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Sep 21, 2023

What makes Sour Rot so challenging for wine grape growers is that it is a disease complex. Hans C. Walter-Peterson, Viticulture Extension Specialist, Finger Lakes Grape Program, Cornell Cooperative Extension explains that Sour Rot comes in late season after ripening.  Yeasts get into the berries and ferment the sugar out in the vineyard. Bacteria follow up, feasting on the alcohol, converting it into acetic acid – an unwelcome component in winemaking. And, the disease is spread rapidly by fruit flies.

In this interview Hans shares methods to reduce Sour Rot disease pressure by managing increasingly resistant fruit fly populations, leafing to encourage fewer berries at fruit set, the correct way to drop fruit, and timing antimicrobial and insecticide sprays to Brix to maximize effectiveness.

Cornell Cooperative Extension is trialing non-chemical control practices including UV light for sterilization and hormonal sprays plus a disease model is under development with Penn State University.



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Craig Macmillan  0:00 

Here with us today is Hans Walter-Peterson. He is a viticulture extension specialists with the Finger Lakes Grape Program, part of Cornell Cooperative Extension. Thanks for being our guest today.


Hans C. Walter-Peterson  0:12 

Thanks for having me. Glad to be here.


Craig Macmillan  0:14 

You've been doing a lot of work on a situation I'll call it called Sour Rot on grapes. And that's what we're gonna talk about today. Let's start with some basic definitions. What exactly is Sour Rot?


Hans C. Walter-Peterson  0:24 

So sour rot is pretty much what it sounds like. It's one of the late season rots that can afflict grapes comes in after ripening starts so much like Botrytis, bunch rot some of these other types of rots that that growers might be familiar with. So it's another version of that, but it comes along with the bonus of acetic acid, every rot kind of brings its own different compounds to the party. Sour rot brings one that really is not terribly welcome in winemaking, you know, essentially the the main component of vinegar. It's a particularly rough type of rot. We really are getting some more challenging years with it past several years. So my program has really started to focus in on what we can do to try to keep it under control.


Craig Macmillan  1:09 

You know, I understand that part of the issue here. Is that sour rot is a disease complex. There's multiple actors involved in all of this. Can you tell us what some of those pieces are of that complex and how they interact to create sour rot?


Hans C. Walter-Peterson  1:23 

Yeah, it's probably the thing that makes sour rot a more difficult thing to manage than kind of the standard diseases, the regular diseases that most growers are used to dealing with like powdery mildew, downy mildew, because those are created those are developed by one type of microbe. So if you find the one thing that can control that one microbe, you've got a control measure. With sour rot it's a like you said it's a complex of multiple organisms that bring it about. So basically, there are yeasts, the yeasts get into the berries and take the sugar that's being developed in there, and they do exactly what we use yest for in winemaking takes the sugar and turns it into alcohol. So we'd get a fermentation starting within the berries out in the vineyard. The second part of it that happens then is that there are bacteria that follow up and also arrive in there most notably Acetobacter, but also some other things like Gluconobacter and Henseniaspora. This is some great work that was done by Wendy McFadden-Smith in Ontario a number of years ago. So they all kind of come in and feast on that alcohol and convert that alcohol into acetic acid. So thereby there's the sour of sour rot. The piece that comes after that, then is not just the sour rot. But then the thing that probably is really characteristic of it also, as with some of these other rots, but it spreads really quickly in a vineyard if the conditions are right. And that's mainly done by fruit flies. And it's not just the one that we've been hearing a lot about lately, the Spotted Wing Drosophila, Drosophila suzukii but it's also just your plain old Drosophila melanogaster, the ones you used in your your high school genetics classes, or college genetic classes and see on your fruit around the sink and stuff like that. Those fruit flies, for the most part, mostly fruit flies are a couple of other suspects in the mix, too. But they're the ones that spread it from berry to berry and cluster to cluster and block the block.


Craig Macmillan  3:13 

Are they spreading the yeast, the bacteria are both.


Hans C. Walter-Peterson  3:16 

All of the above.


Craig Macmillan  3:17 

Okay, so that's it,


Hans C. Walter-Peterson  3:18 

They're gonna freeride. So that's, that's the difficulty with it. If it was just, you know, like I was saying earlier, if it's something like black rot, or botrytis, where it's just one single causal organism, that's one story. And that's hard enough to control when you've got multiple types of organisms that aren't even directly related. I mean, yeast and bacteria are very different types of organisms, for example, we don't have a spray or a single thing that control that. And so that's the real difficulty with managing it year in and year out.


Craig Macmillan  3:48 

So this just made me think of something. One way of thinking about disease complexes is if I can remove one of the elements, or two of the elements I can at least reduce if not prevent or treat the disease is that the case with sour rot if I had no bacteria, if I didn't have a yeast or something like that, can I get rid of one of them and and help with this?


Hans C. Walter-Peterson  4:09 

Yeah, that's that's a really good point. As I said earlier, you need the two micro organisms to cause the sour of the sour at the acetic acid development, but then you need a vector to move them through the vineyard. And that's the fruit flies. So if you can control the fruit flies, you have less chance for those microorganisms to move through the vineyard. If you create a less hospitable host for the microbes, there's less of them to be moved around by the fruit flies. So the management strategies that we're looking at are trying to come at it from both directions. Some of the original work that was done on this recently here at Cornell by a grad student, Dr. Megan Hall, who I believe you had on the show a while back.


Craig Macmillan  4:50 

I had in the show, and I know her yes.


Hans C. Walter-Peterson  4:53 

In Megan's original work here at Cornell. She basically found that it was somewhat more effective to control the fruit flies than to control the microbes that just the microbes by themselves could cause a certain amount of rot. But then if you're controlling the fruit flies, it just you don't get that explosive growth.


Craig Macmillan  5:10 

The fruit flies in the gasoline.


Hans C. Walter-Peterson  5:12 

Right. Exactly. Yeah. The microbes are the fuel. Yes. So that was the impetus of kind of saying, Okay, if you had a control just one thing, it's the fruit flies, because that's really where the explosive nature of the disease comes along. And it's a little bit easier to control a bug than it is microbes that are hiding inside the skins of berries and things like that.


Craig Macmillan  5:31 

Where do the microbes come from? are they hanging out under the bark of the vine? Are they inside of shoots? Are they out in the environment and get blown on?


Hans C. Walter-Peterson  5:43 

They're pretty ubiquitous in the environment, talk about a lot about Native fermentations and yeast coming in from the vineyard. So there's so they're there. And the bacteria are as well, I don't know, it's some of the exact overwintering mechanisms. And if we know all about that, somebody probably does, I just don't, but it's my understanding is they're they're pretty native in our neck of the woods. They just, they're they're pretty much all the time.


Craig Macmillan  6:05 

Are there environmental conditions that are particularly conducive to promoting Sour Rot. And then also are there environmental conditions that will prevent it or retard it?


Hans C. Walter-Peterson  6:16 

So the big thing that gets sour rot going is for some way for the microbes to get into the berries in the first place. Predominantly in grapes, we think about that as either being insects, birds, or water. Here in the east, obviously, we get rain throughout the growing season, including during the harvest season, we have high humidity days, plenty of times. And so those are the kinds of conditions where we see greater incidence of sour rot develop. When the vines take up water, or the berries take up water either through rainfall or just the atmosphere, and then the berries swell up, they can't handle all the water they have and they split or you have a very tight clustered variety, that just the berries start getting forced apart, and they just break by force. So those entry wounds however they're caused, is how it gets started. So we know here in New York that if we have a dry fall days, with not many days with dew points above 70, and all those kinds of things, we don't see very much sour rot develop, we might see a little Botrytis here and there. But for the most part, we don't see it. And a lot of that is because we just don't have the humidity to kind of build up the water in the berry to cause it split the years where we have it bad. On the contrary, that's that's when we see more water, more rainfall, more high humidity days, that's when we see more splitting and therefore more sour rot. Much like most other diseases, the warmer it gets, the faster it can progress. And the same thing with insects, the fruit flies at a at a lower temperature. It takes them longer for a next generation to develop. And so the warmer it gets, they get faster too. So yeah, so warm and wet.


Craig Macmillan  7:55 

So cool and dry would be the opposite would be the desirable.


Hans C. Walter-Peterson  7:59 

So that would be best.


Craig Macmillan  8:01 

That actually that just reminded me of something. My experience has all been on the Central Coast California. This only happened once. And that was with some Pinot Noir that came in that had quite a lot of Botrytis damage. And the winemaker had us go through and sort then not simply sort out Botrytis and throw it away, but by hand sorted and then smell it for sour is something like Botrytis or a scar from powdery mildew or something like that. Is that Is that also a possible entry for the organisms?


Hans C. Walter-Peterson  8:33 

Yes, very often we see Botrytis and sow rot in the same cluster. Because it's the same thing. Botrytis is a very weak pathogen, it needs a place to kind of get established like a wound. And so same thing with sour rot. We do know that, like you're just saying powdery mildew scars can create micro fissures in the skin. And later on in the season, those can start to tear apart even if you can't see them, especially around the pedicel near the stem where the stem connects to the berry. They're going to be micro fissures that those micro organisms can take advantage of as well. So those conditions are pretty similar for for other kinds of rots as well.


Craig Macmillan  9:11 

Are there cultural practices or preventative or prophylactic practices that growers can use that might help manage this?


Hans C. Walter-Peterson  9:17 

Yes, probably the biggest one that we know of and we're trying to get a little bit better handle on as far as how to use it for this purpose. So we know that if you pull leaves before bloom are right at the very beginning of bloom, you will reduce berry set you basically kind of starve the the clusters, the flowering clusters of carbohydrates and other nutrients and so they don't set as many berries. You have a looser cluster. Those clusters don't swell up they don't like I was talking before kind of force berries off, they dry out faster. All the good things we like about looser clusters pulling leaves at that very early, just pre bloom or very early bloom stage can reduce berries set pretty consistently year in and year out. out and help to reduce that cluster compactness aspect of rot development.


Craig Macmillan  10:05 

I think it's the first time I've ever heard of a intentional shatter. Usually we're all we're all praying that we don't have what you're describing.


Hans C. Walter-Peterson  10:14 

Yeah. Well, I mean, you think about table grape growers do this fairly often, they try to make more room on the cluster so that they can have larger berries, which consumers want. And so we're not worried about it. obviously, for consumer sentiment, we're worried about that for disease pressure, there's definitely a cost to it. You're reducing your yield as a grower from the standpoint of just how many grapes you're going to carry. But you also might be saving more yield later on in the year and not having to drop fruit before you send it off to the winery


Craig Macmillan  10:40 

In your area. You've got wine grapes, obviously, but also there's a lot of Concord production there. And is it mostly for juice is that right?


Hans C. Walter-Peterson  10:48 

Mostly for juice, yep.


Craig Macmillan  10:49 

I'm assuming this problem applies there as well.


Hans C. Walter-Peterson  10:52 

Concords really don't get sour rot very much, partly because their clusters more open, they don't set a tight cluster. If you think like a Pinot Noir cluster, or Chardonnay, or Riesling, they're much more loose like that. They also have much thicker skins, so they tend not to split quite as easily, they can still split, but we tend not to see sour rot develop on them. And I, I'm not totally sure why that is. But part of it from at least on a production level, a lot of our Concord gets picked before it gets much more than 16, 17 Brix. We know with sour with sour rot, we don't see symptoms start to develop until you get to 13 or 14. And I think that's partly a result of just how much sugar is in the berry, but also the relation of sugar and acid because microbes can't tolerate a certain acidic level of environment also. And so this is kind of an educated speculation right now. But I think that's part of the reason we don't see it in something like Concord and Niagara and some of these these juice varieties is that we pick it at a relatively low Brix, as opposed to wine varieties where we're picking 20 Plus.


Craig Macmillan  11:57 

Right, right, exactly, exactly. Continuing on the cultural thing. I one thing that growers do for both try to fend for grape powdery mildew. They may go through and they may drop infected crop when they first see it. This sounds like this gets spread around, can you crop drop with this and control the spread?


Hans C. Walter-Peterson  12:17 

The challenge with this is if and I've seen this happen in a couple of places. If you drop crop that's starting to rot and just leave it on the ground near the vines. What does it do when it's on the ground? It continues to rot. Right? It doesn't it doesn't stop and the fruit flies can easily go from the ground back up to the canopy and back down to the ground back up to the canopy.


Craig Macmillan  12:35 

Find another Fissure or whatever.


Hans C. Walter-Peterson  12:37 

Right, exactly. So another part of the challenge that is ideally you're not just dropping the crop and leaving it there in the vineyard, you got to kind of take it out so that it's not around that healthy fruit. Because otherwise those microbes will be back. You know, they get blown around on wind again or carried by fruit flies. And they'll find another fissure to get into.


Craig Macmillan  12:57 

Can you cultivate it? Can you can you tell it under?


Hans C. Walter-Peterson  13:00 

You probably could. Yeah, we don't do that much tillage in our in our vineyard rows just because we have all the rain we maintain cover crops between the vineyards all year round. Otherwise we'd slide all over the place.


Craig Macmillan  13:11 

Yeah, no, absolutely. Of course. Yeah. I've talked to Justine Vanden Heuvel about undervine cover cropping and things and I was like, This is crazy. Going to California perspective. That's nuts. And she was like, Craig, you have no idea how much water is in the ground. It would be a mess if we didn't which is which is really interesting. So okay, so that's not gonna work. Do we have anything in the chemical realm for prophylactic sprays?


Hans C. Walter-Peterson  13:33 

Prophylactics per se not so much what we've been looking at lately, a colleague of mine out on Long Island Alice Wise for about three or four years now we've been looking at a couple of materials that are designed to enhance the cuticle thickness around the berries basically as a way to try to see if we can prevent cracking. One of them was originally developed to reduce cracking and cherries.


Craig Macmillan  13:54 

What materials are we talking about?


Hans C. Walter-Peterson  13:56 

So the material we've been working with is a combination of materials, some waxes and carbohydrates and some other things that kind of just bind to that cuticle around the berry and just thicken it up. Literally from everything I've seen, it works in cherries to prevent this cracking. We've been looking at two versions of these, this material they both developed at Oregon State actually one produces a thinner cuticle and other one produces a much thicker one. And we've tested them both. And we haven't seen any difference in sour rot from using these materials. Now we've had kind of some kind of weird years when we've been testing this, we've had a couple of years where we had a lot of sour rot and a couple of years where we had almost none. So it it hasn't been the best time to be testing this. But in the two years that we've had sour rot, it didn't seem to do very much in the way of reducing it to the point that you could justify spending, you know the time and effort to do it. The only other kind of sprays that we're looking at at this point are things like hydrogen peroxide and proxy acetic acid, then there's some there's some commercial products that are out there that contain one or both of those ingredients. And those are basically just antimicrobials I mean, they they burn whatever they touch. You know, same thing like when you get a cut on your arm, you put hydrogen peroxide on there it disinfects. So that's basically what we're doing for the microbes. And it works pretty well. The key always is coverage, because it has to hit it. As soon as that material hits, hits that grape hits a microbe, whatever, it starts to convert to water, basically those those materials, if you don't have good coverage, if you can't get the material to where the microbes are hanging out, it's not going to be terribly effective. And so that's the that's always the challenge with those kinds of things. But they they do work to the extent that they can reach.


Craig Macmillan  15:36 

To some extent, yeah, and again, this is going to be another issue with cluster architecture. Obviously, this is terrifying. As I'm sure everybody in the state of New York and elsewhere, certainly not limited to New York, New York, as far as I know. Okay, now I've got it. It's getting started. Maybe I caught it early, maybe I didn't know what what can I do?


Hans C. Walter-Peterson  15:59 

The standard treatment that we have at this point is that either when you get to that 13, 14 Brix number or you start to see it show up, and most growers will wait until they see it show up. The standard practice is basically to start this combination of an antimicrobial and an insecticide to kind of keep it under control and try to keep it from getting to that explosive stage. The challenge with that is that fruit flies under the right conditions. And if it's above 70 degrees or so they're generation time is every six to seven days.


Craig Macmillan  16:33 

Oh, wow.


Hans C. Walter-Peterson  16:33 

New generation of these things at their at their utmost or at their best. Essentially, we need to be spraying every seven, eight days to try to keep this under wraps. What we've found, and this is more good news, what we found is that we are identifying a lot of populations of fruit flies here in New York, not just in the Finger Lakes, but in some other areas that we've been testing to where their fruit flies have quickly developed resistance.


Craig Macmillan  16:59 

That's how they do it, isn't it.


Hans C. Walter-Peterson  17:00 

And so what we've seen is that basically the fruit flies have developed resistance to a couple of these materials. We've tested them on a couple of different pyrethroids, a couple of organophosphates, a couple of other materials and found pretty high levels of resistance in the lab, at least, when we've tested them. It has pointed out to us very quickly that this is not a problem that that chemistry alone can solve. All right, there we go. Okay, that's kind of leading us in the direction of maybe not necessarily replacing chemicals completely. It'd be nice if we could, but at least supplementing some of these other cultural and non chemical practices like the leaf pulling, I was mentioning earlier to try to reduce the need for those sprays, if, again, if not eliminate it all together.


Craig Macmillan  17:42 

And so what kind of research projects do you have going right now on this topic?


Hans C. Walter-Peterson  17:44 

We've got a few that we're that we're kind of looking at, again, kind of tackle this from a couple different directions, we're doing some a little bit more work on that leaf pulling aspect, we've done some work, my colleagues and some other people in around the country have looked at mechanical leaf removal at that pre bloom stage and found that it works pretty well as well. There are certainly hormonal sprays that can be used. We mentioned with like with table grapes, tuberculinic acid can be used to to kind of stretch the racus and give the berries more room basically. So it kind of reducing that cluster compactness. And one of the things that I'm particularly kind of interested in and excited about is the potential for UV light to play a role in this.


Craig Macmillan  18:25 

I am curious about this UV light thing, I'm hearing more about it and I'm getting kind of excited.


Hans C. Walter-Peterson  18:31 

UV light is basically just another sterilant that we use. So almost all of our wastewater treatment plants have UV light to sterilize the waters that's coming through the plant. So it does the same job that these hydrogen peroxide peroxy acetic acid materials do, but we don't have to worry as much about coverage. If we apply it right. One of the pathologists here, Dave Gadoury, has done a lot of work on using UV light to control different plant diseases in grapes, normally powdery mildew, which is very effective against, but one of the things that they found kind of along the way is that they were also reducing sour rot in this test plot that they were working in. And so if again, if you kind of think about it, you're if you use the right dosage and the right retention time and da, da, da, you're basically have an antibiotic material, but it's not a chemical. It's a physical one, I'm very interested in looking at the potential for UV light to not only control powdery mildew, which would be a lovely thing, which is, but also can we use it to minimize the sour rot incidents and those microbes that are causing it, as well. So we've got a small trial is kind of a proof of concept thing we've done last year, and now this year, if it works as well as it did last year, we're going to kind of try to expand that work a little bit further and try to see how do we incorporate that into a potential grower practice, you know, how, how often do you need to do it? What's the what's the light intensity? Do you have to do it a day or at night, which is one of the considerations you have to have. So There's a bunch of things that we still need to look at, to turn it to make it something that growers can be really rely on as a potential possible part of this solution. That UV thing is really kind of exciting to me. We also are a little further down the road, we're really trying to work on with some folks at Penn State and a couple of other places on developing a model based on climatic conditions that promote sow rot. So it just kind of can we predict when it's going to be coming, if we know that we're going to have five days of 80% humidity or whatever, there was actually just a really interesting study that's come out of Uruguay that I just heard about a couple of weeks ago at the GiESCO conference that was held here in Ithica, where they saw an impact on bunch rots, they were looking specifically at Botrytis, by having undervine cover crops, where they had those underground cover crops, they saw less Botrytis and less bunch rot than they did where they had like a weed free herbicide strip. So that's something I'd like to follow up on as well, I'd be curious about and then kind of the I won't even say sci fi because this stuff seems to come along so quickly. Now. We work with a couple of really wonderful pathologists and engineers here at Cornell, I was talking to a couple of them about this last year. And they said, I bet it'd be pretty easy to develop a sensor that we could stick out in the vineyard that could detect acetic acid far earlier than any nose could and just be like, Okay, here's your early warning. You know, it's kind of an early warning sensor, it's starting to develop, let's go find it and and try it, see if we can prophylactically take care of it early on. So there's just some some things that we're starting to bandy about as far as kind of further down the road. But I do think kind of the immediate thing that I would really like to are trying to put together is can we take the practices like UV light, loosening cluster architecture, changing cluster architecture in order to reduce that environment that's promoting sour rot? And then also try what can we do on the chemical end to reduce the need for those sprays?


Craig Macmillan  21:50 

Right, right. So there's some stuff coming down the pike here, that's really good. That's really, really great. And thank you and everybody else who's working on this. How big of an economic impact is this for folks?


Hans C. Walter-Peterson  22:00 

It can be one of the most significant economic diseases in grapes. In 2018, we had a particularly bad year here. And I know some growers who had to drop almost half their crop of Riesling on the ground before the harvesters came through. And so if you think about a three to four ton average crop, that's a few $1,000 an acre that you're losing. So I mean, no diseases are good. But I mean, that's a pretty profound one. And again, as I was saying earlier, the thing that's so hard about it is that you've already put almost all of your work and money into that crop all the way from pruning to spraying and all the handwork and everything. And then in a bad week, to all of a sudden, just as somebody called it go to snark my favorite descriptions of sour rot seems like the perfect word for it. It's just it's a really kind of a, obviously financially, but kind of almost as much emotionally devastating feeling.


Craig Macmillan  22:57 

If there's one thing, message piece of advice. One thing that you would tell growers on this topic, what would it be?


Hans C. Walter-Peterson  23:05 

I'd say probably the biggest and easiest thing you could do right now, to reduce sour rot is that early leaf pulling, we just know that cluster architecture, it makes a big difference in how much rot develops, you might still get some, but it won't be nearly as profound and prolific as it would be otherwise, we have just as a very quick example of it, we have a hybrid variety here called Vignoles we use in all of our sour rot studies, because if you just say the words and it gets sour Rot. Some work that's been done by some colleagues of mine, and some folks at USDA, they basically come up with, they've created two loose clustered clones of Vignoles and so those clusters, obviously, are much less compact than the kind of the standard one. And the amount of disease that is in those clusters is drastically lower than what's in kind of the standard, the standard clone of Vignoles. It's one of those things that just kind of is really illustrative when you see it and just kind of realize that, you know, again, you can still find a few berries here and there that'll have it but you just won't see this entire two panel stretch that's just kind of wiped out by it or whatever doing that that leaf pulling to kind of open up the clusters, I think is probably the right now the biggest thing you can do.


Craig Macmillan  24:19 

Interesting. Well then we're running out of time. I want to thank our guest, Hans Walter-Peterson viticulture extension specialist at the Finger Lakes grape program, part of Cornell Cooperative Extension. fascinating conversation, keep up the good work. I think a lot of people are depending upon you.


Hans C. Walter-Peterson  24:38 

We're doing what we can see. It's becoming a bigger and bigger problem with climate change around here. We know we've seen it increasing in recent years. So yeah, it's it's one we'd really like to get our hands around better.


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