Assessment of the resistance of potato cultivars to Synchytrium endobioticum (Schilb.) Per. in Poland

In Poland the Plant Breeding and Acclimatization Institute is responsible for officially assessing the resistance to Synchytrium endobioticum of domestic potato breeding lines and cultivars from other countries. Cultivation of potato cultivars in Poland requires confirmation of resistance to potato wart disease. The official assessment uses the modified Glynne-Lemmerzahl method (laboratory tests) and pot tests. The full cycle of assessment of resistance to wart disease requires 52 seed potatoes per variety/breeding line. Forty two tubers are used in laboratory tests. To complete the laboratory tests the next 10 tubers are grown in pot tests (in soil with winter sporangia) during the vegetation season. The final results for domestic breeding lines of potato are available after 3 years of investigation. For cultivars from other countries the authorization of resistance to S. endobioticum takes approximately one year. The Polish breeders (breeding lines) or the breeder's representative (cultivars from other countries) receive the certificate only for lines/cultivars with laboratory and field resistance to S. endobioticum .     

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.     

Current situation and prospects of potato production in Ukraine1

Total potato production has been maintained in Ukraine, but the share of production on private plots has recently increased from 65% to 97% and the efficiency of production has declined. Protection of the potato crop from pests has suffered particularly. Scientific institutions continue to breed new cultivars (particularly with resistance to wart and cyst nematodes), and the seed-potato production system is being maintained and improved. However, these systems were designed for large-scale production on collective and state farms and have to be adapted to the realities of private production.     

Assessment of varietal reactions to potato wart (Synchytrium endobioticum) in Germany

A key for official assessment of potato cultivars for their resistance to Synchytrium endobioticum (potato wart) is presented. The margin between resistant and susceptible reactions is determined solely by the intensity of necrosis. Low numbers of ripe wart sori can be tolerated, if substantial necrosis occurs simultaneously. The key is a combination of the testing procedures of the two former German States and has been officially applied in Germany since 1992.     

The Relation Between Field and Laboratory Susceptibility of Potato Cultivars to Wart Disease1)

Potato wart disease has been effectively controlled legislatively by the growing of resistant cultivars. The testing of new potato seedlings for plant breeders is undertaken so that susceptibles can be removed from the breeding program at an early stage. In the laboratory, gradation of resistance and susceptibility is noted. The range of reactions is divided into 6 arbitrary groups, 3 with necrosis and 3 without (Hills , 1965), and Hille's infection patterns have been interpreted as a set of assessment keys. Susceptible cultivars show a variation in degree of susceptibility, but infections persist even though there may be a necrotic hypersensitive reaction of the host. With some less resistant cultivars, the fungus can become established in the host and complete its life cycle, but infected tissue mostly necroses and is sloughed off. These less resistant cultivars (intermediates) grow away from infection when grown on in pots. The cultivar Red Craigs' Royal showed no infection when inoculated in soil tanks (3 soil types) with heavy watering or when grown on infested land. All evidence collected gives indication that intermediate cultivars are field resistant and not affected to any measurable extent by Synchytrium endobioticum (Schilb.) Perc.     

Reactions of potato cultivars to Synchytrium endobioticum and implications for minimum tuber numbers in wart susceptibility tests

Results of wart susceptibility tests on cultivars submitted for UK National List Trials in 1984/1994 are analysed. In cultivars classified as Resistance Group 2 (RG2), the percentage of individual tubers giving RG2 reactions ranged from 1.5 to 76. For most such cultivars (87%), less than 50% of tubers gave RG2 reactions. The percentage of tubers giving susceptible reactions in cultivars classified as susceptible ranged from 10 to 100. For most susceptible cultivars (87%) , more than 60% of tubers reacted as susceptible. It is suggested that slightly susceptible cultivars in which 10-21% of tubers react as susceptible are likely to produce numerous winter sporangia but no wart tissue when grown in infested soil. Those in which > 67% of tubers react as susceptible are likely to produce varying quantities of wart tissue. There was no cultivar in which 22–62% of tubers gave susceptible reactions, indicating a possible natural break in the spectrum between slightly susceptible cultivars and those which produce wart tissue. It is suggested, on the basis of the evidence available. that the number of tubers per cultivar used for susceptibility testing could safely be reduced for most cultivars.     

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.     

Protection of potatoes against pests in Ukraine1

Potato production in Ukraine is now mainly in the private sector, and the phytosanitary state of potato crops leaves much to be desired. Control of Leptinotarsa decemlineata is generally well managed, but potato late blight (Phytophthora infestons) is out of control. Potato wart disease continues to cause problems in areas where it occurs because of the unavailability or unacceptability of resistant cultivars. Potato viruses and the losses they cause are little studied or taken into account. Other fungal, bacterial and insect pests are causing increasing problems in the field and in store. Integrated pest management systems are proposed by scientific institutions, including adequate forecasting of potato blight and use of certified seed potatoes of cultivars resistant to wart and cyst nematodes, but these systems do not adequately reach private growers. The range of plant protection products authorized for sale to the general public is very limited, and this is a serious obstacle to adequate plant protection in potato.     

Traditional and commercial apple and pear cultivars as sources of resistance to fireblight1

In Hungary, fireblight research programmes were initiated on pear in 1999 and on apple in 2000, with the aim of evaluating the susceptibility or resistance of commercial cultivars. Sources of resistance for future breeding were also sought among traditional apple cultivars collected from Ukraine and pear cultivars in the Hungarian gene bank (Szigetcsép). Experiments were done under secure conditions. Inocula were mixtures of characteristic Erwinia amylovora isolates from pear and apple in Hungary. Host responses (symptom development, disease severity and multiplication rate of bacterial cells in host tissues) were assessed on shoots, flowers and fruits. About 30 pear and 30 apple cultivars, and 35 apple hybrids, were tested and grouped into four categories for pear and three for apple. Of the pear cultivars tested, 50% were susceptible, 30% moderately susceptible and only 10% of low susceptibility. Different plant organs occasionally displayed different responses. Members of the last two groups might serve as useful candidates for growing under IPM conditions. Among the traditional Hungarian varieties tested, we found high resistance in ‘Sikulai’ and ‘Szemes alma’, which could be used as sources of fireblight resistance in breeding programmes and also grown in organic orchards. Furthermore, among the offspring of the apple ‘Prima’ (scab‐resistant), we have found highly resistant lines.     

Production of seed potatoes in Ukraine1

The Institute for Potato Research is one of the major producers of healthy potato material in Ukraine. Under its leadership, an increasing proportion of elite seed-potato production (about 70% at present) is based on healthy material propagated in vitro (mainly minitubers and vitro plants). The Institute is investigating the cost effectiveness of elite propagation based on different types of healthy starting material. Against the background of an overall fall in seed-potato production, the Institute has an important responsibility to maintain its collection of healthy potato material, to sustain the national system for seed-potato production, and to support the breeding of new potato cultivars.     

Assessing genetic resistance to spot blotch, <Emphasis Type="Italic">Stagonospora nodorum</Emphasis> blotch and tan spot in wheat from Nepal

Fungal leaf spot diseases of wheat (Triticum aestivum L.) in Nepal cause significant yield reduction. Although field testing has identified a few partially resistant cultivars, most wheat grown in Nepal lacks adequate resistance to leaf spot diseases. During 2009–2010, 116 local and commercial spring wheat cultivars and advanced breeding lines were selected from multi-year field experiments in Nepal and evaluated for seedling resistance to three leaf spot diseases: spot blotch, Stagonospora nodorum blotch (SNB) and tan spot races 1 and 5 (two of the most prevalent races) in the growth chambers at North Dakota State University, Fargo, ND, USA. The wheat cultivars and lines were artificially inoculated with individual pathogens or races at the two-leaf stage and disease reactions were evaluated 6 to 10 days after inoculation (DAI). Results indicated that 30%, 31%, 19% and 10% of the tested wheat cultivars and lines were resistant to spot blotch, SNB, tan spot races 1 and 5, respectively. Six advanced breeding lines (SW89-5422, BL 2127 = DANIAL88/HLB30//NL297, BL 3033, FILIN/IRENA/5/CNDO/R143//ENTE/MEXI-2/3/AE. SQUA (TAUS)/4WEAVER, GAN/AE.SQUARROSA (236)//DOY1/AE.SQUARROSA(447)/3/MAIZ/4/INQALAB91, Mayoor//TK SN1081/Ae. Squarrosa (222)/3/FCT, were resistant to spot blotch, SNB and tan spot race 1. Similarly, two wheat cultivars Chirya 3 and Chirya 7 were resistant to spot blotch, and tan spot races 1 and 5. The resistant wheat lines identified in this study represent potentially useful and untapped sources of resistance to multiple leaf spot diseases and should be utilized in wheat breeding programs in Nepal in order to develop wheat cultivars with broad-spectrum resistance.     

Interlaboratory tests for resistance to Synchytrium endobioticum in potato by the Glynne‐Lemmerzahl method

Synchytrium endobioticum is a major quarantine pathogen of potato causing potato wart disease. In Europe, the pathotypes 1(D1), 2(G1), 6(O1) and 18(T1) are the most widespread and occur locally in almost all countries. Resistance to this disease in potato cultivars is tested for in the majority of the EU countries by the Glynne‐Lemmerzahl method. This paper describes the results of testing two different protocols of this method in five laboratories of three different countries (Germany, Poland, and the Netherlands). The four pathotypes 1(D1), 2(G1), 6(O1) and 18(T1) were tested mainly on the cultivars described in the EPPO Standard PM 7/28 Synchytrium endobioticum. For pathotype 1(D1) and in most cases also for pathotype 18(T1), the results of the cultivars tested were identical to the rating in the EPPO Standard for both protocols. For pathotypes 2(G1) and 6(O1), the cultivars Désirée, Delcora and Miriam showed different results between laboratories as well as between the two protocols. In conclusion, further research is needed to develop one harmonised methodology for resistance testing of potato cultivars to the pathotypes 2(G1), 6(O1) and 18(T1) and to develop a new differential set of potato cultivars for the identification of pathotypes of S. endobioticum.     

Novel necrotrophic effectors from Stagonospora nodorum and corresponding host sensitivities in winter wheat germplasm in the southeastern United States

Stagonospora nodorum blotch (SNB), caused by the necrotrophic fungus Stagonospora nodorum (teleomorph: Phaeosphaeria nodorum), is among the most common diseases of winter wheat in the United States. New opportunities in resistance breeding have arisen from the recent discovery of several necrotrophic effectors (NEs, also known as host-selective toxins) produced by S. nodorum, along with their corresponding host sensitivity (Snn) genes. Thirty-nine isolates of S. nodorum collected from wheat debris or grain from seven states in the southeastern United States were used to investigate the production of NEs in the region. Twenty-nine cultivars with varying levels of resistance to SNB, representing 10 eastern-U.S. breeding programs, were infiltrated with culture filtrates from the S. nodorum isolates in a randomized complete block design. Three single-NE Pichia pastoris controls, two S. nodorum isolate controls, and six Snn-differential wheat controls were also used. Cultivar-isolate interactions were visually evaluated for sensitivity at 7 days after infiltration. Production of NEs was detected in isolates originating in each sampled state except Maryland. Of the 39 isolates, 17 produced NEs different from those previously characterized in the upper Great Plains region. These novel NEs likely correspond to unidentified Snn genes in Southeastern wheat cultivars, because NEs are thought to arise under selection pressure from genes for resistance to biotrophic pathogens of wheat cultivars that differ by geographic region. Only 3, 0, and 23% of the 39 isolates produced SnToxA, SnTox1, and SnTox3, respectively, by the culture-filtrate test. A Southern dot-blot test showed that 15, 74, and 39% of the isolates carried the genes for those NEs, respectively; those percentages were lower than those found previously in larger international samples. Only two cultivars appeared to contain known Snn genes, although half of the cultivars displayed sensitivity to culture filtrates containing unknown NEs. Effector sensitivity was more frequent in SNB-susceptible cultivars than in moderately resistant (MR) cultivars (P = 0.008), although some susceptible cultivars did not exhibit sensitivity to NEs produced by isolates in this study and some MR cultivars were sensitive to NEs of multiple isolates. Our results suggest that NE sensitivities influence but may not be the only determinant of cultivar resistance to S. nodorum. Specific knowledge of NE and Snn gene frequencies in this region can be used by wheat breeding programs to improve SNB resistance.     

Evaluation Of Tests to Determine Resistance of Zantedeschia spp. (Araceae) to Soft Rot caused by Erwinia carotovora subsp. carotovora

Bacterial soft rot caused by Erwinia carotovora subsp. carotovora is a major disease in Zantedeschia spp., particularly in cultivars from the section Aestivae. The disease can be partly controlled by cultural measures, but by combining cultural methods with resistant plant material a promising strategy for control of soft rot can be developed. No tests are available for resistance testing in breeding Zantedeschia spp. Therefore, three tests developed for use in potato breeding were adapted for use on eight cultivars of Zantedeschia spp. Variation was found in all three tests. Resistant control cultivar Zantedeschia aethiopica Crowborough scored most resistant in all three tests. Within the section Aestivae, degrees of susceptibility were identified that were in agreement with each other and with field observations, indicating reliability of two of the methods in which tubers were used. The correlation coefficient of these two tests was high. A new non-destructive test method was developed for use on seedlings which involved immersion of leaf disks in a bacterial suspension. The percentage of decayed leaf area was a measure of resistance and results were in general agreement with the other tests. These methods will be useful for breeding for soft rot resistance and performing genetic analyses.     

Strategy to identify resistance to white mould associated with high yield for irrigated common bean in Brazil

Development of common bean cultivars with partial white mould resistance through breeding techniques has been a challenge in Brazil. As yet, lines/cultivars from breeding programmes have not been investigated for resistance; therefore, this study screened 107 lines/cultivars for their reactions to white mould in 14 preliminary trials conducted under irrigation. Thirteen resistant lines/cultivars (three of Andean origin) and six Mesoamerican cultivars (three intermediately resistant and three susceptible) were selected for further investigation. These lines/cultivars and the resistant control A195 were evaluated in six advanced trials and two straw tests to assess the effectiveness of the screening procedure. In 11 preliminary trials, screenings were performed under moderate/high or higher disease pressure. These pressures occurred in two advanced trials in which, when yields were averaged across moderate/high and high pressures, 10 Mesoamerican lines/cultivars selected for resistance yielded 14%, 23%, and 38% more than intermediately resistant cultivars, A195, and susceptible cultivars, with median disease ratings (1–9 scale) of 4.5, 5.7, 5.7, and 6.7, respectively. In the straw test, three Andean lines/cultivars (A195 included) and two susceptible cultivars in the field were among those with the highest levels of physiological resistance. Thus, field rating under high disease pressure and greenhouse rating did not correlate significantly, suggesting that field trials are critical to evaluating resistance and to identifying high-yielding beans. Therefore, lines/cultivars from breeding programmes assessed in field trials may provide a low cost and fast way to identify high-yielding bean cultivars with partial resistance to white mould in the subtropical southern hemisphere.     

四川省育成小麦品种和高代品系对条锈病的抗病性评价

四川省不仅是条锈菌冬繁区,也是我国东部麦区重要的春季初侵染菌源地和毒性变异地。准确评价四川小麦新育成品种和高代品系对条锈病的抗性水平,能为合理培育和使用抗病品种提供重要依据。利用当前条锈菌优势小种和4个地点的成株期鉴定圃对来自四川省的115份小麦育成品种和299份高代品系进行苗期和成株期抗病性鉴定。结果表明,新育成品种苗期对优势小种的高抗CYR32、CYR33和CYR34的比例发生不同程度的变化,对CYR32的高抗比例为40.9%、对CYR34的高抗比例为17.4%,而供试高代品系高抗比例从52.3%降至46.2%;中抗类型育成品种从20.0%增加到40.9%,高代品系从19.6%增加至33.2%。表明条锈菌优势小种更替后一些材料仍保留了部分抗性,后期选育的高代品系兼顾对新小种的抗病性。育成品种在四川成都和甘肃清水成株期鉴定,高抗品种所占比例较低,分别为13.9%和3.5%,但高代品系高抗类型比例较高,分别为42.2%和8.5%,表明甘肃和四川条锈菌的致病类型和发病环境条件存在较大差异,近年来四川小麦成株抗条锈病育种取得显著进展。     

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.     

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.     

Novel metrics to classify fire blight resistance of 94 apple cultivars

Fire blight (Erwinia amylovora), a potentially devastating disease in apple, can cause floral, fruit and structural damage and even tree death. Most commercial apple cultivars are susceptible and the resistance/susceptibility of many modern cultivars has not been evaluated. Fire blight resistance/susceptibility is difficult to phenotype due to quantitative resistance, impacts of tree vigour and environment on susceptibility, and the erratic nature of the disease. Resistance/susceptibility levels were determined for 94 apple cultivars and important breeding parents. In 2016 and 2017, multiple actively growing shoots per tree (about three trees per cultivar) were challenged with E. amylovora Ea153n via a cut-leaf inoculation method. Proportion of current season's shoot length blighted (SLB) was calculated for each shoot. To classify cultivar responses, estimated marginal SLB means were compared to four controls, representing highly susceptible (HS) to highly resistant (HR), via Dunnett's tests. Cultivar responses ranged from HS to HR with estimated marginal SLB means of 0.001–0.995 in 2016 and 0.000–0.885 in 2017. Most cultivars demonstrated similar resistance/susceptibility levels in both years (ρ = 0.657, P < 0.0001). K-means clustering was used to classify cultivars into three resistance/susceptibility groups based on incidence, average severity (SLB), and maximum severity values (maximum SLB and age of wood infected). Sixteen cultivars were consistently moderately resistant (MR) to HR while the remainder ranged from HS to MR. An updated comparison of susceptibility of important cultivars is provided. Resistance/susceptibility information gained could be used to identify genetic loci associated with resistance/susceptibility and/or inform parental selection in apple scion breeding programmes.