 Good day everybody who's tuning in to take your talks on Tuesdays at 10 Thank you so much to Andy Robinson for this invitation to speak to you about Experiences with Padre Skaab in South Africa. I'm Jackie from Evolts from the Department of Plant and Soil Sciences at the University of Pretoria in South Africa the southern tip of Africa So I would like to start by just Recapping what diseases spongospora subterranean can actually cause on potatoes the classical potato Solonium tuberosum and Then we'll move on to some of the experiences that I've had over the past couple of years with this pathogen in South Africa So there are three main diseases that spongospora can cause on potatoes. The first is root infection This is caused by infection of roots normally Feeding roots or root hairs by zoospores and root infection results in disrupted root function leading to reduced water and nutrient uptake The second disease which is more easily seen with the naked eye is that of root gauze and these result from root hyperplasia which is a result of the pathogen infecting again these fine feeder roots and Excessive growth is caused and we see these root gauze developing. The root gauze also results in disrupted root function However, the disease that we normally associate with spongospora is Padre Skaab and this these are lesions on the surface of tubers It's basically a blemish disease, but it results in reduced quality and rejection of seed So as I've mentioned zoospores are responsible for initial infection of roots and also initial infection of tubers Inside the root gauze and particularly inside these powdery lesions on tubers We find sister story, which are the resistant survival structures of this pathogen So spongospora diseases in South Africa not only in South Africa But throughout the world are steadily increasing in incidents in severity and in their distribution We've seen this increase predominantly over the past decade There are a number of reasons for this. The first is that spongospora and particularly powdery Skaab has previously been underestimated and Often misdiagnosed and I'm referring specifically to the misdiagnosis and the confusion between common versus powdery Skaab Even to the trained eye It's often very difficult to tell the difference between common versus powdery Skaab particularly in the initial phases of these diseases In South Africa particularly there was a stigma related to powdery Skaab It was almost as if Somebody with powdery Skaab on their field had leprosy and they wouldn't want to talk about it fortunately the stigma has So we're slowly dissipating and the farmers are not as reluctant to admit that they have powdery Skaab in their fields Then this pathogen is a very successful pathogen because it is able to survive for a very long time in The soil and I will discuss that in a little bit more detail Potato production has been intensified not only in South Africa, but globally and The problem here is that there is now a limit to the amount of virgin soil which is available in South Africa For example, there is very little virgin soil and arable land in fact that we can still use for potato production most of the cultivars that are produced in South Africa are dictated by the commercial or the Consumers demands and these are often the most susceptible cultivars to powdery Skaab. So this results in an increase in the disease And then another problem is that in South Africa Approximately 85 to 90 percent of the commercial potatoes are irrigated due to our low rainfall and irrigation of the crop definitely Increases the amount of final disease in the potatoes So being a classical applied plant pathologist I always start everything or at least end it with a disease triangle or the disease pyramid So basically what we want to do when we are studying a plant disease is firstly to ensure that the environment is favorable for growth of the host and unfavorable for proliferation and infection of the pathogen of the host by the pathogen We also want to ensure that the host is strong enough to withstand onslaughts by the pathogen to be able to resist infection and Then we want to reduce the amount of pathogen inoculum present reduce the virulence of the pathogen and basically subsequently reduce the final amount of disease So in order to be able to do this one has to understand the disease triangle in the context of a disease cycle and that's where we're going to start So looking at the disease cycle of powdery scab and root gawling on potatoes I always like to start with the initial inoculum. What is it? Where does it come from? How can we reduce it? So the initial inoculum of Spongospora is Propagals known as sister sorry, which are found in predominantly in the soil and these sister sorry contain a number of resting spores and The reason why Spongospora survives so long in the soil I will explain to you It's got to do with the resting spores that are found in the sister sorry Some anecdotal evidence particularly some from Dr. Stuart whale in Scotland has said that this pathogen can survive for up to 50 years in the absence of a host in soil The other way in which primary inoculum is spread and survives in the soil is that it survives passage through the digestive tracts of animals So what might happen is that? When potatoes are harvested that are not fit for human consumption that have a lot of blemish diseases They are then fed to livestock these livestock are subsequently grazed on fields Which are then later put back into potato production and of course in the process of feeding the livestock the contaminated tubers the manure lands on the fields and Subsequently the inoculum is is put back into the fields and the inoculum increases And this we want to avoid So what are the two main sources of inoculum? Well as I've mentioned contaminated soil in which we will find sister sorry But the other source of inoculum is infected seed so One can already imagine that it is critically important To prevent planting or not to plant infected seed Into clean soil So that it's clean that does not have a history of powdery scape should be kept that way as far as possible So to answer what I with a question that I posed earlier as to why these sister sorrows are highly resistant I'd like to show you how they function So a sister sorus looks kind of like a grain of sand when you look at it under the Microscope it can easily be confused with a grain of sand However, if we zoom into these and these are electron micrografts graphs complements of early merits One will see that an individual sister sorrows is composed of a number of Smaller structures and these smaller structures are termed resting spores. So I'll show you what a resting spore looks like These individual structures here you can see each one of these are resting spores Within each sister sorrows, there will be between 700 and 2000 resting spores Now, let's go ahead and look at what the resting spores contain. So the resting spores each contain a Zoological spore In the presence of root exudates of a host plant Certain compounds in the root exudates will stimulate germination of the resting spores I'm sorry of the of the zeus spores inside the resting spores and the zeus spores will then swim To the roots to infect them this process can be as quick as 15 minutes in the case of a non-host or a plant which does not attract these Germination of these zeus spores the the compounds found in the root exudates will therefore not be stimulatory to germination of the zeus spores the other problem with the resting spores is that they are composed of Three layers of cell walls all of which are highly resistant to external factors such as desiccation and fungicides etc What also happens when zeus spores germinate is that not all Say 700 to 2000 resting spores in one sister sorrows will germinate at one time so even if only 10% of these resting spores germinate in one occurrence that will still really release anything from 70 to 200 zeus spores which are able to infect a root and remember that there are thousands millions of sister sorrows in the soil around a potato root So now that we know what the primary inoculum is and where it comes from We need to ask ourselves how we can reduce this primary inoculum So the first would be the choice of field But as I've mentioned it is almost impossible these days to find pathogen free fields with no history of Spongospora if there are such fields these are obviously first prize and why it should plant in fields Free from the pathogen particularly in the case of seed production Fields should be well drained because an excess of soil moisture Allows the zeus spores to swim more easily and infect the roots and the tubers Secondly pathogen free seed should be planted now the best way is to plant certified seed however Certification of seed is not a guarantee that the seed will be completely free from pathogen propagules Certification of seed merely says that the seed is below a certain threshold level of visual disease So what might happen and this is one of the problems with Spongospora one of the reasons why it's been spreading So widely and we've been unable to stop the spread is that in many cases Seed is sorted before certification So growers will remove the visually infected seed from the seed lots leaving the healthy looking seed But which is actually Contaminated with sister story and these seed will then be certified as pathogen free because they are visually clean and healthy but actually one way to test them one would find that there are Certainly traces of pathogen inoculum on these seed in terms of treatments There are some biological control products which have shown some promise and I will discuss this in a little bit more detail Later these include trichoderma and bacillus species These can be applied to the soil or to the seed In the term in terms of chemical seed and soil treatments as with the biological control agents None have been shown to be 100% effective to completely eradicate this pathogen Some products have given a level of control and these include propanib, formaldehyde, mancozeb, Fluazinam, fluosulfamide, methamsodium, calcium cyanamide, which in South Africa is registered actually as a fertilizer and Then zinc oxide and certain sulfur and copper containing compounds. However, the latter three are only Relatively effective when the disease pressure is really low So these products can be used but when we look at the list of these products We will see that the majority of them have either already been removed from the market in many countries particularly in the EU or are on their way out So we cannot rely on chemicals To reduce primary inoculum in the soil or on the seed first. We have to perhaps look at an integrated strategy using Fungicides and biological control products and putting a number of different options in place to reduce the primary inoculum So from 2015 to 2020 we've been conducting some trials having a look at the efficacy of various products in reducing disease incidence and severity of powdery scab and of root galling So initial trials were some pot trials in the greenhouse the study has been published and We followed this up with field trials in in all of these cases we screened a number of commercially registered bio control agents both as seed and soil treatments We looked at fluorescent a methamsodium and calcium cyanamide. So calcium cyanamide as a Bioside which is registered as a Fertilizer methamsodium as a fumigant and fluorescent as a soil applied fungicide and These compounds were shown to have some promise in the reduction of Spongospora or The disease that Spongospora causes on roots and tubers There were other products that we screened as well, but that did not show such promising results However, as I mentioned previously, it would be the best option to combine Biological control products with other harsher chemicals to ensure the best control possible. So Some questions were raised by growers in one of our Areas in South Africa namely the Suntfelt which literally translated mean sandy field as to whether the pathogen can actually be spread by wind and incidentally this was at the same time as Prof. Liatsoar and her team were investigating the same question in Israel. So in order to To test this hypothesis we sampled a few remaining virgin fields and some cultivated potato fields nearby and Exposed the samples to or ran them through a QPCR to determine the level of inoculum in these soil samples and What was very interesting is that although very low there was presence of inoculum in three of the five Tested virgin fields. So these fields had never been cultivated to any crop before however, they were in region of potato Farm and thus it is highly likely that the wind had dispersed Some of the pathogen propagules into these virgin fields So the implications are that spongospora may be spread by wind and that virgin fields may be contaminated with spongospora and This was confirmed by Leah study Leah source study in Israel where they found the same thing that in virgin soils they were able to detect the presence of Spongospora and they also found spongospora present in spore traps, which were placed around potato fields So one needs to just bear in mind that Spongospora can be spread by wind Right, so we have discussed the primary inoculum where it comes from options for reducing it Now I want to look at the secondary inoculum So the secondary inoculum is primarily the zoos spores Which then germinates in the presence of a host and swim to infect the host roots or tubers So obviously because those zoos spores they have flagella they can swim they require water to move which is why I said high soil moisture is actually favorable for disease development So having a look at the infection cycle from the zoos spores There are some critical points in the disease cycle, which are important to remember a Study done by thongafil et al in 2015 Confirmed that the critical period for tuber infection specifically is Soon after tuber initiation and for infection of tubers to occur The pathogen must or development of powdery scap to occur the pathogen must have infected these tubers By 40 days after emergence In other words the first 40 days after emergence are critically important for Infection of tubers if one can prevent infection during this period one will essentially Prevent the development of powdery scap What they also noted is that when infection occurs early in the season more powdery scap is observed At harvest than when infection occurs later on in the season the critical temperatures for Tuber infection are between 9 and 17 degrees C and Although this might seem relatively low will particularly for South African conditions where some crops are grown through the summer Where the temperatures can easily go way above 35 40 degrees C the soil temperature is brought down By irrigation Often with borehole water, so the soil temperature in the area of Initiating tubers can easily be around 15 degrees C Even though the ambient temperature is 20 degrees higher than that Another critical point that we need to remember is the infection of the root hairs Now in contrast to infection of the tubers Root hair infection can take place right throughout the season right throughout the growing season of the of the potato So there is not a critical time specifically to control root hair infection one needs to control it throughout the season However initial infection can occur within two weeks to 20 days after the roots have been formed So that's really quick the roots the Roots and the plants will still be very small and the roots will be infected Spiranger are observed about 15 to set 45 days after Inoculation where as root goals are observed 45 to 75 days And this is important when one is actually studying the disease to determine The efficacy of a control measure or differences among cultivars for example Root infection occurs at a slightly higher temperature or temperature range to that of tuber infection And it's around 11 to 25 degrees Which generally means that we might often be missing root infection if we're only looking for powdery scap Because often there will be a lot more root infection than powdery scap So this is important factor to remember So coming back to temperatures looking at the literature these are some of the important temperatures summarized for Infection of potatoes by Spongospora the minimum temperature is around 9 the optimal temperatures between 12 and 16 to BC and The maximum temperature is about 25 to BC We conducted. Sorry. Let's just say that it's very therefore very important to choose a planting date So that the temperatures during the first root formation, which I said is when the roots are first infected or Above 20 degrees seats so that it's not close to the optimal infection temperature Now to confirm these Temperature ranges We did some studies in South Africa with different soils in South Africa to determine the optimal temperature and soil moisture For infection of roots by Spongospora So our findings showed that the optimal temperature for root infection is a is around 15 degrees C and Second optimal was around 20 where literature says it's 17. So it's pretty close We can see that it's below 20 degrees C and it's in the region of 15 to 17 degrees The least root infection occurred at 30 degrees C So remember this would be in the soils that be a really hot day no irrigation applied at that stage for example and Then at 25 and then at 10 degrees C So one is likely to see more disease at a lower temperature than it would at a higher temperature Interesting results from soil moisture studies where we had a look at the effect of permanent wilting point saturation point and fluctuating moisture conditions basically between Field capacity and permanent moit permanent wilting point and we found That the earliest infection occurred at saturation point so that the roots were infected first When the pots were maintained at saturation point however at the end of the season we found the most infection of roots in Pots that were that had fluctuating soil moisture and this would occur where growers apply irrigation and Then let the soils dry out again and apply irrigation and let the soils dry out so you get Fluctuation between dry and wet soils So to avoid this it's advised that growers use an irrigation scheduling model So that they do not get these fluctuating soil moisture conditions in the soil So those are some of the applications of our results Right back to the life cycle If we look at the life cycle of Spongospora, we can actually divide it into a complete life cycle and an incomplete life cycle in The complete life cycle which is primarily completed in primary hosts such as potatoes and tomatoes Sister sorry in other words the rest the unsurvivable structures are produced which Subsequently increase the soil inoculum So it is important to determine which crops in which crop Spongas Spongospora can actually complete its life cycle and produce more resting spores and Sister sorry because this will essentially increase the soil inoculum these kinds of hosts need to be avoided in rotation with potatoes on the other hand we have an incomplete life cycle where There something is lacking in the plant and the pathogen is unable to complete its life cycle So it cannot produce sister sorry it produces Zeus spores It's able to infect roots sometimes it doesn't even infect the roots But it is unable to produce sister sorry again at the end of its cycle and In this case the soil inoculum is not increased so these types of crops in which the incomplete life cycle occurs are Ideal to include in a rotation cycle with potatoes and these would either be non-host crops or what we call trapping crops So we conducted some studies to have a look at which of the commonly Planted crops that are used in rotation with potatoes are actually hosts to Spongospora and The results were rather shocking rather worrying in That the majority of crops that we are currently planting in rotation with potatoes are Actually proper hosts for Spongospora So these include onion white oat Indian mustard green bean Tomato okay, that's not planted in rotation of potatoes Nonetheless tomato wheat and maize these are all hosts The only two trap crops that we could find which would ideally be planted in rotation with potatoes were black oats and soybean Some of the other crops which were shown to be non-hosts were cabbage carrots or seed radish rye and grazing sorghum however, there were a couple of discrepancies and contradictions with our results and those of some other researchers and This could possibly be due to cultivar differences between some of these crops So this is as I said rather concerning because one therefore cannot really plant any of these Crops that are hosts because they're likely to increase the soil inoculum of Spongospora But then I decided to go and have a look at other pathogens So remember when one is planting potatoes You're not only dealing with one pathogen in the soil. There are a host of pathogens in the soil that can result in tube of blemishes or Yield losses, etc So I took the three main soil born pathogens namely Spongospora streptomyces and rhizotonia saloni And I had a look at these crops that we tested and the host range of these three pathogens to try and find some crops that would be suitable and Again, this gave us an even more worrying picture because there was virtually nothing So the green ticks show us that the crop is suitable to plant in rotation with potatoes the red crosses say no This crop is a host for one of these pathogens So basically when one is choosing a rotation crop one needs to have a look at the Greatest problem in that field in that season and make a decision based on that We also know generally that Spongospora and streptomyces do not infect potatoes under the same conditions. They're actually totally contradictory so Essentially if Spongospora is a big problem You're not likely to have a huge problem with streptomyces and vice versa and one could make an informed decision based on that We are currently expanding the study to other crops and to cover crops and We're also including some different Cultivars of some of these crops to give us a more comprehensive answer of what we can actually plant in rotation with potatoes But basically what we did see is that a number of them cannot be planted in rotation with potatoes So bringing it all back Essentially, we want a healthy plant. We want a healthy potato with no disease So what are some other options that we can look at? To strengthen the plant to ensure that it can withstand infection by Spongospora So I've looked at three main aspects and the first one would be plant nutrition avoidance of high levels of nitrogen is important and this could be in the form of Nitrate or ammonium. It doesn't matter high levels of nitrogen to do two things they keep the plants in a vegetative state and in so doing Increase root matter so increase the number of fibrous roots and this therefore gives the pathogen more Places to infect therefore resulting in a higher disease incidence and severity Secondly, as I mentioned previously, it's important that irrigation is managed well Plants or fields should not be over irrigated or waterlogged especially during chivalry initiation And then cultivar choice is very important. Now that said I realized that in most cases while in South Africa particularly the cultivar that is planted is basically determined by consumer demand and In most of these cases The cultivar the consumers happen to like cultivars that Spongospora likes as well So we sit in a bit of a catch-22 However, we do have a range of susceptibilities to Spongospora amongst the cultivars that are planted in South Africa So over a number of years field trials and pot trials We screened quite a few potato cultivars and this is just a summary of a number of trials and repeats in and which we arbitrarily classified Cultivars grown in South Africa in terms of their susceptibility to Spongospora in terms of powdery scab and root galling and From this you can see that there is definitely a range From susceptible to tolerance only one of the cultivars actually showed up as tolerant None were shown to be Resistant, but we do have quite a long list of moderately tolerant tolerant cultivars So growers can choose some of these if they know that they have a problem with Powdery scab in a specific field it would then be wise to plant one of the moderately tolerant cultivars Rather than a susceptible cultivar Some of the notes and the observations when we did the screening Confirmed a lot of what was already written in literature first of all is that there is no Relationship between root and tuba susceptibility in a cultivar so what I'm saying is that if a cultivar is highly susceptible to powdery scab Development it is not necessarily also highly susceptible to root infection and vice versa We also noticed that there were some differences in our results compared to those of researchers in other countries and in other regions and the differences in these results are Basically due to a gene by environment interaction because the Resistance of potato to Spongospora is Controlled by a number of genes it means that the environmental conditions play a large role on the expression of resistance to the pathogen So in one field trial we might have found that a certain cultivar is say Highly susceptible to the pathogen and in another one with a different environmental condition It appeared that the cultivar was actually for example moderately tolerant But this was actually more a an environmental effect than a gene interaction the gene for gene interaction Agricultural practices also play a huge role in the expression of disease The pathogen distribution all in the field needs to be brought into consideration when one does any kind of field trials Spongospora has an extremely clustered and random distribution in the field So one might find patches in a field that have high inoculum density and other patches with very low or almost no inoculum The time of tuber initiation is important For the reasons that I mentioned previously the fact that tubers are initially are infected early in the season and That temperature plays a major role So based on this again Shifting of the planting date may actually play a significant role in final disease expression So in the initial infection and subsequent disease expression and Then linking up to the passion pathogen distribution in the field the amount of pathogen in a certain field and the pathogen population whether it is all Clonal or whether there are differences in the pathogen population will also play a role in the final disease expression So putting it all together How do we then get a healthy crop where we have to struggle with Spongospora? long rotations with Not hosts or trapping crops clean seed Very important if the seed is not clean treated Clean soil if the soil has a high inoculum concentration treat the soil planting of a tolerant cultivar or if at all possible a resistant cultivar if there is one well drained field Irrigation management and planting dates Important to try and reduce the knock at the infection of the potatoes proper nutrition and avoiding excess nitrogen and Then choice of the correct rotation crops So basically management of Spongospora is not One silver bullet or one simple solution. It's a number of different factors that all have to be put together to eventually Give us a certain level of control that is acceptable We've done some other important research, but I don't have time to discuss it This research is all available in published articles and the results of this research Which I'm showing here on the screen are basically also all very important for breeding and for management of Spongospora globally and in South Africa I've also been very active in But what we would call technology transfer and getting our results out to growers So that the results don't just stay on a shelf or in a scientific journal So that the growers can use the results and many of them have said that they are grateful to the research that we've done because they've changed They're like their practices and they've actually seen a difference in the disease development in the field So some acknowledgments My funding body for the study world all these studies was potato South Africa the students that were involved in the results that I've presented to you and in doing the work Dr. Kennedy Simongo Chris Slavitt, Dr. LeBoheng Lakota Common Rainsburg and Rudolph Stradon and my admin assistant Lindy Rasmus And thank you so much for your time this morning. I think the floor will now be open to questions