if you keep destroying that with tillage. Eventually you reduce the population of AMF, there are a few fungi that seem to be much more tolerant of tillage. And these are some of our favorite lab rat ones, for example. Ones that are tolerant of disturbance have been ones that are most often done well in the laboratory and are easily easy to culture. Again, we're culturing on a plant but still similar kind of thing. So tillage is one thing. The fertilizer issue is I would avoid both high nitrogen and high phosphorus inputs. We have shown in some of my work if you apply phosphorus, for example, to the foliage, which some people like to do, you can reduce mycorrhizal colonization. It's all tied into the whole plant response to these this group of fungi, you know, plants evolved with the fungi, right? It's not just the fungi that were evolving. They know that the main function is phosphorus. So when the plant has high phosphorus status, it down regulates the colonization by this group of fungi. Well, when you get plants phosphorus, especially to the foliage, it sends a signal to the roots, I'm very happy, and it tends to reduce colonization.


Craig Macmillan  29:24 

Specifically, how many units of nitrogen are we talking about?


Paul Schreiner  29:28 

That gets into tricky territory.


Craig Macmillan  29:30 

You know, if I'm putting on a 777 am I am i doing a bad thing?


Paul Schreiner  29:35 

Probably not course, it also depends on the rate, you know, I mean, 777 But you're putting out 200 pounds per acre that's


Craig Macmillan  29:42 

Yeah, that's why use the term units.


Speaker 2  29:45 

Yeah, you know, and the thing about viticulture is we don't need as much nitrogen and as much phosphorus that as we do in other classic farming crops, you know, like the big the big crops corn soybean commodity. Yeah, commodity grapes are super cheap. super efficient at getting nutrients, other work that I do, which is actually more of more of my time spent on nutrition than it is on mycorrhizal fungi. But, you know, we've shown that high nitrogen is not necessarily a good idea in the vineyard, right. And most people know that. And almost intuitively, you don't want a massive canopy, that shading the fruit.


Craig Macmillan  30:19 

In a vineyard, if I'm putting on nitrogen at a replacement rate, so I'm looking at how many pounds per acre I took out, I'm guessing and how many pounds per ton that relates to taking in cycling from canes and leaves that fall on the ground and go into the soil. You know, most vineyards you're looking at not a lot. So I've looked at some organic systems that are putting in, you know, two pounds per acre, the highest I think I've ever seen was 25 pounds per acre. Eight is a pretty good number kind of on average total. It sounds to me like these replacement level rates, not the high rate, but the replacement level rates where we're, we purposely are trying not to get a bigger canopy, we're not trying to bump a vine. That sounds like those are fine.


Paul Schreiner  30:58 

I think so we've done work here, see, it was in Chardonnay, and also Pinot Noir. And we were putting out 20 and 40 pounds of N per acre. These are not high rates in in my view, especially when we look at agriculture as a whole right, we can see a little bit of a depression in mycorrhizal colonization, when we apply, say 40 pounds, or 60 pounds of nitrogen per acre, we don't wipe it out entirely, you know, the vines also can recover. The other thing is, the kind of nitrogen you put out may play a role as well. More soluble classic conventional fertilizers that have more soluble N and especially more soluble P will probably have a more negative impact. If you're putting out more organic sources of those fertilizers, because they're more complex, you know, they don't cause as quick of a response in the plant. And it does seem that that does not have as a negative impact on AMF, as the more soluble forms.


Craig Macmillan  31:58 

There we go. Now we're out of time, unfortunately, this could go on forever, you and I should get together sometime and just hang out talk about phosphorus all you want. What is one piece of advice that you would give to grape growers related to this topic, especially if they want to increase or maintain in AMF population in their vineyard?


Paul Schreiner  32:17 

You know, the most important thing is to be conservative with inputs. I think that's probably the the key thing conservative with both water inputs and nitrogen inputs and phosphorus inputs. You know, the role of fungicides, so far does not really appear to be a big deal. And again, we don't have time to go into all that. But the evidence that we've collected so far suggests that you know our fungicide spray programs, for example, which we're talking about controlling things in the canopy, I'm not seeing a clear effect on microns of fungi because of that. If you overwater over fertilize. That's when you're going to do damage to Microsoft fungi, you know that that's clear. And then the other piece of advice is think about it pre plant, because that's the time you can actually do something put a cover crop in prior to planting the vineyard and I bet 99% of the time, the mycorrhizal population that's there will be sufficient to colonize the vine roots and be healthy goes back to just very briefly the fact that I consider grape vines, a very, very receptive host for mycorrhizal fungi. I've looked at other crop plants, including other woody perennials, and grape vines are so heavily colonized. It's it's truly amazing.


Craig Macmillan  33:28 

That is cool. Where can people find out more about you? And or more about this topic? You mentioned research that's been published recently on some of these topics. Where can we find you?


Paul Schreiner  33:39 

So the easy way to find me is type my name Paul Schreiner. And grapevine will be in the title in the show notes. Yeah, yeah. I mean, if you just put my name and grapevine nutrition, or grapevine and AMS, I should pop up as the first thing on Google. But you can also just email me And I'm happy to provide for those that are more interested in getting into the weeds. I can provide you some lists of good references and whatnot. So I'm happy to do that.


Craig Macmillan  34:05 

That's fantastic. My guest today has been Paul Shriner. He is a research plant physiologist with USDA ARS. He's based in Corvallis, Oregon. This has been really fun for me. I hope it's fun for our listeners, too. This is such a hot topic. And so thank you very much for being on the podcast. Really appreciate it. Paul,


Paul Schreiner   34:22 

Thank you so much, Craig. It was great having this conversation


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