Over-informed on IPM - Episode 011: Fruit IPM - Fireblight, bacterial ooze & the insects that love them

Anna chats with PhD candidate, Matt Boucher of Cornell University, and IPM expert extraordinaire, Kathleen Leahy of Polaris Orchard IPM, about who is to blame for spreading the causal agent of fireblight in apple.
fire blight

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Show Notes

pest cycle
Diagram: Johnson, K.B. 2000. Fire blight of apple and pear. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-0726-01.


From UNH Cooperative Extension, this is Over-Informed on Tree-Fruit IPM.

I’m an entomologist. I study insects that eat plants. But, real talk, at the end of the day, I understand that insect biology is one of the last things an average grower is considering before a laundry list of other things like plant fertility, farm equipment, labor, family disputes…I don’t know…teenagers? Problems with teenagers probably rank higher than insect problems with most growers.  Also, in integrated pest management, we consider management in terms of how all pests - that’s insects, mites, disease, and weeds - affect overall management of a crop. Diseases will outrank insect pests in how crop management is considered. This is particularly true in tree fruit and even moreso in eastcoast tree fruit production, where the pathogens that cause diseases just loooove our humidity. So though it chagrins me to admit it, we must take a moment to talk about one of the most devastating diseases affecting tree fruit in New Hampshire, and that is fireblight.

First the basics, from Dr. Smith’s factsheet, which you can find on the UNH website. Fireblight affects over 75 species of plants, including common orchard crops like apples and pears, but also many related landscape plants like mountain ash, cotoneaster, hawthorn, and the ornamental crabapples and pears. The disease is caused by the bacterium Erwinia amylovora. When the bacteria gets into the tree, it kills plant tissue as it grows and reproduces, affecting blossoms, twigs, leaves, fruit and branches. Blighted branches have the appearance of having been scorched by fire, hence the name, and have a bend to them that gives them a “shepard’s crook” appearance. The more and more the bacteria grows, the more plant tissue is destroyed by the disease and can eventually kill the tree.

Best management practices are generally to reduce inoculum sources in the orchard – in other words – to removed infected tissue full of these bacteria who are willing and able to infect new tissues. During the winter or early spring, orchardist will prune out and burn infected branches. Chemical control is often used at the end of the dormancy period to prevent outbreaks, silver tip to green tip. There are also antibiotic products and biopesticides that can be applied to protect trees from fireblight infection during bloom. Through the growing season, orchardists will also often prune out fireblight strikes as they occur, making cuts at least 8-12 inches below the edge of the infected area or cankers, and making sure to sanitize tools between cuts with rubbing alchohol or a 10% solution of household bleach in water.

However you may ask yourself, how do these tiny bacteria get inside trees? Traditionally we think of two major avenues, “blossom blight” as a primary infection where insects and rain move bacteria from infected tissue into apple blossoms, and then there’s “shoot blight” where bacteria make their way into cracks in bark made by hail, wind whipping, or insect feeding.

Furthermore you may ask yourself, what kind of insects are we talking about here? Well maybe you wouldn’t ask yourself that… but I’m an entomologist and I have a one track mind …and this is my show after all so… let’s get over-informed on the insects involved in fireblight and have a chat with a couple of entomologists that know this system really well.

Matt Boucher, Cornell University

Anna: While honey bees were long thought to be the main culprit is spreading blossom blight, it looks like that might not be the case.

Matt: …well I’m uncomfortable removing the onus from bees entirely but what we’re thinking is they got a pretty bad rap for a long time. Bees can only be important if there is already infection on the flower. So how it works is a new infection will build up [bacteria] in such high numbers in the plant that it will actually burst out from underneath the surface. When it ruptures the surface, it oozes out this sugary matrix, sugar produced by the bacteria, that serves as the inoculum for new infections. In the spring, these hold-over cankers start oozing out and this ooze is moved by wind rain and insects into blossoms and other injuries in young green tissue. And the infection cycle starts over again.

People used to believe that honey bees would pick up this infection and spread it from blossom to blossom and spread it broadly through the orchard. But the problem is that bees don’t go to the ooze in the overwintered cankers so that movement is predicated on the fact that infection has already made its way onto a blossom from some other introduction.

When I first started this project we were pretty perplexed as to why this hadn’t been looked at in over 80 years, because looking back to research from the 1910s and 20s they were looking at all sorts of insects. Until the 1930s when they started using antibiotics to spray on trees, they stopped doing it because they didn’t need to, I guess. So we decided to revisit this, as antibiotic resistance has sprung up here and there over the country.

What we’re finding is that flies are a really important player in this system. What they’ll do is they will go and land on this ooze, and feed on it, and sit in it, and it gets stuck to their bodies, and they can move it to new healthy tissue. In the spring, a lot of these flies are overwintering as pupae in the soil so, when they emerge, they’re looking for nutrients. They’ll drink nectar from flowers, they’ll go after the pollen, and they’ll also go after this ooze if available. Because this ooze is a high in sugar, we believe this is very good nutrient resource.

Anna: So, Matt is hard at work understanding the steps of transmission.  Like how much bacteria a fly might need to pick up in order to transmit the disease? How long those bacteria are capable of causing disease after sitting on the outside of a fly or on the surface of a tree until it gets into cracks in the tree surface?….  I hope you’re picking up on what he’s saying here. The bacteria are creating a source of nutrition for the flies! And the flies are attracted to the bacterial ooze! This is really a remarkable approach to understanding the players in this disease cycle. The take home message for orchardists doesn’t change. You guys know how important it is to prune fireblight out of your trees. But this may emphasize how important it is to get that infected material out of the orchard, perhaps burn that infected wood, because there will be flies coming to it, pigpenning around in it, and spreading it through your orchard.


Anna: So we’re talking specifically about blossom blight here? What about shoot blight?

Matt: That’s one of our big questions. We broadly believe that flies and wind and rain are responsible for the movement of bacteria from those original overwintering cankers to blossoms. Shoot blight is really an unknown, people really don’t know what’s going on there. So this is really the crux of my PhD project: how does shoot blight happen? How does this disease continue to move after petal fall? What we’re interested in is how these flies (that don’t cause damage to the tree) pick up bacteria from the ooze and regurgitate it or defecate it or slough it off their body when they’re grooming onto a surface, which could be opened by any kind of mechanical damage. We think these flies need help for shoot blight but they do it on their own during the period for blossom blight infection.

So the first thing we did, was we went out to the field for a field survey. For a couple years, we would collect insects on sticky cards throughout the growing season and get a relative idea of which flies were the most abundant and which flies were testing positive [for the bacteria] at high rates. That’s how we identified seedcorn maggot as an interesting player. They were the predominant species in the orchards and they tested positive at a relatively high rate.

So we’re interested in this fly because its attracted to fermenting and decaying odors and fireblight-infected tissue, when it starts to ooze, has a fermenty kind of smell. We did an experiment where we gave these flies a choice between a diseases fruit and a healthy fruit and we showed that 80% of the time, they are choosing that diseased fruit. What that says to us is, these flies can find this stuff pretty easily.

Anna: But how are shoot blight infections getting into trees and what can we do to prevent that in the orchard? Apologies for the audio here but I really wanted to ask Kathleen Leahy about this too and I got her on the phone where she spends most of her time out on the road.

Kathleen Leahy, Polaris Orchard Management

Well, we were using MaryBlyt, developed by Paul Steiner to track blossom blight and it’s excellent for blossom blight. It will predict when you see symptoms, which is one of the things I like about it. If you go out on the day MaryBlyt says you’ll find symptoms, if they’re there, you’ll find them. Although, people used to tease Paul that he only went out after the model said so.

He was trying to extend the model to predict shoot blight, and kind of pining it on white apple leafhopper and maybe aphids too. I was just kind of “nooooo!” because we were just getting to the point where people were tolerating the white apple leafhopper and the aphids. I just kind of wanted it not to be true but it seem unlikely that leafhopper would be the likely to be a factor. Then Doug Pfeiffer, I think, and a couple other people came out with some work that potato leafhopper might have some role. That ended up being my master’s project. Basically we looked at potato leafhoppers and Apogee because they had just found out that Apogee itself suppresses fireblight.

Anna: …and Apogee, that’s the ummm?...

Kathleen: Plant growth regulator.

Anna: Oh, so it kind of boosts the immune system of the plant?

Kathleen:  Yeah, and Keith Yoder was one of the first one’s looking at it because they had been thinking: because fireblight affects rapidly growing shoots, maybe a plant growth regulator that slows down rapidly growing shoots might stop fireblight. When they started doing experiments, it was remarkable how much it did. It turned out to not be for that reason at all.

Anna: Oh, wut? What happened?

Kathleen: It was just a few years ago. They figured out that it makes the cell walls a little thicker. Thick enough that it makes it hard for the bacteria to penetrate. It’s really wild!

Anna: But it works…just not for the reasons you thought it did?

Kathleen: Right. It works extremely well. It continues to be the second most effective products after an antibiotic. Streptomycin is the most, Apogee is the next, and I think even some of the other antibiotics are below Apogee.

And then I wondered if Apogee would suppress potato leafhopper, which is also going for rapidly growing shoots. It turns out that, yes, there was significantly less damage where Apogee had been used. If I remember, it was pretty comparable to insecticide.

The thinking is – they’ve always called it facilitating, not vectoring. As far as we know, they’re not carrying it around. Theyr’re punching a hole into the vascular system and the bacteria are hollowing it out.

Anna: So in summary. The bacteria infect the apple trees and trick them into producing sweet ooze, which flies love. The flies, and wind and rain, move the bacteria around the orchard. The bacteria get into new plants through flowers, with the help of rain, or get into shoots, with the help of leafhoppers. The take home here is to be vigilant in removing sources of inoculum and to do all you can to boost your trees’ ability to defend themselves from attack, maybe a growth regulator product like Apogee. Stay tuned for more research on the mechanisms of this disease system, which has been keeping researchers busy for hundreds of years.

Well I think this might be the most over-informy episode I’ve done so far. I know I learned a lot and I hope you did too! Thanks to Matt and Kathleen and thanks to Brentwood’s favorite son, Jason Lightbown who wrote and performed our theme music.