IPM Conference Call: April 24, 2012, with Phil Schwallier from MSU Extension

Today we were joined by Dr. Phil Schwallier, a horticulturalist and entomologist from MSU Extension. He is an expert on fruit thinning and nutrition, and took nearly the whole hour with us to discuss the issue of thinning in this difficult season. Find the recording and full transcript below.


Download the recording of the April 24 call here.

Topics / Table of Contents

  1. Updates on scab – rainfall and scab infections
  2. Update on insects – Two-spotted spider mites
  3. Insecticide applications during this year’s bloom and post-bloom
  4. Visit from Phil Schwallier
  5. Thinning techniques for this season
  6. On determining which fruit will and will not develop
  7. On  maximizing next year’s bud formation
  8. On predicting return bloom, and reasons for low return bloom
  9. On timing of bud formation
  10. On fruit quality in lightly-fruited trees
  11. On pruning to keep costs down
  12. Nutrients: boron and zinc
  13. On calcium and tree nutrition
  14. Using NAA for thinning
  15. On techniques for de-fruiting trees
  16. On root pruning to control vegetative growth

Resources Mentioned in the Call

Articles on boron and zinc:

Call Transcript, April 24, 2012

Updates on scab – rainfall and scab infections

John: I need to update you guys on the difference between scab infection periods and your rainfall. The question: what happens to the ascospores if it rains but you don’t say wet enough for long enough to generate an infection period?

The answer: the spores are shot out at the beginning of the rain event, and if it dries off too soon – before that germ tube can grow down into the leaf – then the spore dies, and you’re home free. That’s the ideal situation: to have fairly frequent rains that dry off quickly so that the spores are continuously being shot out as they mature but never given the chance to infect the leaf.

Update on insects – Two-spotted spider mites

John: I’ve found Two-spotted spider mite at several orchards in the last three days. This is somewhat alarming. We normally don’t even have a problem with these on tree fruit, except in some hot, dry years, later in the summer.

There are high numbers – way over threshold – of Two-spotteds in some orchards. My only guess is that the heat in March got those things cranking. In orchards with a nice herbicide strip, there was basically nothing for them to feed on, forcing some of them to crawl up into the trees.

They’re harder to see about European red mites. Basically, they spread throughout the tree, and they can do a lot of damage, damage that you wouldn’t see until it’s too late.

The best way to look for Two-spots is to pick off a cluster, turn it upside down, and look along the midrib of the undersides of those leaves. If you have Two-spots, what you’ll see from a distance is subtle white-ish or lighter areas. They may be the size of a dime or smaller. That whitish coloration is from webbing. With a hand lens, you’ll see some whitish-yellow mites in those areas.

I don’t know how widespread the Two-spotted problem is, but they could really cause some problems if they’re not controlled. As I said, there are several predator mites out there that I’ve never seen before that are working on some of these orchards. So, not only did the Two-spots come up into the trees abnormally soon, but so have the predator mites that feed on them. So if we don’t kill the predators, maybe they will take care of the Two-spots and my worries will be moot.

However, I’d recommend that you scout for them in the next week, particularly if you have an herbicide strip. If you don’t have an herbicide strip, you’re probably less likely to have this problem.

Insecticide applications during this year’s bloom and post-bloom

John: There is one thing that you can do as an exercise. Take a look at the article by Art Agnello in the most recent Scaffolds issue and see what you get out of it.

I disagree with him. He talks about the petal fall insects and various ways to approach their control. He concludes by recommending that people do something that I think is very unwise: he says that you need to start by using a broad-spectrum insecticide as a complete cover at petal fall, and contradicts what he has laid out as potential problems with such an approach when it comes to things like Plum curculio.

Between now and next week, if anybody removes their pollinators and doesn’t have enough blossoms on there to have to worry about native pollinators being out there, this is a great year for doing a perimeter spray: that might be an OP, Indoxacarb, or Avaunt, or Pyganic for organic growers. The idea here is to create a border around the perimeter that is clean of Plum curculio and give yourself some insurance that PC won’t move further inside those blocks over the next week or so. We know the movement will be slow with the forecasted temperatures. But sometime soon, we will have some hot days and warm nights, and we’ll need to be concerned about PC activity levels.

However [in response to Art Agnello’s article], it doesn’t make sense to go out and do a full cover of a broad spectrum insecticide. Even if you’re past bloom, you still have a whole lot of beneficials out there. It’s counterproductive unless you’ve scouted for and found some other pest problem (other than Plum curculio).

Visit from Phil Schwallier

John: We’re going to talk with Phil Schwallier from MSU Extension, who agreed to come on to talk about thinning, tree health, tree physiology, and so forth.

Thinning techniques for this season

First question: There are going to be a lot of blocks across the region that don’t receive any thinner at all because of the light crop. But, this season, how long should one wait before thinning trees, if they need it? What should we know about thinning, in general?

Phil: Well, I assume that you guys are near the same growth stage as we are. We’re just coming out of bloom here in Grand Rapids. Some early varieties are at 6mm size, but most things are just in petal fall. Are you similar to that?

John: Yeah. Most of the area usually lines up pretty well with the Grand Rapids area. The people further north, toward the Twin Cities, are not quite there yet – they might be more still in bloom. But I don’t see much fruit that’s at 6-7mm, yet, anywhere.

Phil: Okay, then you probably have a lot of frost damage like we do over here?

John: Very much so. Widespread.

Phil: Okay. Yeah, we’re very concerned about the frost damage. We have some blocks in which 50% of the buds are burned, browned. But we can still find plenty of green bud, which is sort of amazing. If those green buds set, we will probably have to thin on some blocks. Most blocks we will not have to thin at all. I assume that that will probably be the case for you guys, too.

Over here where we have higher sites and frost protection – wind machines and things – those sites look quite good – maybe 10% bud damage from the frost. We’ve had 17 nights where they’ve had to run the frost fans. Most of those nights, they had to run the fans between 6 and 9 hours through the night, as well as burning brush and bales of hay to try to warm things up.

So, I’m concerned. Up until the last week, the trees really looked terrible. Last week, when we finally got a couple warm days, the trees were looking yellow and peaked, and they were showing signs of nutrient deficiency, mainly because it was so cold for so long. Now it’s cold again, for us anyway. I know that the jet stream further to the west separated the region into a warmer side and a colder side, and I think that you’re on the Michigan side, the side with a colder climate right now.

John: Pretty much, but in the last week, the trees have really perked up with a little bit of warmth. There was a lot of tree stress apparent in the leaves.

Phil: That will continue. The trees will continue to look better once we get some warm temperatures and more rainfall. Here, we’ve been on the dry side.

Judging by the looks of the trees, I think both you and we have to be very cautious about thinning. Use probably no more than moderate rates at this point. I think they will thin easily. We know that when we have frost damage in pink, bloom, and petal fall, the fruitlets thin more easily because they’re already weakened, and the leaves are probably damaged as well. They don’t have a lot of strength.

The carbohydrate model shows that there’s no stress right now, and there won’t be for the next 10 days, while the cold weather stays with us. But the carbohydrate model does not know how to handle frost; that’s not a part of the model. The model’s prediction is understandable: cooler days will reduce stress and sunny days will increase photosynthesis, so there’s no stress on the trees. That’s a good thing, and maybe we’ll get some fruit to set.

I do have a concern – and you guys will have to judge your area for yourself: we got great pollination for 2 or 3 or 4 days. Even on windy and cold days when the bees weren’t supposed to fly, they were out there working the bloom, I think because they were starving to death during the cold weather. On the warm days in the last week, they were working the bloom like mad: they had 5-10 bees in a tree, and managed to pollinize the tree in 5 minutes. You could smell the nectar in the trees. The beekeepers over here are telling us that they’re pulling out a lot of pollen. So the bees are really working.

I think the bees’ pollination was adequate, both for you and for us.

However, I’m concerned about fertilization. Once the pollen has been delivered to the stigmas, the pollen tube has to germinate and grow down into the bottom of the pistil and has to fertilize the ovules. Old-timers will say that if it’s cold or extremely hot during bloom, your fertilization will not be adequate. That’s another reason that the fruit will probably thin easy. We’re starting to see early drop here in Grand Rapids, where you can brush a whole cluster of buds in petal fall right off the spur.

The message: be cautious about thinning. Wait until you get to 10mm, probably 12mm, and see what you really have setting. Cut open the fruit. If you have brown tissues in the fruit, they will be easy to knock off. If you still need thinning at that time, go with a moderate rate of, maybe, Sevin plus a low rate of NAA on the difficult varieties. Or maybe just NAA alone, or Sevin alone. With MaxCel: you typically want something to go along with MaxCel in order to get a response. On some varieties, you probably can thin with 150ppm of MaxCel, alone. It’s only a mild thinner by itself.

John: But in a year like this, might MaxCel alone be worth trying?

Phil: If you need thinning, I would probably look at Sevin or NAA. But if you need only moderate thinning, then 1 pint of Sevin + 50ppm of MaxCel will give you mild-to-moderate thinning.

In general, I think you should wait, and I think you should be cautious. I think you need to wait and see what’s setting at 10mm. You probably only need to thin the tops of trees, or just your very best sites in your orchard – maybe where you have frost protection. Those sites appear to be good right now, but they still could have fertilization problems. So, be cautious, and make sure you absolutely need thinning before applying a thinner.

In cold years, typically, you can thin later and still get response. On hot years – like last year – you have to thin earlier, because they go out of the thinning window very quickly.

Your orchard probably has some unique sites: places with frost protection, tops of trees, etc: make sure that those need the thinning. The bottoms and frosted locations will probably need no thinning.

Then, there will probably be a varietal difference. For example, over here, it looks like our Ida Reds have set pretty well and will need thinning. So will Gingergold. Galas look like they’re going to set pretty well over here. And I say all this with the caveat that fertilization is an unknown: if there’s no fertilization, we’ll get a fairly heavy drop in the coming days, and then we’ll probably get another drop in June.

So, caution is what I would suggest. Wait, but don’t wait too long to pull the trigger, because once they go beyond the thinning window, then it’ll be down to hand-thinning.

John: In our forecast here, we have only a couple of days with highs in the 60s. Otherwise, it’s supposed to be pretty cool for the next 7-8 days. Even though some of these early-blooming varieties are at or past petal fall, it’s going to take a while before those fruit size. It’s going to seem like it will take a long time to thin, anyway. The Honeycrisp are still way too small to thin. So, the waiting will drag out for a long time if we continue to have these cool temps.

Phil: Absolutely. We have the same thing over here.

You won’t be able to thin until it gets warm, which will be at least 7-8 days from now. I talked to you about considering petal fall thinning. But, heck, the temperatures aren’t going to allow that. You’re at petal fall now, and it’s way too cold for thinners to do anything.

in these conditions, the fruits will probably grow at the most 0.5mm per day. That means that in 7-8 days, you’ll be 4mm bigger than you are now, meaning you’ll probably be at 6-8mm, total, a week from now. If you have some warm temperatures at that point, that would be the ideal time to do something, even if you just thin the tops so that you don’t have to hand-thin the tops.

John: That’s an interesting idea. Not all areas show a height differential in frost damage. Most do, probably, but in a lot of cases – particularly with small trees – the damage is all the way to the top.

Phil: Okay. I’m talking about vertical-axis type trees, central leader-type trees, of 12-14 feet.

On determining which fruit will and will not develop

John: About the apples themselves and testing the apples by pinching or cutting them to look at the seed compartments:

Apples that are brown range from having seed compartments that are total mush to having easily-visible individual seed compartments, but the seeds are mush.

Other apples have a gradation, where the seed compartment is still intact, some of the seeds are still green and other seeds are brown.

In that latter situation, even if the fertilization is poor, I’d imagine that we’d most likely get an apple there. Do you agree?

Phil: You can have an apple where there are brown parts in the fruit. You can even have an apple where the pistil is dead. 90% of the time, that’s not the case; those will typically fall off and be very easy to thin.

John: Okay, so any place where you have a considerable amount of that condition, what do you do?

Phil: Unless you find that in the tops of trees, walk away from it and wait and see what grows.

On  maximizing next year’s bud formation

John: What dangers are we seeing, in terms of tree health and getting next-year fruit bud formation? You were saying that the carbohydrate model doesn’t show any stress over the next period of the time. With a light crop, and without warm, cloudy weather, you would think that the model would indicate low stress. So in what ways could we, as managers, screw up fruit bud formation over the next 6 weeks?

Phil: From my point of view, two things are important for next year’s flower bud formation.

The first is fertility and nutrition. You want a healthy tree. So you want your N levels to be adequate, around 2%. I would assume that most of you have pretty good nutrition on your trees. You have moderate vegetative growth.

The second is crop load. This is where you are most likely to screw up next year’s bud set. This is probably the most important factor, if you consider that everybody’s nutrition is fairly good. If there is a heavy crop load, you reduce return bloom.

But I’m not worried at all about return bloom for next year, except in cases where we might set too many apples in the tops of trees and reduce return bloom in the tops, or if you don’t thin trees that still have a good fruit set.

In general, crop load will be the major factor in a frosty year. The problem is that next year, you’ll have such a snowball bloom that you start getting into a bi-annual bearing cycle – one year on, one year off.

On predicting return bloom, and reasons for low return bloom

John: We had some tree stress last year, with a lot of warm, humid nights where respiration had to continue without any sugar production. So a lot of things ripened late. In a lot of those blocks that were light last year, growers expected there to be a pretty heavy return bloom. But there wasn’t, in many cases. Whether that was due to the extra tree stress last year or not, I’m not sure. Do you have any ideas?

Phil: No, I don’t know. We had the same thing over here, where we had a light crop last year and we thought we would have a better bloom this year. I blame it on climate, I guess, which we don’t have too much control over. I don’t know why the bloom didn’t come back; it should have. We had a couple nights in the winter when it got pretty darn cold – down to 5 below zero in January and down to near 0 again in February  – after all the warm weather. Those cold snaps may have killed some of those flowers.

John: That’s a good point. The chilling-hour requirements were met on these trees, and most of those were probably met by January 1. Cold temperatures like that could certainly have played a role.

Phil: The only other thing that could have been at work: if the trees were low in nitrogen or something else, then they might have been more prone to injury.

On timing of bud formation

John: Can you give us a ballpark figure: are the initial stages of fruit bud formation concluded about the time that shoot elongation stops?

Phil: [Inaudible] it’s different based on every variety. In most varieties, flower bud formation starts just shortly after thinning, maybe in late May. It is typically done in July, somewhere around July 10. But in some varieties, you can continue to get flower bud formation into the summer. We’ve eve en seen that some varieties can be encouraged to switch in the Fall and get more bloom based on some Fall conditions and Fall treatments.

It’s mostly in May, June, and the first part of July in a typical variety. But I’m sure that different years and different climates and different varieties will lead the trees to respond differently.

John: The reason I ask is because if the crop turns out to be a little better than they suspect right now, and they haven’t applied a thinner to the frost-damaged blocks, then they may have to go in and do a little hand-thinning later on. So the question arises: how late is that effective? Obviously, it depends on when those fruit buds are being formed.

Phil: Hand-thinning is effective any time of year, all the way up to harvest – not for return bloom, but for fruit quality at harvest time. Hand-thinning usually doesn’t promote more than 10% return bloom. It’s chemical thinning that really promotes return bloom. So to promote the most return bloom, use NAA in the summertime.

John: So the organic growers really don’t have a tool?

Phil: That’s right, they don’t have a tool, except to blossom thin with lime sulfur.

John: That wouldn’t have been recommended this year.

Phil: Well, they use that for fungicide, too.

John: Right, but I didn’t recommend that anybody use lime sulfur during bloom for scab. It’s not worth it with the damage that was out there. But I understand that that’s about all organic growers have for thinning.

On fruit quality in lightly-fruited trees

John: Regarding fruit quality on lightly-fruited trees: if you only have 10% of the apples that would normally be on a tree for a reasonable load, and you decide not to de-fruit it, and you carry the tree out to harvest, will those apples likely have a shorter shelf life, need more calcium, need less calcium, or something else?

Phil: They will be a problem child, most certainly. You’ll probably need to spray them all summer long with calcium. You probably won’t overcome bitter pit, if that’s a problem. You’ll probably have internal problems.

John: Could you explain why having such a small crop on a tree leads to those fruit problems?

Phil: When you have a small crop, usually the tree converts into a more vegetative growth pattern. The vegetative growth is favored, which makes it difficult for the fruit to get adequate nutrients – especially calcium. Then, if you go into harvest time with inadequate calcium – even if you sprayed it all year long – you’ll usually have a 3-4” apple with little or no shelf life. It’s just a problem fruit, and it would be best to get rid of them as soon as possible in the Fall, if you can take them that far.

Phil: Applying Apogee will help with fruit quality in the Fall. It controls vegetative growth and reduces the “stealing” of nutrients from the fruit. It will also reduce russetting. You will need more Apogee on a light-crop year.

But ProVide is much more valuable for reducing russet.

John: Growers who use Apogee annually for shoot growth control as well as Fire blight control will probably use Apogee anyway, because they’re in that mode. But many other growers who are looking at a light crop, and even no crop in some cases, will be trying to cost-control pretty severely. So, I suspect that most people will probably not put Apogee on these lightly-cropped varieties.

Phil: That wouldn’t surprise me, either. If they haven’t started Apogee by now, they’ll just have more vegetative growth. If they’ve been using Apogee in the past, I will tell people that apples have a mad, pre-response (withdrawal symptoms) to going off of Apogee. When you have trees that have been sprayed with Apogee for 2-4 years in a row, and then you decide not to spray Apogee, the tree releases all its pent-up frustration in excessive vegetative growth. They grow an extra 50% of vegetative growth compared to the tree that hasn’t been treated at all. You’ll get 3ft of growth where, in an untreated tree, you might get 2ft of growth. They’ve been held back all these years, and now they’re release from the Apogee bondage. This extra growth will be even more dramatic in a year like this year, because it won’t have the fruit to hold down the vegetative growth.

What growers need to do is apply more Apogee in a year when they lose their crop. In terms of reducing vegetative growth, a normal crop is worth 60-70% of Apogee with no crop. The cropload is the “cheap Apogee.” Having the cropload controls vegetative growth. When you don’t have the cropload, you need Apogee to replace it.

So, if they have no crop, they probably increase their Apogee 50% over what they had normally been using in the past (as a seasonal rate). If they have a partial crop, increase by 25%.

On pruning to keep costs down

John: If growers are trying to keep costs down on some blocks, I would imagine that there’s nothing that they can do manually – for example, late-summer pruning – that is going to make any difference. Is that true?

Phil: Well, yes, they could do summer pruning all summer long, if they wanted to do some corrective pruning. Summer pruning after July 1. By doing that, they will finish a lot of their dormant pruning completed early and have less to do next winter. That will help with controlling the growth of the tree.

John: If they don’t defruit some of those blocks, it would also help from an IPM standpoint, because if they get 3ft of shoot growth with a 10-25% crop, by late-July and August, it will be pretty hard to get into that canopy with any kind of protective spray.

Phil: Yeah, they’re going to really take off with no crop on there.

Nutrients and fruit quality: boron and zinc

John: Many growers tend to put boron and zinc on every year. Those nutrients are therefore not so dependent on fruit load. Is that correct?

Phil: Yes, there is a good article out of Scaffolds by Ed Stover (see here and here and here). He talks about boron, zinc, and nitrogen to help fruit set, especially in frosty years.

If you don’t have a crop, you can probably back off boron and zinc, and most certainly nitrogen as well. You probably shouldn’t back off on calcium.

You also need to maintain good tree health. You can usually measure that based on terminal shoot growth. You want to target around 16-18” of terminal shoot growth. With no crop, it will probably be 30”.

So, you probably don’t need much in the way of nutrients – except for calcium – because the tree isn’t carrying a heavy crop. Trees will always be short of calcium.

On calcium and tree nutrition

John: Has anything changed in calcium research/knowledge in terms of better formulations for getting the calcium to go into the apple itself?

Phil: In my opinion, no, nothing has really changed. Some companies have new formulations out now. I haven’ t tested them. There are no silver bullet calcium formulations out there that I’m aware of.

If you have a light crop on Honeycrisp – say, a 20% crop – and you spray calcium all summer long: you will reduce bitter pit in the fall, compared to an untreated tree. The untreated tree might have 30% bitter pit this year, and the tree that you spray all summer long might have 20%. It does reduce the problem of bitter pit, but it doesn’t cure it. Nothing that I’m aware of will cure it.

Certain years, blocks, and varieties will overcome the applications of calcium, and those factors are out of your control.

So, spraying Apogee, spraying calcium – all of it will help reduce bitter pit, but they won’t cure the problem.

John: If it’s a dry summer, we will have more problems with bitter pit, is that right?

Phil: That’s right, yes.

John: Well, some people do irrigate, and those people are better set up to survive.

Phil: Yes, the climate is important. A lack of rainfall will exacerbate bitter pit. So, even if they have irrigation, if it’s still hot, they will reduce the bitter pit, but it’s still not a silver bullet. Perhaps over-tree sprinkling would help, but we don’t do that here in the east.

John: No, I think that would be counterproductive in the long run.

Phil: Out west they do that, but they don’t have the scab.

Using NAA for thinning

John: What do you mean when you say to use a low rate of NAA in thinning? Say you’re using a pint of Sevin, how much NAA would you recommend using on something like Macintosh or Cortlands?

Phil: Those are typically easy-to-thin varieties. Oftentimes, what we do here is spray with Sevin and/or we use NAA at 10ppm. Maybe, if the fruit is setting well, 15ppm.

But on a year like this, we’d probably use 7-8ppm. We’d probably put in no Sevin at all.

In a heavy year like last year, a pint of Sevin plus 5ppm of NAA did it. Easily-thinned varieties typically don’t need much. If you over-thin them, you typically get pretty large fruit. So we approach those varieties pretty cautiously and with a mild to moderate thinning program. We’re not very aggressive on those at all.

On techniques for de-fruiting trees

John: Say a grower wants to de-fruit a block because the crop is so light that the cost of protecting from direct fruit pests is too high for that small number of apples. Are there better and worse ways to de-fruit a block? What is the best timing?

Phil: The best timing would be right at the 10mm or 12mm stage with temperatures near 80F. We’d like to have three days of warm temperatures after our spray application. If you can find that window, probably the combination of between 1 pint and 1 quart per acre + NAA at 15ppm will work.

If you’re on Honeycrisp, some people like to use a 6-BA (like MaxCel) with the Sevin instead of the NAA because NAA can sometimes be a harsh thinner. But MaxCel at 150ppm + Sevin will do the best at reducing the crop load.

Now remember, if the crop is already light at that time, the fruits will resist thinning, because they have no competition on the tree. If they’re injured by frost, then you might be able to knock them all off.

John: So, the earlier that decision is made, the easier it will be to get the fruit off?

Phil: Yeah, 10mm – 12mm will be the ideal time; they will be the most susceptible. If you run into a temperature of 80-85F at the 8mm stage, I think I would go then. I would go with (favor) that warm temperature. The warm temperature is so critical.

John: It is, except I think that most growers are not going to want to jump the gun. They’re going to want to be convinced that none of the fruit are going to set, or at least that 98% of the crop is gone.

Phil: I was referring to where they want to thin off a crop.

John: Yeah, I understand. But at 6mm, 7mm, or 8mm, they may not know, yet, whether they want to de-fruit.

Phil: Right, okay. I don’t think they should go at 6mm or 7mm. Between 10mm and 12mm would be ideal. But if the ideal temperatures arrive at 8mm and you know you want to thin them off, then I’d do it with the warm temperatures and not necessarily wait for the ideal fruit size.

Of course, if they don’t know what fruit is setting, then they’ll probably wait to 12mm – 15mm before they do something.

John: Yeah, that’s probably the case.

On root pruning to control vegetative growth

Grower: Several years ago, I remember seeing in New York some highly-vigorous, high-density plantings where there was root pruning being done. I don’t remember when they were doing it or what the effects of it were, although I can certainly understand cutting roots to slow growth down. Is it possible to do something like that now?

Phil: Root pruning has a kind of “Apogee effect” on the trees. And you can do it at a severe level or a mild level. It’s sometimes a little bit of a challenge to calibrate your pruning accordingly.

But we do root pruning during the full boom stage. We’re right in the window, right now – in the next 7-10 days, you could still do it.

It’s like Apogee: you want to do it soon enough to get a grip on the trees that are highly vigorous. So you have to do it early.

What will root pruning do? It will reduce fruit size. It will reduce the vigor of the tree. The tree will look like it’s nitrogen-deficient for about 60 days – it will turn somewhat yellow.

They’ve been doing root pruning on vigorous Jonagold, and they really like their response.

Let’s talk about technique, and let’s go by trunk diameter. Your pruning should be a distance of 3-4x the trunk diameter away from the tree. So, if you have a 6” diameter trunk, you want to prune 18-24” away with your root pruner, cutting straight down.

By the way, you’ll need to do some experimentation to get the optimal settings. You need to cut 60% of more of the roots to have an effect on the tree. If you go further from the tree – say 30” instead of 18” – you’ll only cut about 35% of the roots, so you won’t have an impact on the tree. You have to be close to the tree. So, like I said, I think that the optimal distance is about 3-4x the diameter of the tree. (By diameter, I mean the diameter at 12-18” above the soil line.)

You want to prune on both sides of the tree. And you want to do it every year, down the same trench.

Now, when we were in Chile at the International Fruit Tree Association tour of Chilean orchards, they showed us how they were using a double-row root pruner. The pruners cut into the ground at a 45-degree angle. They would go down the alleys, pruning both the row on the left and the row on the right with one pass. The pruner would reach and cut underneath the tree. In one year, they would only prune one side of the tree; the next year, they would go down the other row and do the other side of the tree.

That technique results in a more severe pruning. Down there, they liked the response; it gave them a similar response to what we see up here, with our technique. Anyway, that’s another possibility.

The trees will jump when you cut the roots, and sometimes your tractor will jump, too, if you cut a big root.

John: Do you have growers who do this on Jonagold regularly?

Phil: Yes, we have about three growers who will do it on Jonagold and on other overly-vigorous semi-dwarfs like Gala.

Root pruning is better than Apogee, because you get the whole tree under control. Then you can use Apogee on top of it to hold the local growth in check. Apogee is locally-systemic, so wherever you spray is where you get the effect. So if the tops of your trees are vigorous, then put an extra spray on just in the top.

John: Well, there are some pretty vigorous older blocks – of M9s and other things – that the root pruning technique might be a good idea for.