MENDOCINO Co, 10/13/22 — Pierce’s Disease is a deadly grapevine disease, transmitted through the bacterium xylella fastidiosa by bugs called sharpshooters. Historically its effects have been more prominent in Southern California, but Mendocino County has its own Hopland-specific strain. Scientists at the Hopland Research and Extension Center (HREC) are working to learn more about how the disease spreads and how climate mitigates its impact.
The Mendocino Voice caught up with Andrea Brown, the Hopland Scholar for the year 2022-23, and Monica Donegan, who does research on a grant through the California Department of Food and Agriculture. Both are studying Pierce’s Disease in the Almeida Lab at the University of California, Berkeley and conducting climate-focused experiments at HREC — Donegan by focusing on the bacteria xylella fastidiosa, and Brown by focusing on the immune processes of grapevines.
The Mendocino Voice: Can you tell me about your area of field work and how you came to it?
Andrea Brown: I’m Andrea. I’m a second year PhD student in Rodrigo Almeida’s lab at Berkeley, and I’m generally interested in how climate mediates plant response to disease.
Monica Donegan: I’m Monica, I’m a third year in the PhD program. Before grad school, I was working in biotech in Boston, and I was at an agriculture company that was studying plant diseases in citrus. So I was interested in plant diseases, so when I was applying to grad school, I came across Rodrigo’s lab. I’m particularly interested in how the pathogen evolves, so how xylella is adapting to climates in different areas. It does also affect citrus, olives, coffee. But for this project in Mendocino, we’re focused on strains that infect grapevines.
TMV: Can you break down the methodology for your fieldwork? Andrea, I know you’re a bit earlier in the process.
AB: What distinguishes our research is I’m looking more at the plant side of how climate is affecting disease progression, and Monica is looking more at how climate affects the bacteria itself. Part of both of our research is looking at this overwinter curing phenomenon of Pierce’s Disease, which is a bacterial disease caused by xylella fastidiosa. When plants are exposed to cold weather in the winter, they come out of the winter in the spring, without the disease sometimes, but it’s not really understood what the plant’s immune mechanism is that’s causing that curing phenomenon. So I’m interested in looking at the plant side of that, and then Monica is also looking at how the bacteria is evolving in regards to temperature.
MD: With overwinter curing, it may be that the plant responds to the cold, and then the plant is producing some sort of chemical or immune response that then eliminates the bacterium. Or it could be that the bacterium is sensitive to the cold itself. So whether it’s a direct response to the cold, like the bacterium is not surviving X temperature, or it’s an indirect response in that the plant responds to the cold, and then that cures the plant of the disease, isn’t really known. We’re approaching it from both angles. It’s likely both — like there’s probably a plant response, but then there’s also differences in the bacterium of their ability to survive the cold. So I think it’s kind of cool that Andrea and I are working on different sides of this.
AB: I just started this experiment — so no results or anything, but I started a field experiment putting plants out over the winter at Hopland and at Berkeley, and then also at Blodgett Forest Research Station, which is out towards Tahoe and has very cold winter temperatures. And so one part of the project is tracking temperature during the winter and seeing how that correlates with curing at the different places and then part of it is trying to get at when during the winter we see the curing phenomenon happening. So we’re going to bring plants back to the greenhouse at different times during the winter, and then look at those cohorts of plants and compare curing rates between those different cohorts of plants so that in the future, we can go in and do more experiments, knowing what part of the winter is important.
MD: Andrea also infected all of these plants in the greenhouse before. So they all have Pierce’s disease. And so then when they’re being put in the field, we already know that they’re infected.
TMV: How about your project, Monica?
MD: At the Hopland Research and Extension Center, there was this vineyard that was initially set up to look at how Mediterranean grapevine cultivars do in California. They wanted to evaluate these different grapevine cultivars, potentially, to grow here. And so at the end of that experiment — that was a different researcher — we were interested in this because we could actually infect these vines with Pierce’s disease. And that’s a very rare opportunity, because for obvious reasons, a lot of commercial vineyards don’t want you to infect their vines — but it’s also really important to learn about how the disease progresses in the field. We wanted to compare two different strains of Pierce’s disease — and when I say strains, they’re a little bit different genetically, but they’re still the same species. One of the strains is local, from Hopland. I isolated it from there the previous year. It has been circulating in this area, and potentially it’s adapted to these colder winters. The other strain that we used is from Bakersfield, California, which is in the South Central Valley. It’s really warm there during the summer, and during the winter it also doesn’t get very cold, and in the past, there hasn’t been as much overwinter curing there. So this pathogen, the strain that’s circulating in Bakersfield, may not have been exposed to these cold winter temperatures. So we infected all these different grapevine cultivars in May of 2021 with the strain that’s local from Hopland, and the strain from Bakersfield. The hypothesis was that the local strain would survive the winters better if it had adapted to these conditions.
In September, we didn’t see any difference in the infection rates — both the Bakersfield and the Hopland strain in the first year were able to colonize the vines, and we were able to detect the infections. And then this year, we started to evaluate symptoms and also to detect the pathogen again, and we actually are seeing a pretty stark difference in between the two strains, which is really interesting. When they had never been exposed to winter, the disease rates were similar between the two strains, but the strain that’s from Hopland seems to be surviving the winter at greater rates than the strain from Bakersfield. But that doesn’t necessarily mean that there’s no plant response involved. We’re also going to look at between all these different Mediterranean cultivars whether you see different rates of overwinter curing, but it does indicate that there’s some difference in these two bacterial strains that affects their ability to survive the winter. It is some support for the fact that genetic differences in the bacteria are important to this overwinter curing, but I do want to emphasize that it doesn’t eliminate something on the plant side as well. They can both be true.
TMV: What does the process of overwinter curing look like for the plant?
AB: That’s not really known. That is the big question that I’m interested in exploring — maybe not answering, you never know how far I’ll get. But what we see is you’re able to say like cured or not cured, because they do the qPCR [molecular detection of the pathogen] in the fall, and then again, at the end of the next growing season, so you don’t see symptoms, and you also can’t detect it with the molecular tools as well.
MD: With grapevines, they prune them year to year. And so when the plant gets infected, say that bacterium is in one shoot, that shoot might get pruned off over the year. So it’s important that the bacterium moves down into the trunk and into the cordon — usually grapevines are trained with a west and east cordon. They’re bilateral. The bacterium has to move down into this trunk, so it doesn’t get pruned out – and so the movement of the bacterium is important for surviving the winter. That’s something we’re also tracking in this study is how quickly that happens, because we know exactly where it got infected, which is kind of unique. Usually, this disease is transmitted by insects, so it could be anywhere on the plant, but because we infected these plants, we know exactly where on the vine it started.
TMV: Do you have reason to believe that changing climate and changing characteristics of average temperature could contribute to higher rates of Pierce’s Disease here?
MD: We don’t know how recently this strain evolved, or if it’s been circulating for a while, but even if it was able to survive the winters, you may see less symptoms, because it was colder, there was less of the bacteria. And so with climate change, you might start to see more symptoms and more noticeable disease prevalence, even though the bacterium has been in Hopland for a long time. Twenty, 30 years ago, they were able to detect it there, but the disease wasn’t really much of a problem. There could have been evolution on the bacterium to survive the winters, but the actual expression of the disease and the prevalence of symptoms in the plants could be related to climate change, as well. Those things are really hard to tease out because you need a lot of data over time — something that would be really nice to have is historical disease surveys in different parts of California. But there’s not all of that data, and Andrea and I would both love to have some of these historical surveys so that you can look at climate change. But I think both bacterial evolution and climate change are probably contributing to increasing disease prevalence in Hopland.
AB: Yeah, folks have published a few papers trying to model disease range, with temperature as often one of explanatory variables. If those models are correct, then one of the papers says you need x number of hours below six degrees Celsius, and then the plant is more likely to cure. So if that is the case, then you could expect as temperatures warm, that there might be a little less curing of the disease. But again, there’s very little known about this curing process, and so it’s hard to say anything definitive. It’s a lot of “might”s and “may”s.
MD: What part of the winter? Is it the absolute minimum, if it gets below zero? Is it number of hours below six degrees Celsius? In these models, they’ll use different explanatory variables that are all dealing with winter temperatures, but what part of winter temperatures is contributing?
TMV: What has been the most interesting or exciting finding or breakthrough moment in your research so far, or in the lab generally?
MD: Our results this summer were really exciting, because in the past people have done some strain experiments, some USDA scientists, and they compared strains in climates that were a little bit warmer than Hopland, and they didn’t actually see any differences in the overwinter curing. Even if you look at overwinter, carrying in like the Central Valley between strains, you might not see a difference. When we were doing this in Hopland we thought we might see a difference, because it’s a lot colder there, so it might matter more. But, you never really know, it’s a hypothesis. So this year, when I started to do the qPCR results, and I looked at the rates, and almost all the positives were from the Hopland strain, I was actually really excited, because it’s nice in your PhD to have something that actually is what we expected. There’s a lot of questions to ask after this, but it’s a starting place.
TMV: How do you envision that the Department of Agriculture or specific local vineyards could use your eventual findings?
MD: I think specifically with the Hopland site, I mentioned there’s 13 Different Mediterranean cultivars, and some of them are more susceptible to Pierce’s Disease, some of them are less susceptible. So on that level, I think it’s cool that we’ll be able to have a lot of data about these cultivars and their susceptibility to Pierce’s disease. That could be important for evaluating which ones to grow in California and having some baseline data on disease susceptibility. When people try to predict where a disease is going to spread over the world, they lump all of these different strains together, as one homogenous group to say, this is how the species is going to spread. But it’s actually really important to know differences between the strains. So I think, for the Department of Agriculture, they’re always worried about what quarantine measures you have to enact for a specific pathogen. But knowing differences between strains of a pathogen is important because you might not need to do quarantine measures if you have a strain from Southern California that’s not as good at surviving the winter. But if you know you have this other strain that’s resistant to overwinter curing, it might be more of a risk. So I think understanding these genetic differences and what they actually mean for the bacteria surviving winter and adapting to climate is important for predicting where disease will be in the future.
AB: One of the things Monica mentioned earlier that we’re trying to tease apart is kind of what winter factor is actually affecting overwinter curing, and I think knowing that in the future could help vineyard managers inform their disease management strategies. So if you know you went through a really cold winter specifically — say it seems like it’s hours below a certain temperature that’s most important — if you know you went through a winter like that, maybe you’ll expect less of the disease to come back the following year, and you might need to manage less intensely. Whereas if you have a really mild winter, you might expect more disease prevalence in these kinds of Northern regions where curing is affecting disease prevalence. So as Monica mentioned, in the Central Valley where there’s less curing happening, variation in winter temperature there is probably not going to affect curing because it’s just not that cold. But in somewhere like Mendocino where it’s on that borderline of some plants cure, but not all plants, knowing what’s happening over the winter could help people with disease management.
TMV: How does working with the HREC help you and what are the benefits of that?
AB: For me, I mean, it’s made it possible for me to do this field experiment. You can try to emulate winter, as best you can in say, a cold chamber at Berkeley, but getting the plants actually out into the field and the real temperature conditions that commercial growers are experiencing is way better. Having the chance to do that at Hopland, where Pierce’s Disease is locally present especially is huge for my project.
MD: It’s a great collaboration, because they had this vineyard that was going to be retired and it allowed us to actually infect mature grape vines, which is a really rare opportunity. They also have experienced research associates on their staff that manage the vineyard and do a lot of that work. We come up to do the sampling, but it takes both pieces of that to manage the vineyard over the whole year. So they do a lot of work also to keep things running. For these overwinter curing experiments, we’re only going to be there some of the time and they’re going to help monitor the plants and tell us when the buds break and stuff like that.
TMV: Is there anything I didn’t ask about that you think would be interesting for people to know?
MD: Just the history of this. Our lab is now the Almeida lab, Rodrigo Almeida is in charge right now — and the professor that he studied with before was Sandy Purcell, and he’s still around Berkeley as an emeritus professor. But he was the one who actually discovered this whole overwinter curing stuff, and a lot of this work was done in the early 2000s. And when he was doing these experiments, I wanted to say the overwinter curing in Hopland was around 100%. And so even from like the early 2000s to now, you’re seeing differences in what the overwinter curing is — and I also just think it’s interesting that like, our lab has this long history of being interested in some of these questions, and we’re like picking up a little bit on it now.
AB: He did really great experiments also in the ‘80s — he brought plants out to Blodgett Forest Research Station back then, and so one of the reasons I chose that location was to mirror those results and try to compare to temperatures now. And then Hopland I chose because it is in this in-between area, climate-wise, and also because we have this relationship now with the Research and Education Center. They’re great to work with. So there’s a combination of good climate conditions and a great support system to work with.
Note: Kate Fishman covers the environment & natural resources for The Mendocino Voice in partnership with a Report For America. Her position is funded by the Community Foundation of Mendocino, Report for America, & our readers. You can support Fishman’s work with a tax-deductible donation here or by emailing [email protected]. Contact her at KFishman@mendovoice.com or at (707) 234-7735. The Voice maintains editorial control and independence.