Nicholas Greatens Talks Emerging Biocontrol For Warding Off Invasive Plant Species in Minnesota

January 30, 2020

Interview by Ryan Murphy, University of Minnesota, Department of Forest Resources

Generally speaking, us plant folks don’t like to hear about rust fungi infecting our beloved photo-synthetic cousins… until now. TreeIQ sat down with Nicholas Greatens, a graduate student in the Department of Plant Pathology at the University of Minnesota, to discuss his research exploring whether a newly discovered strain of rust fungus could be used for safe and effective biocontrol of glossy buckthorn and reed canary grass.

TreeIQ: So I’ve heard about a rust fungus which infects common buckthorn, maybe you could start by explain-ing a bit about that, and then how that relates to your current research?

N.G.: That is oat crown rust. Oat crown rust has two different hosts. It lives on oats throughout much of the summer, and once the oat plant starts to senesce, the rust fungus will start to produce overwintering spores called teliospores, which then germinate in the spring to cause infection on common buckthorn (Rhamnus ca-thartica). The rust life cycle is pretty complicated - with five different spore stages. This is a very devastating dis-ease for oats in Minnesota, sometimes causing up to 50% crop loss.
We are working on a related rust also in the crown rust complex. It’s called crown rust because the overwinter-ing teliospores look like they’re wearing a little crown. For a while, it has been considered one or two species, but in the past 20 years or so, a few new forms have been identified. It’s all a bit of mess right now. They’re all fairly difficult to differentiate just based on morphology. There are a couple of differences such as size of the teliospores, but without looking at DNA sequences or inoculating the rust onto its host plants, it’s difficult to identify.

TreeIQ: So the species of rust you are working with is more recently discovered?

NG: Yeah, about two years ago, my advisor and some other people started to see it widely on glossy buckthorn (Frangula alnus).

There are a couple of species of buckthorn. There’s common buckthorn, Rhamnus cathartica, which is the awful one that you see everywhere in the cities and our forests, and there’s also glossy buckthorn, which is actually rated as a worse invasive by the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC) here at the University of Minneso-ta. Glossy buckthorn is frequently found in the Great Lakes region and New England. It’s a really awful invasive that tends to invade wetlands more than common buckthorn does.
We knew that there were some rust species associ-ated with glossy buckthorn previously, but this one seems to be totally different. It causes much great-er infection, to the point where almost all the flow-ers are infected and can cause pretty severe defoli-ation—especially in areas where both hosts are abundant.

TreeIQ: Do you think this was some sort of muta-tion or an introduction of a new rust species?

N.G.: A key region of the rust’s DNA suggest it is likely an introduction of a European form of crown rust. A couple of papers out of England also de-scribe a rust with a very similar host range. But we can’t yet say definitively.

TreeIQ: So it only severely infects glossy buck-thorn, not common buckthorn?

N.G.: It doesn’t seem to infect common buckthorn as far as we can tell. We’ve tried to infect it and it doesn’t work. We do have a native buckthorn here in Minnesota, Rhamnus alnifolia, but we’re not sure if it infects that yet. It does infect a US native buck-thorn species called Frangula caroliniana, which doesn’t grow here in MN.

TreeIQ: So what about the other host for this rust? 

N.G.: Reed canary grass is the other host.

TreeIQ: And reed canary grass is a problem species?

N.G.: It’s a huge problem – probably one of the worst invasive plants in Minnesota. If you go to some of the wetlands around here, it almost com-pletely dominates them along with glossy buck-thorn, cattails and the common reeds.

TreeIQ: So we’re just getting lucky and it’s infecting two of these really bad invasives?

N.G.: Yeah, it is kind of lucky. Usually, we think of rust diseases as being really harmful to plants and our forests. For example, there is a bad rust disease on white pine (white pine blister rust), Cronartium ribicola, which also affects currants. There’s other rusts that infect hawthorns pretty severely, apples can get cedar apple rusts, quinces get bad rusts – yeah, there are lots.

TreeIQ: Are you hopeful this rust can be utilized as a potential biocontrol?

N.G.: We’re not certain about its biocontrol poten-tial at this point. Certainly this is going to keep in-fecting these two invasives naturally. It will proba-bly negatively impact both of these plants, especial-ly the glossy buckthorn, since there will be a reduc-tion in seed production.
There is potential though. We don’t know this for sure yet, but we think it’s somewhat promising for controlling reed canary grass, which is also inter-esting, because there are very few controls of this grass that we can use right now. The idea is to har-vest spores from infected plants, grown in a green-house for example, and infect reed canary grass earlier in the season than would normally occur naturally. This could be especially useful when you are trying to ward off reed canary grass in places without a lot of natural infection, for example in an ecological restoration. In that type of situation, you have this invasive grass competing with all the na-tive grasses and wildflowers you just planted, so spraying this rust everywhere and infecting young reed canary grass seedlings could help reduce its competiveness.

TreeIQ: Are you also considering off-target effects such as negative impacts on other species?

N.G.: We are looking at a lot of native grasses and important forage grasses that could be affected. So far we haven’t seen too much infection - it seems to be pretty specific to reed canary grass. But this all very preliminary data.

TreeIQ: So these new strains of rust that you have, did you collect them from campus, greater MN, other places?

N.G.: Yeah, all over. We have some from Duluth, some from southeast MN, a lot from the metro ar-ea, and we have some collaborators in New Eng-land who’ve sent us samples of infected F. alnus which we infected reed canary grass with and made isolates from. 

If anyone sees particularly severe infection on ei-ther glossy buckthorn or reed canary grass, we’d love to see it or get a sample. If anyone sees rust on the MN native buckthorn, Rhamnus alnifolia, that would also be fascinating.
Other forms of crown rust affect Elaeagnus (Russian olive) and Shepherdia (silver buffaloberry) in west-ern Minnesota. If anyone finds rust on those spe-cies, we would love to see that!
If you think you may have an interesting sample for Nick, you can get in contact with him via email at

PDF icontiq_winter_2019_for_review.pdf