Making a Fuss about Red Star Rust

Junipers, crab apple, and apple trees are all important tree species in Minnesota. In fact, apple varieties of Minnesota origin are among the top sellers in the country, making the health of these trees a priority to many. Recently a new danger to these trees, red star rust, was found in Minnesota. Plant disease experts Michelle Grabowski from the MDA and Tiffany Enzenbacher from the University of Minnesota discuss the basics of red start rust, new research being done, and steps to protect these tree species. 

Smarty Plants is a podcast of the Minnesota Department of Agriculture. Visit www.mda.state.mn.us/plants-insects/smarty-plants for more episodes. Look for a new episode of Smarty Plants every month.

Transcript

Jennifer Burington: 0:19

Welcome to Smarty Plants, a Minnesota Department of Agriculture podcast. Inform citizens about invasive species that affect our environment and agricultural resources. My name is Jennifer Barrington, and I will be your host. As Minnesotans, we pride ourselves on our trees and we are lucky to have such a large variety, including junipers, crab, apples and apple trees in 2022 red star rust. An invasive plant pathogen was found for the first time in Minnesota. Red star rust could be a particularly concerning problem for our apple trees. Michelle Grabowski and Tiffany Enzenbacher are here to discuss this new plant pathogen and to tell us about ongoing research and how to manage the disease in nurseries, orchards, and home landscapes. Michelle is a plant pathologist with the MDA's Plant Protection Division. Her work focuses on prevention and early detection and invasive plant pathogens that threaten crops and native plant communities in Minnesota.

Michelle Grabowski: 1:13

Happy to be here.

Jennifer Burington: 1:15

And Tiffany is a researcher in the Department of Horticultural Science at the University of Minnesota in Dr. Brandon Miller's lab. She's working on several projects related to Red Star Rust, which you'll hear about during this podcast.

Tiffany Enzenbacher: 1:27

Thank you for having me.

Jennifer Burington: 1:29

So before we get started, talking about Red Star Rust, Tiffany, can you tell us a little bit about the importance of apples and crab apples grown here in Minnesota.

Tiffany Enzenbacher: 1:39

Well, due to Minnesota's low winter temperatures breeding cold, hardy apple trees has been at the forefront of the states and the University of Minnesota's priorities. The 1991 University of Minnesota released Honey Crisp has actually risen among the top four apple fruit grown in the us. Sweet tango is another popular university variety and triumph and kudos are newer releases. In 2022, the value of US Apple production was just over 3 billion with sales specific to Minnesota at over 20 million in 2017. And as far as crab apples, they are grown here in Minnesota because of their spring flowers, their fall fruit, various tree forms suitability to most landscapes. And most importantly for US cold tolerance. Minnesota is actually the second highest producer of crab apples in the US with sales here at just over 3 million in 2019.

Jennifer Burington: 2:31

Then. Michelle, could you remind us what a plant pathogen is?

Michelle Grabowski: 2:36

Um, so it's important to realize that plants get sick too. Um, so a plant pathogen is, uh, an organism that causes a disease in a plant. And when we're talking about plant pathogens quite frequently it's a fungi, but there's also bacteria and viruses and other microorganisms that can cause a plant to be sick.

Jennifer Burington: 2:56

So red star rust is one plant pathogen. So what or what type causes red star rust and how can it affect tree health?

Tiffany Enzenbacher: 3:06

So red star rust is caused by a fungus, uh, gymnosporangium yamadae. It's essentially the counterpart of our native North American cedar apple rust. Uh, red star rust is also similar to other rusts that affect apple, uh, like Hawthorne and Quince rusts. Uh, as we talked about earlier, red star rust, uh, affects apples crab, apple, and some species of landscape juniper. The reported juniper hosts include Chinese juniper, sergeants Juniper, flaky Juniper, Garden Juniper and pfitzer juniper. Um, and as far as how can it affect tree health for apple and crab, apple red star rust may cause early defoliation or leaf drop, which reduces the health and vigor of the plant. Uh, and this may affect yield, which we be concerned about for apple. And of course, the flour and fruit display for crab apple and actual fruit infection is possible. As far as Juniper, we wouldn't expect any significant permanent damage. And the thing to remember with rusts is that they're obligate pathogens, which means they require living tissue in order to complete their own lifecycle. And because of that, they need to grow on live plant tissue. So we wouldn't anticipate plant death.

Jennifer Burington: 4:18

So this seems like a really new plant pathogen to our state. Um, Michelle, could you talk a little bit about when we first learned about it in Minnesota?

Michelle Grabowski: 4:28

Sure. Um, so red star rust is, uh, as Tiffany mentioned, a a fungal plant pathogen. And it was actually first described in Japan in 1904. Um, it's known to occur in a couple different countries in Asia. And it was first, um, identified in the United States in 2009 on the East coast, uh, so in Delaware and Pennsylvania. And once researchers started looking for it, it was found widely throughout the eastern states. But at that time, the Minnesota Department of Agriculture and other Midwestern states looked for the pathogen here in our apple producing areas. And we didn't find it. Um, then in 2021, Wisconsin reported finding the red star rust in nurseries, um, on crab apple trees. And so Minnesota realizing that this pathogen was now closer to home, um, started looking here as well in apple orchards as well as in nurseries and landscapes. Um, and in 2022 we found it, um, and we found it in all of the seven metro counties. Um. As well as Rice County, Le Sueur County and McCleod County. Um, and we were able to find it in apple orchards. Uh, we found it on, uh, nursery trees and we even found it in some landscape trees. So we found it in a couple of different places where it was out there, um, causing disease. Um, but it doesn't seem to have spread very far beyond those, uh, metro counties and a a few counties out from there.

Jennifer Burington: 6:04

So I have a backyard crab apple tree, and my neighbors have a couple apple trees that they have recently planted. Um, what does red star rust look like? The symptoms on crab apples and apple trees versus what's in juniper.

Tiffany Enzenbacher: 6:21

Yeah. So on apples and crab apples, we see a very large irregularly shaped lesions on leaves. And by irregularly shaped, I mean, they're not perfectly circular. Um, the lesions sometimes have a starburst shape, hence the name red star Rust. Uh, so red star rust is also known as Japanese apple rust. However, the literal Japanese translation is red star rust. So for accuracy sake, uh, we go for red star rust for the common name of the disease. Um, and these, these lesions or spots are typically brightly colored in shades of yellow, orange, or red. Uh, this is really dependent on the cultivar. So typically, white flower plants usually have yellow lesions or lighter lesions, and then red or pink flowered plants tend to have darker lesions or, or red lesions. Um, and these, these spots or lesions are usually always bordered by, uh, a red color. And then in late summer, um, typically July into August, we see bristly spore structures develop on the underside of the leaf. Uh, it almost looked like toothbrush, bristles. Uh, and on Juniper, red star rust causes very small galls or a perforation of tissue. And, um, when I say small, I mean very small. They're just about a quarter of an inch, uh, about the size of a pea. And they're woody and brown and they camouflage very well with the Juniper branch and virtually undetectable until early spring. And in spring, coinciding with warm temperatures and rain. These galls are signaled to germinate or sporulate. They release these bright orange, uh, very noticeable gelatinous spore masses, and we can think of spores as the reproductive units of fungi. Um, so basically they're fungal seeds that cause disease. And these spores on the juniper look like someone flung orange jello at the plant. They're very noticeable and they contrast very well with the green foliage of the Juniper plant.

Jennifer Burington: 8:17

Yeah, I've noticed, um, in the past I've seen some, like cedar porus, like you guys talked about, there's some other common rusts that happen on crab apples and junipers and stuff. So I've seen that and it's very bright orange. Like you'll, you can see it from a good distance away when they have those bright gelatinous, just like you said, looks like jello. Um, so to go a little bit more with the lifecycle and like how it looks, um, different on these different plants, but what is. The lifecycle of red star rust. And how does it spread? Michelle, if you wanna take a shot at this.

Michelle Grabowski: 8:49

Sure. Um, so the important thing with, uh, the rusts, like red star rust is they need both plants to complete their lifecycle. So the spores from the juniper. Um, when those gelatinous orange spore masses get formed in early spring, they get splashed by rain onto the nearby crab, apple or apple trees. Um, and they create a different type of spore structure on the apple. So the, the, it moves from the juniper. To the apple. And then late in the season, those, uh, bristle brush structures that Tiffany was describing. Um, if you touch them, sometimes you'll see a, a powdery, almost, uh, brick red type spores coming out of it, and those spores will get carried by the wind back to the juniper. So if one of those two plants is not present, then the rust isn't able to complete its lifecycle. It needs to go from the crab apple or the apple to the juniper, and then the juniper back to the crab apple.

Jennifer Burington: 9:51

Perfect. So I mentioned, I've seen cedar, apple, rust and stuff, and this is red star rust. It's different. So why is this plant pathogen distinct from, from the other native, native species that we have?

Tiffany Enzenbacher: 10:04

Uh, yeah, for apple and crab, apple, red star rust lesions tend to be larger than cedar, apple, rust, and also irregularly shaped. And we can expect our native cedar, apple, rust to have more circular lesions. And the lesions may also be smaller on that leaf. And if you turn the leaf over, those bristly spore structures are actually diagnostic. So if you have a small hand lens, or you could even zoom in with your smartphone camera. You can see that the red star rust bristles look different from the cedar apple rust. So the fungal structures for red star rust are long and tapered, and cedar, apple, rust bristles are curved in different directions and radiate outward from the center. And the bristles even look fuzzy, uh, without even using a hand lens. Uh, and for Juniper, currently the red star rust host range does not include our native eastern red, cedar or other North American species that we can grow in Minnesota. Uh, creeping juniper, common juniper, or rocky mountain juniper. Uh, so this is one way to distinguish between the two. Um, the second way to. Distinguished cedar apple rust from red star rust is the size of the gall, uh, red star rusts. Galls are very small, again, about the size of a pea or just a quarter of an inch in diameter. And cedar apple rusts. Galls can be quite large, uh, one and a half to two inches. And a third giveaway is the size and shape of those orange structures that protrude out of the gall, they're called horns. Uh, red star Russ has broad, flat, short horns, whereas cedar, apple rusts are pointed and long.

Michelle Grabowski: 11:34

And the cedar, apple, rust gulls because they're so big. And then those gelatinous orange spore structures are really long. Um, they're very dramatic and noticeable. Um, we do see them quite frequently and I remember, uh, getting phone calls and question. About, you know, orange aliens that had landed in people's tree or these weird tentacle octopus type things. So they're very dramatic looking. Um, whereas I think, uh, Tiffany's description of jello splats, um, in your juniper is, is a, a better description for the red star rust. Um, I'd also wanted to point out that, uh, one of the reasons we are interested. In, uh, looking more into red star rust is we know that many of our, uh, the apples grown here in Minnesota and crab apples have pretty decent resistance to cedar, apple, rust. So we may see spores, we don't see a lot of leaf drop, but we're not sure, um, how our native varieties or our cold hardy varieties, uh, I'm sorry. We're not sure how our cold, hearty cultivars of apples and crab apples will respond to red star rust. Um. In some of the original research work that was done in Japan, uh, and other Asian countries, there were apple trees that had some resistance and were able to tolerate the infection. There were also apple trees that when they became infected, they lost a significant amount of their leaves, and this resulted in lower fruit yields and, um, the trees struggling to be productive. And because the red star rust is a new disease here to Minnesota. We don't know how Minnesota's trees are going to react to this new fungal plant pathogen.

Jennifer Burington: 13:25

Since Red Star Rust is so new to Minnesota, um, Tiffany, what are you, what research are you doing, um, to learn more about this plant pathogen?

Tiffany Enzenbacher: 13:35

Yeah. We have multiple red star rust investigations underway with the ultimate goal of creating best management practices for the nursery and fruit industries here in Minnesota. And this is a joint project between the Minnesota Department of Agriculture and the University of Minnesota. The first investigation that we're undertaking is scouting. So scouting is a systematic way to inspect plants for disease, and we're doing this to see what cultivars are susceptible in a field situation. We scouted in 2024, and we'll look at the same plants again this growing season. And we're doing the scouting work at the Minnesota Landscape Arboretum in Chaska, which is in Carver County. And if you aren't familiar with the arboretum, it's a 1200 acre public garden focused on education, conservation, and research. We chose this location because it does have so many juniper and crab apple specimens. There are actually 45 reported Juniper hosts at the arboretum. We mapped them out and developed a route to scout them all. We began scouting in April, 2024 as the temperature started to warm up and as the rain began and we continued through the end of May, and as you probably remember, there was plenty of rain last spring, which out. Which meant we were out scouting quite a bit. We collected dozens of galls and branch samples. We would take a photo of each gll, put it in a paper bag, and then we submitted it for identification to a lab to confirm that it was indeed red star rust. So out of the dozens of samples, one gall came back positive for Red Star Russ, and it was found on a Chinese juniper. And in the summer we spent days and days at the arboretum scouting all the crab apples because there are so many over 500. We visited every single crab apple tree from mid-July into August, looking for those very characteristic red star rust lesions on the leaves. Large trees could take up to five minutes to scout, and small ones may just take a minute. And over 75% of all the crab apples at the arboretum ended up having red star rust. Uh, we were surprised at the results just because red star rust was just found in Carver County in 2022. And we also observed fruit infection for the first time to our knowledge in the US on two plants at the northern edge of the arboretum. And then we'll be out scouting again in 2025. So we'll have two full years of data.

Jennifer Burington: 15:57

So what about Junipers? Um, is there anything that can be done to manage red star rust on Junipers? Yeah,

Tiffany Enzenbacher: 16:04

so we are working on determining which fungicides are effective in preventing galls from forming on Junipers. We're testing several different fungicides on susceptible juniper species. We started our foliar fungicide applications in July when we could see apples and the surrounding orchards of the horticulture research center forming those briskly spore structures on the undersides of their leaves. And the horticulture research center is a research and outreach center in Excelsior, uh, with a lot of different apple research. And we treated the junipers at two week intervals until frost and to hedge our bets just in case natural infection wasn't enough. From the nearby orchards, we actually made a spore suspension, so we mixed spores with water. Uh, collected from Red Star Russ infected apple leaves and sprayed that on Junipers. So it actually takes, uh, 20 to 22 months for the galls on Junipers to fully mature and release those orange jello like spore masses. So we will evaluate the Junipers in spring 2026, and at that point we will look at several branches from each Juniper plant. And note how many galls are present to determine which fungicide is most effective.

Jennifer Burington: 17:15

So is there any other information about how Red Star Rust will act in Minnesota that growers need to know? Especially for like Junar purrs and um, there are a lot of people that do grow crab apple and apple trees.

Tiffany Enzenbacher: 17:29

Yeah, we're interested in finding out when the most red star rust spores are out in the environment. Uh, both in spring from Junipers and in late summer from Apple and crab apple to better time fungicide spray programs for growers. And we're doing this by spore trapping. Um, so how do we actually trap the spores? We have three Wind Sox spore traps out at the arboretum and the associated Horticulture research center. And these wind socks are exactly the same ones that you might see at an airport to determine wind direction. And then how do we physically trap the Red Star Rust spores? There are small clips attached to the inside frame of the wind sock, and these clips hold microscope slides which are coated with a very gooey petroleum jelly. So when we place the slides and the clips on the wind, so you know, any spore particle of soil, pollen, grain, or insect gets stuck to the petroleum jelly. And then we have a protocol in the lab to confirm the presence and the quantity of red star rust spores. We're working on this lab portion now with our 2024 season slides and we'll trap spores again in 2025 and we'll have two years of data to compare.

Jennifer Burington: 18:38

So there are a lot of junipers planted, um, in landscapes around houses, and then there's a lot of, like I mentioned, crab apple trees and apple trees around in the neighborhoods. So who should be worrying about red star rust and what management practices are recommended?

Michelle Grabowski: 18:54

So red star rust can in affect several different groups of, uh, plant growers here in Minnesota. Um, we have our apple growers who have orchards and they're raising apple trees for fruit to be sold at market. Um, we also have nurseries, which may be raising crab apple trees, uh, apple trees to be sold, um, to home gardens or other landscape settings, as well as raising junipers. Then we have home gardeners and people that are caring for trees that are already in their landscape. And again, that may include apple trees, crab, apple trees, and juniper trees. What we're seeing right now in apple orchards, as well as in home landscapes, um, and other, uh, settings. Homes, gardens, um, and parks. Um, we're seeing red star, rust present, but not at levels that would hurt the tree. Um, so although it's present, um, there's really no management action that is necessary in landscapes at this time for home gardeners to take if they do see red star rust. Um. Probably the biggest group that needs to be focused on managing red star rust at this time are nurseries here in Minnesota that are producing crab apple and apple trees for sale, as well as juniper plants for sale. And that's because here in Minnesota we do have a nursery statute that plants that are being sold perennial plants need to be free of. Pests and pathogens before they're able to be sold. Um, and so, uh, what we are recommending for nurseries right now is that they do use fungicides to protect, uh, apple and crab apple trees in, um, the spring when those galls may be s. Boulting on the Junipers. Um, so that's coming up soon. Um, because we wanna make sure those fungicides are there to protect those apples and crab apple trees, um, before they're offered for sale. So anything that you go to a garden center or a nursery to buy this year is healthy and disease free, and we're not gonna be moving it to new areas of the state or outside of the state. Um. We do know that the fungicide sprays that are used for cedar, apple rust are believed to be effective, um, in protecting our trees against red star rust as well. Um, but we are hoping to update those recommendations with some of the research work Tiffany and her team are doing at the University of Minnesota. Because of course the environment here in Minnesota, um, is unique to Minnesota and so we wanna make sure our recommendations match, um, the weather conditions that we have here so growers can do, uh, the minimum amount of treatments and still end up with healthy plants. We also, um, are really excited about this work going on at the university in protecting the Junipers. Um, what's tricky with the junipers is because that infection, um, is hard to see when the gulls are formed and it's in that plant without forming gulls for so long, it's very easy to, uh, look at a Juniper plant and believe that it's healthy. And move it or sell it, um, when it could actually be carrying red star rust. So that research is going to be really important in helping our nurseries protect their junipers to make sure the junipers that are going out are healthy. Um. But at this time, we don't have recommendations for that. So the focus right now is really on nurseries protecting their crab apple and apple trees to make sure that they're healthy for sale, um, for apple orchards and for lawns and landscapes and gardens. It's really more monitoring at this stage, um, and making sure that, uh, you are reaching out to the MDA if you do feel that it's moving to a stage, um, that it's, it's harming the health of the tree.

Jennifer Burington: 22:51

So what should Home Gardeners do about Red Star rust if they think they have red star rust?

Tiffany Enzenbacher: 22:58

Well, the first thing we recommend is to start healthy. So if you are, are buying a new plant at a garden center or nursery, always make sure the plant has no visible symptoms of disease. So you're starting clean. Uh, the plant doesn't have any disease spots or stem caners doesn't appear stunted. No chlorotic or yellow leaves. Uh, no insect pests or evidence of insect feeding. And overall, the plant should appear healthy and vigorous. Um, the second recommendation is to not plant reported Juniper hosts within a several mile radius of apples. Um, those junipers again, are Chinese juniper sergeants, flaky garden, and pfitzers. Uh, this of course, isn't always possible, but ensuring that these junipers are several hundred feet away, or at the very least, you don't have a mix of these junipers with your apple and crab apples in your own home landscape. And in your home landscape, make sure your apples and crab apples are healthy and properly pruned. So if you have a lot of inside branches and water sprouts, the leaves inside may not be drying out. And after a rain event, uh, infection can occur because of that sustained leaf wetness. And if your tree does have a minor red star rust infection, as Michelle indicated, chemical intervention isn't typically necessary. And for Juniper, we recommend that you prune off bag and discard the galls that you find in spring.

Michelle Grabowski: 24:19

Right. And I think it's important for, uh, home gardeners to understand that on a apple or a crab apple tree, uh, we've seen the infection on leaves is the primary source of in where we're seeing the disease. Uh, as Tiffany mentioned, we've ha found it on fruit, um, in some cases. When, uh, fall happens and the tree drops, all that leaves, um, that fungus is going to break down as those leaves break down. And so it typically will not survive on the crab apple tree from one season to the next. Um. We are coming up on our spring season, um, as we have warmer, wet days, A home gardener could go out if you have junipers in your landscape. Um, go out after it's been raining for a couple of hours and closely look at those plants and you might see, uh, either some of our, our native rusts, like the cedar, apple rust, or the Hawthorne rust. Um, you might see something that looks like a red star rust. And as Tiffany mentioned. It's pretty easy to clip those off, stick 'em in a bag, um, to make sure that those spores don't get spread around through the landscape. Um, once we do reach our warmer, drier summer weather, those gulls will naturally dry up and die. Um, but if you find them early in the season, um, it's not a problem to clip them off. Um, you won't be hurting the juniper and you can reduce the amount of spores that are spreading to nearby apple and crab apple trees.

Jennifer Burington: 25:50

Okay, so what um, should we do if we find, if we think we have red star rust, should we report it in to anybody? Should we. Um, contact the university or the MDA.

Michelle Grabowski: 26:02

So at this time, the Minnesota Department of Agriculture is monitoring where red star rust is found throughout the state in Minnesota. We know that it occurs in the seven metro counties as well as Rice, Le Sueur and McLeod County. Um, so if you're finding it and you live in one of those. Counties. Um, we know that it's there and we don't need it to be reported to us. Um, however, if uh, you are in one of the counties in greater Minnesota, farther from the Twin Cities metro area, and you do see something that you suspect is red star rust, please do take a picture. Um, it's really helpful for us to see. Either a picture of the gall on the juniper, um, with the spoilation or without whatever you can get a picture of. Um, if it's on a crab, apple or an apple, please take a picture of both sides of the leaf. Um, so if you remember Tiffany mentioned there's those, uh, bristle, like fungal spore producing structures on the lower surface, and that's one of the things that we look for when we're trying to identify which of the rust we're we're looking at. So taking a picture of both of those and then reporting it through Report Aest, um, to the Minnesota Department of Agriculture and someone will get back to you, um, about any questions that you have, um, and possible suspects through Red Star rust on report A Pest.

Jennifer Burington: 27:28

And that report a pest, um, information is, uh, we have an online form that you can fill out and attach a picture. Um, just give your, your name and email address and stuff. And then we also have an email address, um, as well, so the website to get more information on how to report in. If you see some, uh, possible red star rust on some crab, apple and apple trees or on juniper's, uh, this spring here is www do MDA state mn us and forward slash and then this is all one word Together, report a pest. And then our email address is, again, one word report a pest at state mn us. And like Michelle mentioned, please attach pictures, um, of what you're seeing. Give us a general idea of the location. Um, and we'll get back to, Michelle will probably get back to you. She's the one that usually takes all the, all the plant pathogen questions. Thank you Michelle and Tiffany for bringing this issue to our attention. Uh, your work to control this and other plant pathogens is very important for our state's trees and everything that depends on them. This has been Smarty Plants, a podcast from the Minnesota Department of Agriculture. Our producer is Brittany Raveill, our editor is Larry Schumacher, and I have been your host. You can learn more about pests and other invasive species that affect our environment at www dot mda dot state dot. MN us and while you head to our website, we'll be working on the next episode of Smarty Plants. See you there.