High-Temperature Adult-Plant Resistance with Tim Murray

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Episode Transcription:

[ Music ]

Drew Lyon: Hello, and welcome to the WSU Wheat Beat Podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. We have weekly discussions with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production. If you enjoy the WSU Wheat Beat Podcast, do us a favor and subscribe on iTunes or your favorite podcasting app and leave us a review while you’re there so others can find the show too.

[ Music ]

Drew Lyon: My guest today is Tim Murray. Dr. Murray is a professor and an extension plant pathologist who has been on the WSU faculty since 1983. Prior to joining the Extension Small Grains team in 2013, he taught introductroy plant pathology. His research program focuses on integrated control wheat diseases, especially eyespot, cephalosporium stripe, specled snow mold, soilborne wheat mosaic, wheat streak mosaic, and other diseases as needed. Hello Tim.

Tim Murray: Hi Drew. How are ya?

Drew Lyon: I’m doing well. I asked you to come in today to talk a little bit about high-temperature adult plant resistance and why it’s important for stripe rust. It’s a topic that I think comes up often but I don’t think is very well understood by a lot of people so I’m hoping you can help us understand it a little better.

Tim Murray: Yeah. Well, you’re exactly right about that. It is not well understood, and I get asked about it a lot, and the scientific literature is not much help. And so I went to our resident stripe rust expert, Dr. Xianming Chen with the USDA-ARS and had a conversation with him about it to try to get the facts straight. So high-temperature adult plant resistance is all, often called HTAP resistance, or some people just call it H-TAP, but it is one of tw0 types of disease resistance that is used in wheat to control stripe rust. The other type is what we call all-stage resistance, or also called seedling resistance. So stripe rust is the most common and destructive foliar disease of wheat in the Pacific Northwest, both winter and spring wheat. And that’s been true for a long time. Stripe rust has become more important since about 2010, where it has occurred most years at a destructive level, and I think probably that’s due to winter temperatures becoming generally milder and allowing the stripe rust fungus to survive better from year to year. When stripe rust is bad, yield losses can exceed 70% if it’s a susceptible variety and if the disease is not controlled. And consequently, growers spend a lot of money spraying fungicide to control this disease, and I think anything we can do to reduce that is good. Improving effectiveness of disease resistance in wheat varieties is the best thing we can do.

Drew Lyon: Okay, so you mentioned two types of resistance, high-temperature adult plant resistance, and all-stage resistance. How do those two differ from one another?

Tim Murray:  Well, as the name high-temperature adult plant resistance suggests, this is a type of resistance that is sensitive to temperature and to plant growth stage. So adult plant resistance or the HTAP resistance doesn’t really start to become effective until plants are close to jointing stage at jointing or after jointing stage. So that’s the adult plant part of it. The high-temperature part of it is the temperatures need to reach a certain level for the resistance to become effective. So if it’s too cool, those temperatures or that resistance is not going to really kick in. Seedling resistance is really kind of a misnomer. Seedling resistance is more accurately described as all-stage resistance because it is effective throughout the life of the plant. It’s just most easily detected at the seedling stage when adult plant resistance is not effective. And so, you know, in the greenhouse, they test for seedling resistance when wheat plants are about 2 leaves. There’s some other important differences between these types of resistance. With adult plant resistance, HTAP resistance, it is nonspecific resistance, and that means that it is effective against all races of the stripe rust fungus. All-stage resistance, on the other hand, is specific and effective only against some races of the pathogen. So it’s effective against some, ineffective against others. And consequently, plants that have all-stage resistance only are either resistant or susceptible.

Drew Lyon: Okay, so races. Describe to us what a race is, and how many are there out there?

Tim Murray:  Yeah. Yeah, that’s a good question. Well, I usually start this description by saying the thing that we probably relate to most when we think about races are strains of the flu virus. You know, we hear about H3N1 and all these different combinations. Well, a race is one type of pathogenic specialization that occurs in plant pathogens, okay. There’s more than one, but with races, a single race of a pathogen can infect some varieties of a plant but not others depending on which resistance genes it contains. The other way to think about this is that a single resistance gene is going to be effective only against some races. That’s what we call specific resistance. And this is the type of resistance that we get with all-stage resistance. Other resistance genes are effective against all races of a pathogen, and we call that nonspecific resistance, and that’s more of the HTAP type of resistance. It’s a nonspecific resistance. So this is the great arms race with Mother Nature. Plants have resistance genes to allow them to overcome the pathogen. Pathogens develop virulence genes to overcome the resistance genes in the host. And so what that means is that races can change from year to year, and they do change from year to year, and in different parts of the country, different wheat-growing areas, you will find different races of the same pathogen. And that depends partly by the varieties that are grown and the resistance genes that they contain. So that’s something that’s important to monitor from year to year to know which races are present in a particular area so we know which resistance genes we should be using.

Drew Lyon: So it’s, in a way, evolution in progress. It’s– No species or organisms wants to die out, so they’re looking for ways to survive.

Tim Murray:  That’s exactly right, yeah.

Drew Lyon: Okay. So I asked you here because you recently wrote a timely topic on this issue of high-temperature adult plant resistance. Why is this topic important at this time?

Tim Murray:  Well, we’re at that point in the growing season where we’re starting to transition into the warmer spring temperatures. We’re kind of late spring now. It’s been warm. It’s been cold. But now, we’re sort of starting to ramp up. And so I often get the question from growers and field consultants: “How warm does it need to be for HTAP resistance to really kick in and become effective?” And there really are no clear guidelines for this, which is why I went to talk to Dr. Chen to clarify about how all of this works. And the answer is that it’s complicated. [laughter] And it’s complicated because there’s a large number of genes involved. There’s more than 30 different genes involved in high-temperature adult plant resistance. Those genes have different levels of effectiveness. Some are very effective. Some are less effective. Some are more sensitive to temperature. Some are more sensitive to growth stage. And so it depends on the combination of genes that you have in a particular variety. A variety may have 1 or 2 genes, or it may have 5 or more genes for resistance. And so the resistance of that variety and how it expresses that HTAP resistance is going to be different. And this timely topic goes into quite a lot of detail on stripe rust resistance, but hopefully it answers some of those questions about HTAP resistance.

Drew Lyon: Okay, so we kind of got to this, but how warm is warm, and how much warmth do you need to – where’s kind of the threshold? Are we talking the 40s, 50s, 60s? At least give us a little bit of a guideline realizing that there’s a lot of play in there.

Tim Murray: Right. Yeah. So when I asked Dr. Chen this question, he said, well, you know, in most years, if we have temperatures that are close to our seasonal averages, those are good enough for the HTAP resistance to kick in. Okay, so more specifically, though, the wheat plant — again, the adult plant part of this — the wheat plant needs to be near or beginning to joint, that stem elongation phase. Some varieties may begin earlier, some later, again, depending on which genes are present. But nighttime temperatures need to be in the 50s, and daytime temperatures need to be in the 70s as a general rule. Okay, the interesting thing about this — it’s kind of like your tomatoes and why we sometimes have a hard time getting them to ripen is that nighttime temperatures are as important as daytime temperatures.

Drew Lyon: Okay.

Tim Murray: So you can have daytime temperatures that might reach into the 70s, but if the nighttime temperatures don’t get into the 50s, then you’re not going to see this expression of the high-temperature adult plant resistance coming on as quickly. The other thing about it is is that these are what we call quantitative genes, and so the resistance, if we get conditions that allow it to increase and become effective and then it cools off after that, the resistance will fade away.

Drew Lyon: Okay, I was going to ask about that. Interesting.

Tim Murray:  So it’ll be, it can become less effective if the temperature doesn’t remain favorable. So interestingly and maybe not surprisingly, these same temperature are also the temperature range that is good for the stripe rust fungus. And, going back to your question about or your comment about evolution, the plant and the pathogen evolve together over a long period of time. So temperatures in that range of 46 to 54 degrees Fahrenheit are good for infection, and temperatures from 50 to 75 are good for disease development. So the same temperatures that are good for the plant are also good for the pathogen.

Drew Lyon: Okay, very interesting. If our listeners want to get more information on stripe rust and high-temperature adult plant resistance, is there a place they can go to find that information?

Tim Murray:  Yeah. We have, well, we’ll have this timely topic posted online, and that will be available. We also have a fact sheet on our website, smallgrains.wsu.edu, that talks about stripe rust, talks about the biology of stripe rust, and talks about control options, including fungicides and the options available for control. Dr. Chen also has a website. It’s striperust.wsu.edu, where he has information as well.

Drew Lyon: Okay.

Tim Murray: And you can always give us a call, email, or Twitter.

Drew Lyon: All right. Yes, you’re one of our Twitter users here on the Small Grains team, and so leading the way there. We’ll make sure we get those links into our show notes for our listeners. Any last comments before we say goodbye?

Tim Murray: Well, I think, judging by the forecast for the next week, I think we’re getting to those temperatures where we definitely ought to be seeing the high-temperature adult plant resistance kicking in, and that’s good news, you know. In general, this year, stripe rust incidence has been low, and if our conditions continue like this, Mother Nature’s going to help us out with temperatures that are favorable for that adult plant resistance.

Drew Lyon: All right, Tim. Thanks for making the time to come talk to me today.

Tim Murray: Yeah, you bet, Drew.

Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat Podcast. If you like what you hear, you can subscribe on iTunes or your favorite podcasting app so you never miss an episode. And leave us a review while you’re there. If you have questions for us that you’d like to hear addressed on future episodes, please email me at drew.lyon@wsu.edu. You can find us online at smallgrains.wsu.edu. You can also reach out on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat Podcast is a production of CAHNRS communications in the College of Agricultural, Human, and Natural Resource Sciences at Washington State University. I’m Drew Lyon. We’ll see you next week.