History of potato wart disease in Europe – a proposal for harmonisation in defining pathotypes

Potato wart disease, caused by the chytridiomycete Synchytrium endobioticum, was first introduced into Europe in the late 19th century. It spread quickly, and today is reported in 15 European countries. Initially, only one pathotype was found, and the disease was efficiently controlled using resistant cultivars. In 1941, however, formerly resistant cultivars showed wart formation in the field simultaneously in Germany and South Bohemia (Czech Republic), indicating the occurrence of new pathotypes. New pathotypes have since been reported from Germany, The Netherlands, Czech Republic, Ukraine and Canada. Today the pathogen is present in The Netherlands (only in fields for ware and starch potatoes) but restricted to two demarcated areas and subject to official control. Outside these areas, the pathogen is absent. For pathotyping, different countries have used different sets of differential cultivars, and the usual system of numerical coding of pathotypes has not been consistently followed. In this review we propose a new standardised code to be used for the 43 pathotypes currently known and described in Europe. The code is a combination of a numerical and letter code, combining the two terminologies used by former West and East Germany, respectively. We also plead for harmonisation in the choice of differential cultivars used for pathotype identification. The set of differentials described in the international standard for diagnosis of S. endobioticum issued by the European and Mediterranean Plant Protection Organisation (EPPO), should serve as a basis. Through close collaboration of European countries dealing with new pathotypes of potato wart disease, a final agreed upon set of differentials, combined with a set of reference isolates, should ultimately be established, allowing a clear distinction between the most important pathotypes occurring in Europe.     

Differential cultivars for aggressive pathotypes of Synchytrium endobioticum in Ukraine1

The use of a internationally agreed standard differential set of potato cultivars to identify aggressive pathotypes of Synchytrium endobioticum is reviewed. In recent years, many new cultivars and hybrids have been tested in Ukraine for their reaction to the Carpathian pathotypes (11, 13, 18 and 22). Because of the specificity and stability of their reactions, and their availability, some of these potato cultivars are now recommended as part of a new differential set for use in Ukraine, especially with respect to the Carpathian pathotypes.     

Definition and distribution of aggressive pathotypes of Synchytrium endobioticum in Ukraine1

Aggressive pathotypes of Synchytrium endobioticum have been detected in Ukraine since 1963, mainly in the mountainous western part of the country. Potato crops are regularly surveyed to detect possible outbreaks. It has been observed that the aggressive forms attack the above-ground parts of the potato plant, as well as the underground parts, and this can assist detection. Possible foci are investigated by planting a standard replicated test with a susceptible cultivar and a cultivar resistant to the common pathotype. The distribution of S. endobioticum in Ukraine is summarized. Outbreaks are particularly found in potato plots adjoining farm buildings.     

Occurrence of potato wart disease (Synchytrium endobioticum) in Bulgaria: identification of pathotype(s) present

Potato wart disease caused by Synchytrium endobioticum was detected for the first time in Bulgaria in 2004. Laboratory tests, pot tests and field tests were carried out to determine which pathotype is present, specifically in Prodanovtsi near the town of Samokov. In addition, potato cultivars were tested for levels of resistance, in order to be used in buffer zones surrounding infested fields. Based on the results obtained in this study, it was found that most probably pathotype 8(F1) is present in Bulgaria. Most of the commercial potato cultivars tested were found to be susceptible to this pathotype. The local cultivars Koral and Bor were resistant, but these are not very important cultivars commercially. The potato cultivars Panda, Désirée and Amorosa were recommended for cultivation in the fields adjacent to infested plots.     

National control measures for Synchytrium endobioticum1

Although Synchytrium endobioticum has been found in almost all potato-growing countries of the EPPO region, it is absent from the great majority of fields. A few plots scattered across the potato-growing areas have been ‘scheduled’ as infested but potatoes are not grown on them. The disease is accordingly hardly ever seen. This is the result of continent-wide multiyear campaign of potato wart control within countries, according to which it is prohibited to grow potatoes on infested plots, and only resistant cultivars may be grown in a safety zone around them. S. endobioticum can persist for several decades in soil, as resting spores, but it is now dying out in plots that have been subject to the control campaign for a sufficiently long period. Procedures are now being agreed for officially ‘descheduling’ such plots.     

Pathotypes of Synchytrium endobioticum in Germany

Results are given of a survey on the occurrence of pathotypes (races) of Synchytrium endobioticum (potato wart) in both parts of re-unified Germany. An assortment of six differential cultivars appears to be sufficient to distinguish between the seven presently important German wart pathotypes. A total of ten pathotypes was recorded. Two West and two East German pathotypes seem to be identical, based on the results obtained with presently available cultivars. In contrast to pathotype 1 (common race) which prior to 1945 was distributed over the whole German territory, the 'new' pathotypes seem to prefer the central and south German mountain areas.     

Variability of Synchytrium endobioticum in the Carpathian region of Ukraine1

The appearance of aggressive pathotypes of Synchytrium endobioticum in the Carpathian area of Ukraine is reviewed. An analysis is made of the factors that favoured their appearance in this area. Possibilities for distinguishing the pathotypes by characters other than their behaviour on differential cultivars are considered.     

Identification of the pathotype of Synchytrium endobioticum,causal agent of potato wart disease,present in Georgia

Potato wart is one of the most important quarantine diseases of potato in the world. In Georgia, symptoms of wart were first seen in 2009–2013 during observations on potato in home gardens in Khulo district, although they may have been present since 2006. However, potato wart presence was only confirmed by PCR in 2013. Isolates pw13N and pw13 of Synchytrium endobioticum collected from the infested area of the village Didadjara, Khulo municipality were tested using laboratories in Georgia and the Netherlands in the framework of collaboration between scientists of Batumi State University and the National Plant Protection organization of the Netherlands. Additionally, pot tests were carried out in Georgia. Based on the results obtained, the pathotype found in Georgia is not known from Europe, and based on its reactions this pathotype resembles pathotype 38 (Nev?ehir) detected in (the non‐European part of) Turkey.     

Biotechnology of production of virus-free seed potatoes in Bukovina (Ukraine)1

In the past, the Ukrainian Plant Quarantine Research Institute used to produce true-to-type virus-free potato cultivars by in vitro propagation for its research on pathotypes of Synchytrium endobioticum and Globodera spp. Use of virus-free seed potatoes has considerably declined in the 1990s as potato production has passed almost entirely into private hands. The Institute now has a role to fill in supplying virus-free seed potatoes to local growers and has greatly increased its production in consequence. The techniques used for seed-potato production in Bukovina are described.     

Identification of pathotypes of Synchytrium endobioticum found in infested fields in Turkey

Pathotypes of four isolates of Synchytrium endobioticum collected from the infested areas of Turkey were determined using Glynne–Lemmerzahl and Spieckermann test methods. Two isolates (Ordu 1 and Nevşehir 1) were tested by the Glynne–Lemmerzahl method in Germany. Ordu 1 was assigned as pathotype 1(D1) whereas the other isolate gave different results in 2 years of testing [pathotype 6(O1) or 18(T1)]. Two other isolates (Nevşehir 2 and Nevşehir 3) were tested using the Spieckermann method in Netherlands. Neither of these last two isolates tested were found to belong to the pathotypes known from Western Europe. The isolates Nevşehir 2 and Nevşehir 3 were given a new, unique code: 38 (Nevşehir).     

Euphresco Sendo: An international laboratory comparison study of molecular tests for <Emphasis Type="Italic">Synchytrium endobioticum</Emphasis> detection and identification

An international test performance study (TPS) was organised to generate validation data for three molecular Synchytrium endobioticum tests: van den Boogert et al. (European Journal of Plant Pathology 113, 47–57, 2005), and van Gent-Pelzer et al. (European Journal of Plant Pathology, 126, 129-133, 2010) for the detection of S. endobioticum, and the pathotype 1(D1) identification test described by Bonants et al. (European Journal of Plant Pathology, 143, 495-506, 2015). Two TPS rounds were organised focussing on different test matrices, i.e. round 1: warted potato tissue, and round 2: resting spore suspensions. When using the tests for detection and identification of S. endobioticum in warted potato tissue, no significant differences were observed for diagnostic sensitivity, diagnostic specificity, overall accuracy, analytical sensitivity and robustness. When using the tests for detection and identification of S. endobioticum in resting spore suspensions, the van den Boogert and van Gent-Pelzer tests significantly outperform the Bonants test for diagnostic sensitivity and diagnostic specificity. For overall accuracy and analytical sensitivity, the van Gent-Pelzer significantly outperforms the van den Boogert and Bonants tests and is regarded as the test of choice when identifying S. endobioticum from resting spores. Tests regarded fit for purpose for routine testing of wart material and resting spore suspensions are proposed for the update of EPPO standard PM7/28(1) Synchytrium endobioticum.     

Problems of potato cyst nematode pathotypes in the Czech Republic

During 1983–1989 about 1,200 infested plots in the Czech republic were investigated for the occurrence of new pathotypes of the potato cyst nematodes (PCN) Globoderu rostochiensis and G. pallida. Only eight populations of the pest were suspected of containing another pathotype than Ro1. Glasshouse pot trials were performed with those eight natural populations, two populations of Ro1, 10 potato cultivars resistant to Ro1 and 10 cultivars resistant to more pathotypes, G. pallida included. The multiplication rates (Pf/Pi) showed that four of the PCN populations were probably mixtures, with pathotype Ro1 prevailing. Three populations were suspected of containing G. pallida , but their morphological characters corresponded only with G. rostochiensis. One population corresponded with pathotype Ro5. The extent of the occurrence of pathotypes or virulence groups different from Ro1 can be estimated at 0.3–0.4% of infested plots.     

Potato Wart in the Federal Republic of Germany

A survey shows the present situation of the occurrence of potato wart (Synchytrium endobioticum) in the Federal Republic of Germany. By 1960 five different pathotypes (races) could be detected. Only 1 % of all foci identified after the Second World War belonged to the old pathotype 1 (← common race →). Questions of determination and emergence of new pathotypes as well as those of official restrictions on cultivar release are discussed.     

The 2019 Potato Wart Disease workshop: shared needs and future research directions

Potato wart disease is caused by the fungus Synchytrium endobioticum. A workshop on this disease was held in June 2019 at the Dutch National Plant Protection Organization (NPPO-NL) in Wageningen, the Netherlands. Over 60 participants from 20 countries and 36 organizations came together to share their knowledge via presentations and posters, to strengthen professional networks, and to identify shared needs and future directions for S. endobioticum research. The three major topics of the workshop were disease occurrence and management, understanding the pathogen, and host breeding and resistance. Many aspects of this disease were discussed and summarized in a shared statement. Future directions identified were (1) research initiatives are currently being initiated to develop standardized molecular viability tests and a molecular pathotyping test, (2) coordination is needed to come up with better standardization of the pathotyping bioassays and the set of differential cultivars described in EPPO PM 7/28 needs to be better available for testing, and (3) stacking of resistance genes is strongly recommended but resistance breeding alone is not enough and should be combined with other management strategies to contain potato wart disease. Furthermore, the attendees remarked that organizing a workshop every 2 or 3 years would be of great benefit to the potato wart disease research community.     

PM 7/28 (2) Synchytrium endobioticum

Specific scope

This Standard describes a diagnostic protocol for Synchytrium endobioticum. 1 This Standard should be used in conjunction with PM 7/76 Use of EPPO diagnostic protocols.

Specific approval and amendment

Approved in 2003‐09. Revision approved in 2017‐06.     

Resistance of potato cultivars to Synchytrium endobioticum in field and laboratory tests, risk of secondary infection, and implications for phytosanitary regulations

Laboratory (Spieckermann) tests, pot tests and field tests provided concordant evidence for the partial nature of resistance of potatoes to pathotypes 1 (D1) and 6 (O1) of Synchytrium endobioticum . Susceptible potato cultivars produced large warts (> 16 mm in diameter) in Spieckermann tests and had low field resistance levels (1–6). Field-resistant cultivars (levels 7–9) produced small warts or no warts at all in Spieckermann and field tests. In pot tests, at low inoculum levels (1 sporangium per 25 g soil) susceptible cultivars still developed warts, whereas field-resistant ones did not develop any warts below 25 sporangia per g soil. Above 35 sporangia per g soil, 100% disease incidence was observed in susceptible cultivars but only minimal wart development in field-resistant ones. Tests with continuous cultivation of potato cultivars in infected soil during three consecutive years showed that field-resistant cultivars will not support build-up of inoculum in soil. It is concluded that field-resistant cultivars do not create a risk of secondary infection, the criterion given for resistance in EU Directive 69/464/EC.     

Development of PCR-based Detection Methods for the Quarantine Phytopathogen <Emphasis Type="Italic">Synchytrium endobioticum</Emphasis>, Causal Agent of Potato Wart Disease

PCR-based methods were developed for the detection and quantification of the potato pathogen Synchytrium endobioticum in soil extracts and in planta. PCR primers, based on the internal transcribed spacer region of the multi-copy gene rDNA were tested for specificity, sensitivity and reproducibility in conventional and real-time PCR assays. Soil extraction procedures compared included the Hendrickx centrifugation (HC) procedure, nested wet sieving (NWS) and a method used by the Plant Protection Service (PPS). The primers amplified a 472 bp product from S. endobioticum DNA, but did not amplify DNA from other potato pathogens, other plant pathogens, and related species. Standard cell disruption and DNA extraction and purification methods were optimized for amplification of S. endobioticum DNA from resting sporangia. DNA was successfully amplified from a single sporangium and equivalent DNA preparations from soil extracts. Low levels of target DNA in water did not amplify, possibly due to DNA loss during final purification steps. A real-time PCR assay, developed for soil-based extracts using primers and probe based on the rDNA gene sequences, involved co-amplification of target DNA along with an internal DNA fragment. Both conventional and real-time PCR methods performed well with HC- and NWS-extracts having a threshold sensitivity of 10 sporangia per PCR assay. Of the three soil extraction methods, only with the HC method could 100 g soil samples be efficiently processed in one single PCR assay. Such a high capacity assay could be useful for routine soil analysis in respect to disease risk assessments and to secure de-scheduling according to EPPO guidelines.     

Characterization of Verticillium dahliae Isolates from Potato on Hokkaido by Random Amplified Polymorphic DNA (RAPD) and REP-PCR Analyses

Verticillium dahliae isolates from potato on the island of Hokkaido (potato isolates) and those belonging to pathotypes A (eggplant pathotype), B (tomato pathotype) and C (sweet pepper pathotype) were divided into three distinct groups by RAPD and REP-PCR. The three DNA groups I, II, III consisted of pathotypes A and C, pathotype B and potato isolates, respectively. The potato isolates were assigned to pathotype A on the basis of pathogenicity. Another set of potato isolates was further collected from eight potato cropping regions on Hokkaido to further examine the relationships among them in detail. Only one of these isolates was identified as DNA group II, but all the others were classified as DNA group III. Isolates from daikon, eggplant, and melon on Hokkaido also belonged to DNA group III. These results suggest that V. dahliae isolates from Hokkaido are unique at the DNA level and different from other pathotype A isolates in Japan. Received 28 February 2000/ Accepted in revised form 6 November 2000     

Relation between pathogenicity and genetic variation within <Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis>

The relation between diversity of pathogenicity on clubroot-resistant (CR) cultivars of Chinese cabbage (Brassica rapa subsp. pekinensis) bred in Japan and DNA polymorphisms in 17 populations of Plasmodiophora brassicae from cruciferous plants was examined by inoculation tests and random amplified polymorphic DNA (RAPD) analysis using 18 arbitrary primers. Four pathotypes (A–D) were identified after inoculation of six CR cultivars of Chinese cabbage in the 17 populations from cruciferous crops. A relatively high level of genetic diversity was also detected among these populations in the RAPD analysis. Although the four pathotypes could not be clearly differentiated using the RAPD data, most populations of three pathotypes had a consistent location on the dendrogram. All pathotype B (virulent on five cultivars except Utage 70) and D (avirulent on all cultivars) populations, which were common in incompatible interactions with cv. Utage 70, were located in a single subcluster. All five pathotype C populations (virulent only on cv. Utage 70) except for one population grouped in another single subcluster. Because four pathotype A populations (virulent on all six cultivars, races 4 and 9) fell in different subclusters, the populations may be genetically polyphyletic. Populations from cruciferous weed Cardamine flexuosa differed remarkably from those from cruciferous crops in pathogenicity on common cultivars of Chinese cabbage and turnip and C. flexuosa, but they grouped in a single cluster with all race 9 populations from crops. Race 9 populations from crops may thus be closely related to populations from the weed rather than to races 1 and 4 from crops.     

Identification of Pathotypes of the Potato Cyst Nematode1

Since the discovery of pathotypes of the potato cyst nematode (Heterodera rostochiensis Wollenweber, 1923), surveys on the distribution of these pathotypes have been carried out in different European countries. Test varieties of potato were selected according to findings resulting in the present situation of varying distinctive host-plants and of diverging programs for breeding potatoes resistant to the potato cyst nematode. With respect to rational selection and use of resistant potato varieties, there is a great need for standardization in pathotype identification (nomenclature), maintenance and supply of appropriate genotypes of Solanum species and methods for testing resistance in new potato varieties. Anticipating a cross-experiment with pathotypes and test varieties from Great Britain, the Federal Republic of Germany and the Netherlands in 1974, a preliminary test with all recognized or presumed pathotypes from these countries, but with only part of the potential differential host-plants was carried out by the Plant Protection Service at Wageningen. The results of this experiment are discussed.