Today we were joined by Dr. George Sundin, a plant pathologist and fire blight expert from MSU, to discuss fire blight, streptomycin, and the effects of the recent frosts and freezes on the disease. Find the recording and full transcript below.
Recording
Download the recording of the April 17 call here.
Call Transcript, April 17, 2012
Topics / Table of Contents
- Announcement: WAGA workshop on thinning, Friday, April 20, Kickapoo Orchard
- Insect updates
- Disease updates
- On Fire blight with George Sundin from MSU
- On disease management options for orchards with no crop
- On using squash mount data, again
- On the forecast for streptomycin’s future and its alternatives
- A frost damage update for the region
- On insect management options for orchards with no crop
- If a blossom is white, does that mean it’s undamaged?
- When should we start thinking about applying first insecticides?
Announcement: WAGA workshop on thinning, Friday, April 20, Kickapoo Orchard
ANNOUNCEMENT: WAGA is holding a workshop on thinning with Duane Green from Massachusetts: Friday, April 20 at Kickapoo Orchard.
Insect Updates
John: On either next Tuesday’s call or the following Tuesday, we’ll be talking about aspects of pesticide use at petal fall and first cover for a variety of possible pest complexes and fruit loads.
John: A heads-up on insects.
Let’s start with some of the insects that I talked about on last week’s call, like Green apple aphids, Rosy apple aphids, Spirea aphids, and Western flower thrips. These insects are extremely common at green tip. They’re secondary insects that we usually don’t worry too much about, except for Rosies, but as of last week’s call they were ubiquitous and in large numbers.
Both the aphids and the thrips have largely disappeared in the last week. There are still some aphids, but not in the numbers that they were at. I think there’s been a fairly large die-off. Some of these things don’t have a lot of reserve energy and need to feed constantly, but haven’t been able to. So, that’s good news.
Tarnished plant bug: I mentioned this last week. I still think people need to be paying close attention to this one. I’ve been in a lot of orchards where Tarnished plant bug is damaging flower buds and causing them to blast off or not open. In a normal year, the damage would be significant. Compared to the freeze damage we’ve gotten over the region, though, it’s not. But the evidence suggests that Tarnished plant bug will be there at petal fall. So, we may need to address it in that petal fall or first cover insecticide.
Plum curculio: There have been some big questions about under what conditions PC migrates into the orchard. It’s been cold, which might suggest that they haven’t moved much. But I can tell you that over the last week I’ve seen PC feeding on stone fruit. I’ve seen both the insect and the damage. The stone fruit I was looking at were on the edges of two orchards, so it doesn’t show that they’ve moved all the way in or that the migration is close to finished. But the fact is that some are here, and some are taking the opportunity on warm days to climb up and feed. They have to feed prior to starting to lay eggs. They usually need about a week or 10 days of normal weather before oviposition begins.
Anyway, you may not have stone fruit lining your apple blocks, but I think it’s clear that there are already some PC around the edges of peoples’ orchards.
Codling moth: I did not catch any CM on Friday or Saturday nights. The conditions weren’t all that great for their flight. If your CM traps are up and your mating disruption is going up, you should be in good shape – at least from the monitoring standpoint.
Oblique-banded leaf rollers: There are a lot of OBLR in peoples’ orchards. They range all the way from just coming out of hibernation (very, very small, slender, 2mm-long) to others that have moulted and are into the 3rd or 4th instar. That second group is a lot easier to see in the buds. OBLR are easy to monitor for: if you’ve got them, you should be able to see them.
Beneficials: There are beneficials of all sorts in peoples’ orchards, despite the cold weather. There are a lot of parasites (parasitic hymenoptera). They’re hard to see because of their small size. Even though it might seem like nothing has been going on in the orchard over the past couple weeks, beneficials are active on warm days.  Things are slowly developing. Beneficials are already out there trying to find worms. Others are leaf-miner parasites.
European red mite: A lot of European red mite populations have hatched. But I’ve also seen Predator mites, which is unusual at this stage.
Scouting: Scouting is certainly much more do-able this year, at this tree stage, than it typically is. Even though the trees are so early, many of the insects seem to be right in sync with the trees.
Disease Updates
Apple scab: I have seen Apple scab in orchards over the last week. I picked up the first lesions over the weekend. Normally when I talk about looking for the first lesions, I’m talking about doing it during bloom. I usually mention how difficult it is to find them. Why? Because they’re usually very rare at this stage. The scab infections for this first generation tend to occur around green tip to ¼-inch green. That’s already a number of weeks ago for us . Ordinarily, the lesions are difficult to see, but this year I have been able to find scab lesions relatively easily in some orchards and in some varieties. They are on the first leaves at the base of the cluster. All the lesions I’ve seen are on the leading edge of the leaf. If the those first base leaves is 1†long and ¾†wide, the lesion will be right at the tip of the leaves.
Powdery mildew: We talked about this at length in the first two conference calls. The worry was that because it’s been so warm this winter, we’ll have relatively more of this disease popping up this year. From what I’ve seen so far, if you have it in your orchard, the mildew has already encompassed much of the growing points of entire clusters. It’s not difficult to spot.
On Fire blight with George Sundin from MSU
John: Our first question for you came from a grower. Are the flower buds killed or damaged by frost more or less susceptible to Fire blight infection?
George: Well, the good news about the dead flowers is that they need live stigmas to incubate the Fire blight bacterium. That’s the real issue that leads to the high populations we end up with on a normal year. This year, we won’t get that population build-up.
Sometimes, though, you can get infections through that wounded, frozen tissue. And you also have to pay attention to the live flowers that remain in your orchard, if you still have them.
I don’t think the FB bacteria has been very active.  Our conditions have been pretty cool. We’ve been having temps in the 50s. Today it might get into the mid-60s. In recent weeks, we did have that one warm day, but the temperatures have been pretty cool. We had that really warm spell early in the season, and at the time I thought that might create chaos for us, but now I think things have settled  back down.
My guess is that we’re not going to see a ton of FB activity in the window of time these flowers can be infected. In the questions you sent, you said that the grower got a spray on. I agree with the grower, and I think that’s fine – I probably would have done the same thing. But I think we’re in decent shape because we’re getting temps in the 50s with lows in the 30s – not good weather for Fire blight.
John: You’ve always talked about how streptomycin can give you 72 hours of protection going ahead or 24 hours backward. How does that work? Does it have to adhere to the plant tissue? Can it get washed off? Or does it just do an initial kill of the bacteria to give you the effect?
George: Streptomycin is mildly systemic. You know you can cause streptomycin injury if you put on a lot of applications, and you can see it on the leaves. But really, we’re targeting the flower, we’re killing the bacteria on the surface and at the base of the flower where it’s going to infect. So if you get some streptomycin that goes in at the base of the flower, that will have some effect on bacteria that are trying to enter there. That’s what’s giving you the 72 hour control.
About the backward action: if you heat a stigma that already has a large number of Erwinia cells on it, the streptomycin will still kill those. It is quite effective. Even after the rain, you can come back with it.
As far as wash-off: if you have a rain, a lot of the strep will wash off, but if you get enough in the base of the flower – which I think you normally would – you’d still expect some activity from that.
Regarding alternatives, for comparison: Oxytetracycline is much more sensitive to degradation by sunlight, for example. Oxytet doesn’t last more than a day and a half before it has degraded.
John: Oxytetracycline isn’t as effective as streptomycin is at backwards-effectiveness, is it?
George: No, it’s not. The other difference is that Oxytet doesn’t kill the bacteria; it just inhibits its growth. Streptomycin will kill the bacteria. So if you put a drop of streptomycin into a stigma full of Erwinia cells, it’s going to kill whatever it comes in contact with, which is most of the cells. The Oxytet will stop them from growing. That’s fine, and you might be able to prevent the infection that way, but if you already have the bacteria at the base of the flower, ready to infect, they are probably going to infect. The strep, by comparison, will still be able to enter into the flower and kill them.
John: We have lots of wounded flowers and early apples that are going to, presumably, begin dropping off. In the meantime, they are going to slowly decay. So, they won’t lead to an increased FB risk? They would, rather, lead to a decreased risk, simply because they don’t support the Erwinia growth?
George: Yeah, the issue with Erwinia is that the population gets such a huge jump by growing on the stigmas of the flowers. So, there is still a risk, but the risk is not nearly as great as, say, if you have 10% live flowers: that would pose a bigger risk than these killed flowers would.
John: Do those dying flowers and small apples pose any increased risk for fungal introductions to the tree?
George: It’s likely that the dying apples could be colonized by things like Black rot fungus. That could possibly infect your crop, later, if you still have a reasonable crop. Black rot is a good colonizer of fruit mummies that occur after thinning. So, that could be an issue.
I noticed in your email that you said growers were putting on strobilurin plus Captan or Luna Sensation plus Captan. That would be good control for Black rot, as well, and should take care of it.
John: Some growers haven’t finished pruning and, because they either want to manage fruit load or are concerned about next year’s return bloom, want to go back in and clean out more of the stuff. If one of these growers hasn’t had Fire blight in several years (meaning we assume they have low inoculum), how dangerous would it be for them to resume pruning?
George: Pruning again would not be ideal. If you have to prune now, the best tactic is to do it when it’s dry. If it’s wet, you don’t want to prune even if you feel you have to. The best chance to get away with it would be under dry conditions. Like I said, if we stay in the 50s, the weather is not very conducive to Fire blight.
John: Well, that’s about the only bright side we’ve seen on the disease front this year.
George: The weather is the best control for Fire blight that we have. We get a lot of years with bloom temperatures in the 50s; that really helps us more than we might realize.
John: Some growers have been thinking about Fire blight management as a numbers game – bacterial numbers – rather than following the models. Growers who do that tend to apply streptomycin a little earlier than a model might recommend. Do you think that approach is too conservative? What is your feeling about it?
George: I tend to be more conservative, so I’m fairly in agreement with them. For example, this past weekend, we had warm weather – something in the 70s – but the EIPs for the Mary Blight model were only in the 30s – 39 in most places, 26 in other places. Looking ahead, we saw it was going to be cool, but we saw we were going to get rain. I would have put a streptomycin spray on, because otherwise, you wouldn’t have put one on at all for 6 or 7 more days.
My approach resembles that â€numbers†approach. I would want to get material on to hold those numbers down.
About resistance: What’s led to resistance issues in Michigan, Washington state, and California? That happened back when growers were spraying streptomycin all year long, and they were spraying it 10-12 times per season. I think that if we hold the number of applications down to 2-4 during bloom, and the only other time we’d ever consider it would be after a hail storm (which hopefully doesn’t occur every year) – if we keep the total number of applications down to that level – I don’t think we have a huge resistance risk.
In Michigan, we found that we had resistance develop in SW, and those strains have spread to other parts of the state. We haven’t had new developments of resistance; it was mostly just bad luck that these strains somehow got spread around the state, likely through human activity like nursery trees. In the other regions, resistance didn’t develop on its own due to grower spraying. Rather, we brought in inoculum that was resistant and then it spread from there.
John: Ah, okay. I thought I remembered that a few years ago you said you had identified three spontaneous resistance developments in the three areas. But that wasn’t the case?
George: No, what we found was that the resistance was due to strains spreading from one region to the other. It wasn’t new development.
So for you guys in Wisconsin or Minnesota: I think if you keep the total number of applications down each season, it’s more likely that any resistance you see will come from the spread of already-resistant populations than from your developing new resistant strains in your region.
John: A question on trying to assess what your inoculum levels are. In some orchards that haven’t FB flare-ups for several years, growers will see quiet, symptomless susceptible trees for 5 years, and then suddenly, the FB will flare up again out of the blue. Do you have any clue what’s going on there?
George: Yeah, you know, we really don’t. We don’t have a good understanding, I think, of blight that just sits in the tree. We don’t understand why that happens, because it’s usually highly virulent, a lot of things will be happening like oozing.
John: You can’t cut out all the cankers, and yet in many cases you still see susceptible varieties that are clean for a number of years. But I imagine that the flare-ups must come in conditions very conducive to rapid bacterial growth during bloom.
George: Probably, but we really don’t know much about those cases of quiescent FB.
On disease management options for orchards with no crop
Grower (from La Crosse area): I have a question about Apple scab and other orchard management issues in this post-freeze time. A lot of us are thinking that we might not even have a crop this year. We’re looking at ways that we might be able to reduce our costs and minimize our spray plans. Most of the orchards have had really good scab control over the last several years. Should we just look at doing a good job at controlling the primary infection, keep an eye on infections, and then stop everything after that?
George: You know, as far as I can tell, the only risk would be to see a tree get an infection that defoliates the tree and affects its long-term health. And if you can escape primary infection pretty clean, you should be in pretty decent shape. You might throw a Captan on, but I think you definitely don’t need to spray that much to keep going for the rest of the season.
You might get scab flaring up again later, in the Fall, but the idea would be: you’d want to treat those leaves next Spring on the orchard fall if you do have some infection issues in the orchard this year.
Grower: Are the orchards in Michigan looking pretty similar? Has there been a lot of frost damage?
George: Yeah, we’ve got a lot. It doesn’t look great, I can say that. Some areas are looking relatively better, but I think overall, it’s not good. And the cherries are even worse. Even the tart cherries. We were up in NW on Friday where I’m trying to do Brown rot trials, but I can’t find any fruit to work on.
Grower: What are your thoughts on going to a more reactive approach? For a lot of us, things like Rally and others are still effective here. What if we treated a scab infection when we see it and let things go otherwise?
George: It’s another idea. And I think it’s fine. Ultimately, it’s not the approach that we like to take. But you could say that this is an exceptional year. From a resistance standpoint, it’s not something that we’d like to see, but at the same time, you’ve got to take into account economics and other aspects of things. I’m not opposed to that, if that’s the way that it has to be.
John: I’d interject with one caveat and would like George’s response. By this point in time, we’ve spent a lot of money on fungicides, and you’re fairly well along in primary scab release. The forecast is for us to be in average or a little above-average wetness. Whether those wetting periods are going to result in infection periods is unknown. But I’m just wondering what the costs would be for you, [Grower], to continue with a fairly steady protectant program for another 3 weeks, until you’re out of that primary, compared to the risk of further resistance development? And if you end up with four infection periods for which you have to go in with Indar or Rally or something else, is that better than making a couple decent applications of Captan?
Grower: Yeah, that’s definitely a balancing act, isn’t it?
John: It kind of depends on where you are in terms of your scab modeling and what the forecast is. I would definitely stretch it out if you’re not getting any significant rains on the previous cover.
George: Definitely. If you can, the protectant approach is still the best, and you definitely could stretch the intervals, if you’re not looking at really bad infection periods. And, this year, the scab fungus has been pretty well timed with tree development. So we should be getting close to the end, here. If you can stick it out, obviously, that’s better. Because if you do go reactionary and all of a sudden we do see a lot of infection periods, then you probably will have been better off just having the protectants on to begin with.
John: I think I should reiterate that we’re close enough to the end of primary that you shouldn’t risk shooting yourself in the foot. What I’d like to see happen in those blocks is that we really stretch the envelope after primary. Obviously, you’re going to let Sooty blotch and Fly speck go. You’re going to hope for the best in terms of Black rot spread, which occurs during that first month after petal fall. And, basically, if primary scab hasn’t shown up by what would normally be your thinning period, I can’t see the downside of not putting on a protectant for the rest of the year if you don’t have a crop.
All that is true, as long as you’re still monitoring. Â
George did say an important thing: If you’re getting Black rot infections right now in some of these flowers or small apples, the rot might not show up until much later.
But our level of concern depends on what kinds of infections we see. If we end up with Black rot-infected fruit out there in July, there’s not a whole lot to worry about. If we end up with Bitter rot lesions, instead, then even in a low-fruit-load scenario where you’re trying to cut costs, you would probably have reason to go back in with additional protectant.
On using squash mount data to monitor Apple scab, again
Grower: John, have you talked at all about the issue with confusing information from squash mount data? (We discussed this with Dave Rosenberger from Cornell on the conference call from April 3 – LINK)
John: That’s a good point. We should get your feedback on that, George. Two weeks ago, Dave Rosenberger was on the call. We spoke mostly about canker fungi. But there was considerable discussion about using squash mount data vs. degree-day modeling for ascospore development. Dave comes down on the side of the model, mainly because of the user error that doing squash mounts introduces. What is your feeling about the reliability of squash mounts? And more importantly, are you guys monitoring spore counts? Are you counting spores during infection periods anywhere in Michigan?
George: First of all, Dave is the man for scab. So I totally back everything that he says.
We don’t use the squash mounts. A couple of our agents do monitor spore counts. We use that data more (along with the model) than we do squash mount data.
John: In some of those early infection periods we had in March (I think there was one around the 24th or 25th), your agents were catching Apple scab spores?
George: Yes, we caught a lot of spores early, as I predicted, because we didn’t have snow cover leading up the early development of the tree. I figured the scab fungus would be ready to go, and it was. We did catch a lot of spores early.
John: Well, I said before you came on that I found my first scab lesions over the weekend. I hardly ever see anything at this stage, but I was able to find these pretty easily, on the first leaves at the base of the cluster and on the leading edge of those leaves.
George: Yeah, and the problem is that those lesions will produce more spores than what’s on the ground.
John: Yeah, I know. So if you’re not paying attention to those and are using infection periods, for example, that second wave of infections from conidia would generally show up around thinning time. This situation seems to coincide with historical bouts where all of a sudden people see scab flaring up in their orchards at first or second cover.
On the forecast for streptomycin’s future and its alternatives
John: Because of the USDA getting involved in the issue of streptomycin in animal agriculture, do you think that streptomycin will still be in use in fruit 5 years down the road?
George: I certainly hope so. It’s a good question. We’ve been wrestling with this for organics. We’ve been wrestling with the sunsets for antibiotics. I hadn’t heard anything about conventional plant ag and antiobiotic issues. So, right now, it’s almost like we’re hope that it stays quiet, that no storm comes up. With the organics, we’re trying to get funded for a project to study what alternatives we have if we’re without antibiotics. And we really don’t have viable alternatives, especially for our region of the country.
John: That leads into the final question. I know you’ve published some work on this, as have Virginia Stockwell and probably other people over the last 10 years: on biological antagonists. They haven’t looked very good compared to streptomycin, at least in the Midwest. A lot of this research was done several years ago. Are there any new approaches on the horizon that show more promise?
George: Right now, I’m going to say No. There’s a new yeast organism called Blossom Protect that was just registered. But the reality is that, at this point, you’ve got to put that on at least 3 times, maybe 4, and the control has not been great.
Serenade Max is coming out with a new formulation called Optiva. I don’t know when. We’re testing that this year. But I don’t know how effective that’s going to be.
That’s about it. Besides antibiotics, there’s not anything out there, really, for our conditions in the Midwest and Eastern US. Any of these will be effective under low to moderate pressure. But it’s those years when we have high pressure and good Fire blight conditions that it can get away from you – those products are not going to stop the problems.
John: That relates to my final question. Are you aware of any “field trials” of these products, that is where a grower has used them w/o active inoculation? (I guess I’m asking: is it probable for a grower with low inoculum to “successfully” use one of these products for years, and have “field efficacy”?)Â
George: Yeah, in a year like this, you could get pretty decent control from Serenade Max, for example. That is, if the conditions stay the way they are. I think that’s really possible. It’s when we don’t have this kind of year that the problems occur.
A frost damage update for the region
John: Regarding the frost damage across the region: I have to tell you guys that everybody has lost some. I don’t think anybody has escaped. There seems to be a lot of serendipity about where the damage is most severe. It’s not all concentrated to the north or to one region. You can see incredibly wide differences in the amount of dead blossoms in a very short drive down the road from one orchard to the next. It doesn’t make anybody feel better, but there’s an awful lot of company.
On insect management options for orchards with no crop
Peter Werts: Related to the grower’s question about cutting back on fungicide sprays, I’d be curious to get thoughts on appropriate cutbacks in insecticides, and what types of novel approaches we might employ so that we’re not taking a step backwards by using cheap, broad-spectrum insecticides that will destroy all our beneficials.
John: That’s a good point, Peter. I didn’t think of that. I was thinking more along the lines of not putting on anything.
When we talk next week about these petal fall or first cover insecticides and your alternatives based on your pest complexes, we have to address orchards or blocks that have really low fruit load, where minimizing cost is paramount. There are better ways to minimize the cost than going out and buying the cheapest pyrethroid and putting it on over the summer. Doing that would be like shooting yourself in the foot.
If a blossom is white, does that mean it’s undamaged?
Grower: If the blossom petals are still intact, does that mean that you’ve missed the freeze damage?
John: No. Sometimes on frozen flowers, the petals are still white. A lot of times they turn brown, but not always. It’s confusing this year because some of those blossoms are 2 weeks old. They’ve been sitting out there, so there’s normal ageing that’s going on as well. But, no, I see lots of flowers where the petals are snow white but the stamens are dead and brown.
The take your thumbnail, pinch the ovum, and look at the seed pocket. If it’s damaged, it won’t be green in the center. Oftentimes, under the glass, you’ll see that the entire thing has turned to mush.
What I don’t know is whether some of the blossoms that have been around for a while and were fertilized will end up turning into apples. There are some repairs that can be done to some things, and I have no idea how badly that center can be damaged and still produce a decent cider apple.
When should we start thinking about applying first insecticides?
Grower Question: We have some blossoms left here. We’re just getting into petal fall, but it’s cool. With the petal fall spray, we usually spray for PC. Should we be skipping the insecticide spray at petal fall and put it out at first cover, instead, when we get some more heat?
John: Good question. You’re right: I definitely do not want people putting insecticide on in the next week (according to the forecast). I think it’s far more important to get a fungicide on, if you have stretched that out as far as it will go and you still need it.
There are still too many beneficials out there, and especially if you’re going to have a light crop load, it’s even more critical that you not go out and eliminate these beneficials. In some cultivars, you have significant numbers of flowers that are still at pink.
Even if you have tons of Tarnished plant bug, they’re not going to do a lot of damage at the temperature that is forecast. And Plum curculio cannot be very far into orchards if they’re there at all. And, again, they can’t be doing any kind of physical damage in these temps. The amount of feeding damage I’ve seen is really minimal, and those were on some pretty nice-looking stone fruit on the edges of two orchards.
Keep an eye on your peaches. It wouldn’t hurt to take a look at the edge of the peaches and make sure there’s no fruit damage.
Otherwise, hold off on the insecticide sprays.
My feeling is that the first thing we should do when the bees are gone and pollinator damage is not a problem is to put something on the perimiter to buy a little time to assess what’s going on inside.
