 Hello, I'm Julie Pashie, potato pathologist at NDSU. Thank you for your time and attention today. Also, thanks to Andy Robinson for putting together this series of presentations. Andy has asked me to talk about the emergence and prevalence of potato mop top virus. Potato mop top virus is present in many potato growing countries around the world. It's been a serious problem in Europe for decades. Studies that were conducted in 2001 and 2002 in the US and Canada looked at about 3,000 seed lots and found that approximately 4% of them were positive for PMTV. Unfortunately, in this study, no province or state distribution data was presented. So at that time, we did not know where the pathogen was. Another interesting finding with this study was that they looked at the virus strains in the US and Canada and compared those, found that they were 100% similar to each other and then compared the North American strains to European strains and found that they were 97% similar. This is important because when we look at things like genetic resistance and other control measures, we know that we're dealing kind of with the same pathogen population. PMTV has been officially reported in seven states in the US, starting with Maine in 2003. In addition to that, it has been recently detected in a couple of other states, but these have not been officially reported. PMTV has a complex genome with three parts, but not all plant tissues may contain the completely assembled virus particle. The incomplete virus particle may still be infectious and capable of movement in the plant, but they might not cause symptoms or be moved to all tubers or stems. This is important because when we look at detecting the virus, one tuber may test negative, but other tubers produced by that same plant may be infected. PMTV can cause foliar symptoms including stunting, modeling, chevrons, and yellow blotches or rings, but these are only rarely seen in the field. Foliar symptoms occur from seed infections. They do not develop from soil-borne infections. These foliar symptoms can be easily confused with those induced by alfalfa mosaic virus and typically occur on lower leaves. Once ground cover is achieved, these leaves senesce due to lack of sunlight. Therefore, they essentially disappear as the season progresses. Little is known about differences in foliar symptomatology among cultivars. PMTV does not typically reduce yield. What is most important is the reduction in tuber quality caused by internal tuber necrosis. Typical symptoms of PMTV tuber necrosis are shown in the lower right-hand photo. These include internal rust-brown arcs, rings, or flecks. They may be single or concentric arcs, and these may sometimes appear on the tuber surface. PMTV symptoms can be confused with symptoms of other tuber necrosis viruses. PVY in the upper left-hand corner produces raised dark brown or redden rings on the surface that can progress into sunken necrotic areas. Necrosis may extend into the tuber flash. Tomato spotted wilt symptoms in the upper right can include rings or general discolored areas on the tuber surface or flash. What are most commonly confused with PMTV tuber necrosis are symptoms caused by TRV. Tuber necrosis symptoms caused by tobacco rattle virus appear as rust-brown arcs, concentric rings, or extensive browning of the tuber tissue as seen in the photo on the lower left. TRV may also produce concentric rings or arcs externally. I'm in a lab today with Obseda Malik. Hello. Obseda's a molecular diagnostician. She has developed and runs molecular diagnostics for petal pathogens for several years. Obseda received samples from growers and based on the rower recommendations, as well as the symptoms she's seeing, she determines what assays should be run to diagnose the problem. Obseda will go through for you today. The difficulties in discerning symptoms between a potato mocktop induced tuber necrosis and tobacco rattle induced tuber necrosis. The tobacco rattle virus is vectored by nematodes. Here we can see the tobacco rattle virus symptoms in tuber can range from necrotic spots to arcs. In this tuber, for example, we see kind of spots and here it's kind of a necrotic arc. Sometimes the tubers can be totally asymptomatic, meaning that they are not showing any symptoms. On the other hand, the PMTV, which is vectored by powdery scap pathogen, sponges porous subterinia, produces the characteristic necrotic arcs. Sometimes the arcs are not fully formed and ends up looking like the necrotic spots, just like the TRV symptoms. Hence, you can see like this are so similar that they are caused by different pathogens. This one is tobacco rattle virus and this is the potato mocktop virus. Hence, it's very important after visual diagnosis, we need to confirm the pathogen by PCR that's caused for molecular diagnosis. PMTV is a complex pathogen, so I just want to talk a little bit about some plant pathology. On the right hand side, we have what we call our disease triangle. We have the pathogen interacting with the environment and in this case, you know, any production practices or the soil environment because that's where PMTV resides as well as the host, including the potato cultivars that you grow. With PMTV, we have to add a fourth side to make a pyramid, if you will. And that is for the vector, Spongospora subterrania. Spongospora is important in its own right because it is the causal agent of powdery scab. Powdery scab gets its name from those scabby sort of irrompant lesions that you see in the tuber in the top photo. The pathogen also infects roots forming the galls that you see in the lower photo. This pathogen can survive as resting spores in the soil for more than 10 years. These release swimming zoa spores, which cause further infections. And this pathogen is found generally wherever potatoes are grown. Powdery scab. It's caused by Spongospora subterrania. Tubers infected with powdery scab develop small pistules to begin with, which eventually becomes raised and walked like and ruptured the tuber peritone. The pathogen produces resting spore aggregates known as spore balls. The powdery scab lesions on tubers can sometimes look like common scab as it's evident here. And hence must be verified and confirmed with molecular diagnostic procedure like PCR. Let's take a look at the disease cycle of powdery scab. Starting the lower left-hand corner, you see those are rumpet lesions on the tubers as well as the galls on the roots. What's inside there are those resting spores that the arrows are pointing to. Those resting spores release primary zoa spores that swim through the soil water profile. They infect the tubers, they infect roots, but then they also release secondary zoa spores that can be released and cause further infections as the growing season goes on when the soil environment is conducive. That favorable environment includes cool moist soils, especially high soil moisture during tuberization. These conditions favor the pathogen and can increase disease severity. Powdery scab has increased in importance, partly because of global potato markets and banning of mercury seed treatments, but the importance of powdery scab has also increased due to what it vectors, PMTV. These pathogens, both Spongospora as well as PMTV, spread with tubers and soil. Anytime soil is adhering to tubers that are being moved, any machinery that is being moved, wind-blown soil, these are all important means by which the pathogen moves from field to field. As I indicated earlier, powdery scab is really spread throughout the potato growing regions. These graphs represent the distribution by state. The numbers and parentheses at the bottom are the number of fields from each state that were evaluated. On the left-hand side, you can see that in most states, greater than 50% of the fields are infested with the powdery scab pathogen. On the right-hand side, shows the level of infection of each field, the level of pathogen in the soil, where the green bars are low and the yellow bars are moderate. And here, really in most states, more than 40% of the fields have that moderate level of infestation. Effective management strategies for these two pathogens are extremely limited. Therefore, if your fields are clean, you really need to work hard to keep it that way. Plant clean seed into clean soil. PCR assays are available for both of these pathogens and are effective for testing both soil and tubers. Do not spread soil from infested fields. If the soil is infested with spungospora, that means that it has those resting structures in it and those resting structures may be carrying the virus. Russet skin cultivars are generally more resistant to tuber postules than red and white skin cultivars. It is important to note that research has shown that insensitivity to PMTV tuber necrosis is not necessarily associated with resistance to pottery scab. As indicated earlier, pottery scab produces tuber postules and root galls. On this table, tuber reaction is on the left-hand side, resistance at the top, and susceptible as you move down. Root gall reaction is across the top. If we start in the top left, these russet skin cultivars are generally more resistant to both the formation of tuber postules and root galls. As we move diagonally, right and down, susceptibility increases, where ivory, crisp, cannebec, and chepatee are susceptible. But I really want to call your attention to those cultivars in the upper right-hand corner. Some cultivars, including Russet Burbank, Umatilla Russet, Snowden, are resistant to the tuber phase, so you may not see postules on the tuber surface, but they produce root galls. These root galls are important because they contain the pathogen-rusting structures. And so if you remember back to the disease cycle, those root galls contain resting structures, which might not be important to the potato crop that you're currently growing, but the next time that you grow potatoes in that field, there will be a higher pathogen population and higher disease pressure. As I mentioned earlier, there are not a lot of effective management tactics for these pathogens. But one of the things we can do is reduce soil moisture, especially during tuberization. And we know that low soil moisture during the early part of the growing season will increase common scab, but that should not overlap with this management tactic. These are results from studies designed to demonstrate that effectiveness of reducing soil moisture. Here we're looking at pottery scab incidents on the left and severity on the right. There are four treatments. They compare wet, which is 90% field capacity and dry, which is 60% field capacity. And we have dry early, and early indicates the first 50 days, and late indicates the second 50 days. Where you see bars with different letters, we have statistical differences. And so with pottery scab incidents on the left-hand side, we see that the treatment where we have dry late and dry throughout the entire 100-day time frame have significantly decreased pottery scab incidents. On the right-hand side is showing pottery scab severity. And again, we have lower pottery scab severity when we have dry late or dry throughout the growing period. These are results from that same study, but results of root gauze per plant. We have two varieties here, Dakota Crisp that's resistant to pottery scab and Ivory Crisp that's susceptible to pottery scab. We see basically the same trends where reduction in soil moisture is reducing the severity of the disease. Here, dry early tends to be a little bit better than dry late, but if we think about when roots are formed versus when tubers are formed, that really does make some sense. These graphs demonstrate the results of the same study where they evaluated PMTV tuber necrosis incidents and severity, those dark flax rings and arcs in the tuber flesh. We have the same treatments, wet versus dry, early versus late. We see the same trends as we did with pottery scab and root gauze, where decreased soil moisture is associated with decreasing tuber necrosis incidents and severity. These trials were done under controlled conditions but similar trials have been done that demonstrate the same outcome that managing soil moisture can help to reduce economic losses due to tuber necrosis. Generally, all cultivars are susceptible to infection by PMTV, but there are either sensitive or insensitive to PMTV tuber necrosis. A second management tactic is to plant cultivars insensitive to PMTV induced tuber necrosis. The pathogen can still be spread in these insensitive cultivars, but they will not develop the dark spots and arcs and rings, and therefore you will not suffer the economic losses. A study was conducted across two years to evaluate PMTV induced tuber necrosis over cultivars from several tuber types. They evaluated 15 white, 13 red, 6 specialty, and 4 yellow cultivars. The table here is representing 21 cultivars among those that were insensitive to PMTV tuber necrosis, which here means that 5% less than 5% incidence was observed in those cultivars, so less than 5% of the tubers produced by each of these cultivars had those dark necrotic spots or arcs. These are results from that same trial evaluating non-resid cultivars, but here we have starting at the left 7 cultivars that were rated as moderately insensitive to tuber necrosis, so tubers produced by these cultivars were either between 5% and 10% incidence of tuber necrosis. In the next column we have moderately sensitive between 10% and 15% incidence, and on the right-hand side, 6 cultivars that were sensitive to tuber necrosis, greater than 15% incidence. Here are the results of the 24 russet skin cultivars evaluated for PMTV tuber necrosis during those same experiments. Among them, 23 were insensitive to tuber necrosis, or less than 5% of the tubers produced by each cultivar developed symptoms. One cultivar, alpine russet, was moderately insensitive. Again, these cultivars are not resistant to infection by the virus. They do not produce symptoms, therefore you do not suffer chronic losses. It is also important to note with the russet cultivars, as well as with the red, white, yellow, and specially cultivars shown in the previous two tables, evaluations were conducted after 20 and 60 days in storage. The reason that I mentioned that is that we know that limiting storage duration in cultivars that are sensitive to the tuber necrosis will help limit the amount of tuber necrosis that develops. These results are from that same study where they evaluated the incidence of PMTV induced tuber necrosis, but I want to draw your attention to what happens over time during storage. As I indicated, limiting storage duration can help limit the development of PMTV tuber necrosis. On the left hand side are the results from 20 days in storage, where the dark green bars show cultivars that are insensitive, broken down by red, white, specialty, russet, and yellow, with the number and parentheses on the bottom indicating the number of cultivars of each type evaluated. As we move to the left, these results are from evaluations done at 119 days in storage. It is very evident here that with increasing storage duration you move from insensitive to sensitive as represented by the light green and gray bars. This is particularly true in red, white, and specialty cultivars. In the second year of that study they performed the same evaluations, but did it at 60 days in storage and at 156 days in storage. Again, the dark green bars are showing the percentage of cultivars that are insensitive to that tuber necrosis stage. But even after 60 days in storage we can see that with red skin cultivars many of them are moving into that sensitive category, where greater than 15% of the tubers produced show that internal necrosis. When we look at 156 days in storage we can see that shift to sensitive is even more dramatic, particularly with the red skin cultivars where approximately 75% of them are sensitive or are producing tubers with greater than 15% incidence tuber necrosis. This is also true with white specialty and yellow cultivars and russet cultivars to some degree, but the swing is not as drastic. Again, these results are from the same study, but I wanted to show them to you in a little different way. We are looking at tuber necrosis caused by PMTV, but this is an average of the cultivars within each skin type. On the left, for example, with the 13 red cultivars, the average tuber necrosis incidence at 20 and 119 days in storage is less than 10%. There is a slight increase with storage duration, but really that increase is much more dramatic when we look at the 2016 data on the right. Here, we are going from 60 to 156 days in storage. And again, for example, in the red cultivars, there is an increase from approximately 13% to nearly 30%. More than doubling the incidence of tuber necrosis during that time in storage. There are other studies that show similar results as these. They all highlight the need to understand the cultivar reaction to the virus to be able to effectively manage storage duration in cultivars sensitive to PMTV-induced tuber necrosis. I just want to summarize what we've talked about today. Certainly, PMTV is an emerging pathogen potatoes, and we know that it's in at least seven potato producing states in the US. Disease management strategies are limited for both the puttery scab pathogen and for PMTV. And I can't stress enough, if your soil is clean, you really want to keep it that way. Seed and soil tests are available for both of these pathogens, but there are no current seed certification standards in place in the United States. I want to reiterate, PMTV, the virus, moves with its vector spongospora subtrania. The vector spongospora moves with soil via those resting spores. So, anytime you move infected seed or infested soil, you're moving these pathogens with them. So, anything that you can do to limit the spread of infested soil is very, very important in the management of these diseases. Also, as we talked about, we can manage moisture where possible, and especially as those studies showed during the tubarization phase. Also, I think I demonstrated that understanding the cult of our reaction to both of these pathogens can be very important. We can plant cult of ours that are insensitive to that tuber necrosis phase in infested fields. Certainly, we're not managing the virus there, but we are managing the economic losses, which really at this point is all we can do. We can limit storage duration in cultivars that we know are sensitive to that tuber necrosis phase as well. Thank you for your time and attention today. I'm happy to answer any questions that you have or I can answer those questions at a later date. Please just contact me at the information provided. Thank you.