The potential for the Rapid Screening of Potato Cultivars (Solanum tuberosum) for Resistance to Powdery Scab (Spongospora subterranea) using a Laboratory Bioassay

Powdery scab of potato, once established in a field, is difficult to control because of the longevity of the resting spores (cystosori) of the causal organism, Spongospora subterranea f.sp. subterranea. Host resistance is likely to be the most efficient in a long-term control strategy for preventing build-up of field inoculum and spread of the disease. Resistance screening of potato cultivars is mostly done in laborious field trials where disease development is likely to be unpredictable. A bioassay with potato tissue cultured plantlets and cystosori as inoculum is described and was tested for its potential to screen potato cultivars at an early stage for their relative susceptibility to powdery scab by comparing the lab results with field data. With cystosori inoculum of Swiss origin, the laboratory test showed clear differences between the potato cultivars in the severity of zoosporangial root infection which correlated better with ranked tuber infection data, compared to root galling. There are apparent differences in the relative trends in susceptibility between roots and tubers of five selected cultivars when using naturally infested soil instead of prepared cystosori as inoculum in the lab bioassay. Furthermore, differences in the severity of zoosporangial root infection of two selected cultivars were found when cystosori from different countries where used as inoculum. A possible host genotype × pathogen interaction is discussed. The bioassay has the potential to screen and select for resistant material at an early breeding stage thus making field trials not unnecessary but more economical. It will allow the use of a standard set of pathogen collections and facilitate testing for inoculum virulence in infested soils.     

Relationship between Spongospora subterranea f. sp. subterranea soil inoculum level,host resistance and powdery scab on potato tubers in the field

The relationship between initial soil inoculum level of Spongospora subterranea f. sp. subterranea (Sss) and the incidence and severity of powdery scab on potato tubers at harvest was investigated. In all experiments soil inoculum level of Sss (sporeballs/g soil) was measured using a quantitative real‐time PCR assay. Of 113 commercial potato fields across the UK, soil inoculum was detected in 75%, ranging from 0 to 148 Sss sporeballs/g soil. When arbitrary soil inoculum threshold values of 0, <10 and >10 sporeballs/g soil were set, it was observed that the number of progeny crops developing powdery scab increased with the level of inoculum quantified in the field soil preplanting. In four field trials carried out to investigate the link between the amount of inoculum added to the soil and disease development, disease incidence and severity on progeny tubers was found to be significantly (P < 0·01) greater in plots with increasing levels of inoculum incorporated. There was a cultivar effect in all years, with disease incidence and severity scores being significantly greater in cvs Agria and Estima than in Nicola (P < 0·01).     

云南省马铃薯粉痂病的发生与危害调查分析

粉痂病严重影响马铃薯的产量和品质,是我国马铃薯产业健康发展的重要制约因素之一。为进一步探明马铃薯粉痂病在云南省的发生、分布及危害情况,本研究于2018年-2019年,在云南省内马铃薯不同生态种植区,开展了马铃薯粉痂病的发生与危害情况调查。调查范围涉及19个县(区)、涵盖了19个当地马铃薯主栽品种(系)。调查结果显示:粉痂病在云南发生普遍,发病率在0%~51.72%,平均发病率和病情指数分别为15.01%和5.32。其中春作区马铃薯粉痂病的发生较为普遍,平均发病率为25.08%,病情指数为8.24。冬作区马铃薯粉痂病在部分产区有发生,平均发病率为4.93%,病情指数为2.39。与2004年-2008年云南省粉痂病调查数据相比,粉痂病发病率下降,病害有所减轻。     

<Emphasis Type="Italic">Spongospora subterranea</Emphasis> soil contamination and its relationship to severity of powdery scab on potatoes

We investigated soil contamination by Spongospora subterranea f. sp. subterranea (Sss) and disease severity of powdery scab in 29 potato fields in Hokkaido, Japan, using a hydroponic culture method with tomato seedlings as bait plants. The quantity of Sss infection on the roots of bait plants was evaluated using the polymerase chain reaction (PCR) and expressed in terms of the infection potential in the soil. The infection potential was positively correlated with the disease severity of harvested tubers, whereas the spore ball density determined using PCR had an indistinct relationship with disease severity. The infection potential can be useful in evaluating soil contamination and in applying countermeasures against powdery scab.     

The importance of the infected seed tuber and soil inoculum in transmitting Potato mop‐top virus to potato plants

Potato mop‐top virus (PMTV), the cause of spraing in potato tubers, is transmitted by Spongospora subterranea, the cause of powdery scab, and by planting infected seed tubers. This study was undertaken to determine the relative importance of these sources of infection in seed potato production in Scotland. The transmission of PMTV from tested seed tubers to daughter plants was examined over 2 years and six cultivars. The development of foliar symptoms varied with year and cultivar. Infection of daughter tubers derived from PMTV‐infected seed tubers was more prevalent on plants affected by foliar symptoms than those without symptoms. The rate of transmission of PMTV from infected seed tubers to daughter tubers ranged from 18 to 54%. Transmission was affected by cultivar and by origin of seed tubers used for a cultivar, but not by a cultivar's sensitivity to PMTV infection. The incidence of PMTV in daughter tubers of cv. Cara grown from seed potatoes from one source (common origin) by more than 25 seed producers was examined over two successive generations. The incidence of PMTV in daughter tubers was not correlated with that in the seed tubers but appeared to be strongly associated with soil inoculum. The incidence of PMTV was correlated with powdery scab in those crops in which both were present. There was some evidence from soil tests conducted in 2006 using a tomato bait plant and real‐time RT‐PCR that planting PMTV‐infected seed potatoes could increase the risk of introducing the virus into land not infested by PMTV.     

Root infection of potato by Spongospora subterranea: knowledge review and evidence for decreased plant productivity

Information is reviewed on root infection of potato by the plasmodiophorid Spongospora subterranea f. sp. subterranea. This pathogen has long been recognized as the cause of root galls (hyperplasia) and the economically important disease powdery scab on tubers (modified stolons). The significance for plant productivity of the zoosporangium stages of the pathogen in potato roots has only recently begun to be documented. Two experiments are described that assessed effects of S. subterranea root infection on potato plant root function and productivity. A greenhouse experiment measured root function and plant parameters for eight potato cultivars with markedly different susceptibilities to tuber powdery scab. Water uptake and plant growth were reduced by S. subterranea inoculation in all eight cultivars. The magnitudes of these negative effects, and intensities of root hyperplasia, differed among the cultivars, but were not related to respective susceptibilities to tuber powdery scab. A field trial assessed root function and plant productivity for a cultivar (Iwa) that is very susceptible to Spongospora tuber and root diseases. Soil water content beneath uninoculated plants was consistently less than for inoculated plants, indicating that inoculation reduced water uptake (root function). Inoculation reduced shoot and root dry weights, and reduced weight of tubers per plant by 42%. Spongospora subterranea causes three diseases of potato: root membrane dysfunction, root hyperplasia and tuber powdery scab. The root diseases caused by the pathogen are likely to be important both for powdery scab management and for deleterious effects on potato crop yields.     

云南马铃薯粉痂病发生情况初步研究

马铃薯粉痂病由Spongospora subterranea f.sp.subterranea引起,是典型的土传病害,主要危害块茎和根部。2004年调查了昭通市和会泽县7个乡镇,8个马铃薯品种的粉痂病发生情况。在调查的品种中,会-2发病最轻,平均发病率27%。会泽县马铃薯粉痂病平均发病率59%,昭通市马铃薯粉痂病平均发病率35%。分析土壤中氮、磷、钾、有机质含量及pH与发病率的关系。PB06发病率与土壤中氮、有机质呈正相关,与磷、钾、pH呈负相关,其中与钾含量相关系数为0.693。会-2发病率与土壤pH、钾含量呈负相关,相关系数分别为0.384、0.858,其中与钾含量相关性显著;与磷、有机质含量呈正相关,相关系数分别为0.727、0.323;与氮含量呈正相关,相关系数为0.993,相关性显著。     

Effect of soil inoculum level and environmental factors on potato powdery scab caused by Spongospora subterranea

The effects of soil inoculum level and three environmental factors (soil type, soil moisture regime and temperature) on the incidence and severity of powdery scab caused by Spongospora subterranea were investigated in potato plants grown under controlled environmental conditions. Symptoms of powdery scab on tubers were assessed visually, after which DNA was extracted from tuber peelings and quantified in a real-time polymerase chain reaction assay using primers and a TaqMan® probe specific to S. subterranea to establish tuber infection levels. Soil inoculum concentration of S. subterranea did not significantly affect the incidence and severity of either tuber infection or powdery scab symptoms at maturity. No significant differences in disease incidence and severity were found between sandy, loamy and clay soils, although the two lighter soils yielded more powdery scab than clay soil. Constant dampness of the soil resulted in significantly more disease than a fluctuating moisture regime. Infection and disease levels were high at all three temperatures tested (9, 12 and 17°C), but symptoms were most severe at 12°C. The percentage of plants with infected tubers did not increase after tuber initiation, although the amount of S. subterranea DNA detected in tubers and the severity of powdery scab symptoms increased in mature plants. Latent tuber infections were found to be common, especially under conditions suboptimal for disease development. This new information may be important for the prevention of powdery scab in potato-growing areas around the world.     

Biocontrol of powdery scab of potato by seed tuber application of an antagonistic fungus, <Emphasis Type="Italic">Aspergillus versicolor</Emphasis>, isolated from potato roots

An environmentally friendly measure to control potato powdery scab caused by a protozoan pathogen Spongospora subterranea f.sp. subterranea (Sss) was developed by focusing on antagonistic microorganisms that were considered compatible with potato root. Five hundred and eight soil fungi, isolated from potato root cultivated in soil suspensions from four potato fields in Hokkaido, were screened for suppressiveness of root infection by Sss in a hydroponic culture system and for powdery scab severity in greenhouse and field experiments. Antagonistic isolate Im6-50, identified as Aspergillus versicolor, was selected as a potent biological control agent. In a 3-year field test, A. versicolor Im6-50 suppressed powdery scab with a protection value of 54–70 (100?=?complete protection) when applied directly on seed tubers compared with a protection value of 77–93 by the synthetic fungicide fluazinam. A. versicolor Im6-50 was detected from the surface of daughter tubers and from the soil in which the inoculated seed tubers were cultivated by PCR using species-specific primers. The establishment of A. versicolor Im6-50 on the stolon of inoculated potato plants and in the rhizosphere is considered to contribute to the mechanism for disease suppression.     

Relationship of resistance to common scab disease and tolerance to thaxtomin A toxicity within potato cultivars

Thaxtomin A has a central and implicit role in common scab disease expression in potato. Thaxtomin A tolerance has been suggested as a possible rapid means for screening potato germplasm for disease resistance, during breeding selections. We have tested a range of genetically diverse cultivars with varying resistances to common scab disease in both pot and field based studies and measured their mean necrosis response to thaxtomin A. We found no association between resistance to common scab disease and tolerance to thaxtomin A toxicity. For example, disease resistant cultivars ‘Russet Burbank’ and ‘Atlantic’ were sensitive and tolerant to thaxtomin A toxicity respectively. Similarly; disease susceptible cultivars ‘Bismark’ and ‘Tasman’ showed susceptibility and tolerance to thaxtomin A. This demonstrates that whilst thaxtomin A is critical to disease expression, reaction to this toxin is only one component influencing resistance to common scab disease and many other anatomical, physiological or biochemical factors are critical to defence against this disease.     

Somaclonal selection in potato for resistance to common scab provides concurrent resistance to powdery scab

Variant somaclones of potato cultivar Russet Burbank, selected for resistance to common scab using in vitro cell selection techniques, were tested for resistance to powdery scab, another important disease of potato caused by Spongospora subterranea. This pathogen also invades roots, producing root galls. Most variants consistently showed increased resistance to powdery scab, both in field and glasshouse challenge, when compared to the parental cultivar, several significantly so. On average, the best variant reduced powdery scab incidence by 51% and severity (tuber surface coverage) by 64%. In contrast, no improvement in the extent of root infection and root galling was seen. These results suggest host interactions during root and tuber infection are distinct. Correlation analyses of disease indices amongst the selected variants showed no association between Sp. subterranea root infection and gall scores, nor between root infection and tuber disease severity. However, a weak positive association was found between root gall score and tuber disease, and a strong correlation between tuber disease incidence and severity scores. Interestingly, positive correlations were also found between the extent of powdery and common scab resistance expressed and both incidence and severity of these diseases within the variants, suggesting a common defence mechanism. The role of thaxtomin A in selecting for concurrent resistance to both diseases is discussed.     

马铃薯块茎和土壤样品中粉痂病菌快速检测方法的建立

 马铃薯粉痂菌(Spongospora subterranea f. sp. subterranea)是引起马铃薯粉痂病的病原。本研究根据粉痂菌内部转录间隔区和线粒体DNA的保守区域,分别设计了2对适用于普通PCR的引物A5/A9、C3/C8和1对适用于荧光定量PCR的引物QF/QR,用于检测块茎和土壤样品中的粉痂菌。特异性检测结果表明:引物对A5/A9和C3/C8,以马铃薯粉痂菌DNA为模板,能分别扩增出264和367 bp大小的单一条带,而对其他非靶标DNA无扩增;引物对QF/QR对马铃薯粉痂菌有单一的熔解峰,说明三对引物特异性良好。灵敏性检测结果表明:荧光定量PCR灵敏度为13.8 fg·μL^-1,是普通PCR灵敏度的1 000倍。进一步建立循环域值(Ct)与质粒DNA含量的曲线关系,获得标准曲线y=-3.893 9 x+35.228,R² = 0.9966,呈良好线性关系。通过对不同地区采集的18份带菌种薯和18份带菌土壤进行普通PCR和荧光定量PCR检测,引物A5/A9、C3/C8和QF/QR对带菌种薯检测率均为100%,对带菌土壤的检测率分别为44.44%、66.67%和100%。本研究建立的马铃薯粉痂病菌快速检测方法,能及时、准确地检测带菌种薯和土壤,为马铃薯粉痂病的早期诊断和防治提供依据。     

Resistance of potato genotypes to common and netted scab-causing species of Streptomyces

Common and netted scabs are two disfiguring bacterial diseases of potato tubers, caused by various groups of Streptomyces species. Common scab, caused primarily by Streptomyces scabies and Streptomyces europaeiscabiei , is characterized by more or less deep pustules on the tuber surface, while symptoms of netted scab, caused mainly by Streptomyces reticuliscabiei , are superficial, corky alterations of the tuber periderm. Some isolates of S. europaeiscabiei are able to induce both common and netted scab symptoms, and therefore constitute a third pathogenicity group. Like most bacterial diseases, potato scabs would be best controlled by using resistant cultivars. Repeated experiments with soil artificially infested with isolates of three species representative of the three pathogenicity groups showed the level and stability of cultivar resistance, as well as the existence of a range of aggressiveness among different isolates. The distribution of scab severity indexes recorded on a collection of 16 potato cultivars and 27 breeding clones grown in soil infested with common scab-inducing isolates was continuous, suggesting isolate nonspecific quantitative resistance. Least susceptible cultivars were Nicola, BF15, Sirtéma, and Charlotte, while Urgenta, Désirée, Ondine and Bintje were very susceptible. The same genotypes proved either highly susceptible (e.g. cvs Bintje, Désirée or Carmine) or highly resistant (e.g. cvs Charlotte, Sirtéma, Monalisa, BF15 or Belle de Fontenay) to isolates forming netted scab symptoms, suggesting isolate-specific qualitative resistance. The ability was confirmed of some isolates of S. europaeiscabiei to induce one or the other type of symptoms depending on cultivar and soil temperature.     

Prevalence and distribution of Potato mop‐top virus in Scotland

This study was undertaken to determine the current occurrence in Scottish seed potato crops of Potato mop‐top virus (PMTV), which is transmitted by Spongospora subterranea and causes spraing (brown arcs and lines) in the flesh of potato tubers, rendering them unsaleable. In 2004, a stratified survey of four commonly grown cultivars was conducted, while in 2007 and 2008, only samples from powdery scab‐affected crops were collected. The incidence of crops in which infection by PMTV was present was 37·5% in the stratified survey in 2004, but was greater in surveys in which tubers with powdery scab were tested (47·2% in 2007 and 44·6% in 2008). Similarly, the frequency of crops with incidences of more than 10% tuber infection was lower (9·4%) in 2004 than in 2007 (25·4%) and in 2008 (26·2%). Significant differences in crop infection were found amongst the four major seed‐producing regions and the counties within these regions. The incidence of crop and tuber infection was least for class Pre‐basic seed potatoes and greatest for class Super Elite 3 and Elite seed potatoes. The results indicate that the prevalence of PMTV has not increased since surveys in the early 1970s.     

Green manures and crop sequences influence potato diseases and pathogen inhibitory activity of indigenous streptomycetes

ABSTRACT A 2-year field trial was conducted to determine the effects of green manures and crop sequences on potato scab and Verticillium wilt. In addition, indigenous streptomycete densities and in vitro pathogen inhibitory activity were measured and their relationships to plant disease were determined. Green manures (buckwheat, canola, or fallow controls) were tested in conjunction with three crop sequences (alfalfa-potato, cornpotato, and potato-potato). Compared with fallow controls, tubers grown in buckwheat-treated soil had significantly lower Verticillium wilt ratings, and tubers grown in buckwheat- or canola-treated soil had greater yields. Potatoes grown in soil planted to corn or alfalfa the previous year had significantly lower Verticillium wilt and potato scab ratings as well as higher yields than potatoes grown in soil previously planted to potato. Streptomycetes from soils collected from green manure-treated plots tended to have greater in vitro pathogen inhibitory activity than streptomycetes from fallow-treated plots. Furthermore, streptomycete pathogen inhibitory activity was frequently negatively correlated with plant disease and positively correlated with potato yield. These results indicate that green manure treatments may contribute to active management of the pathogen inhibitory activity of the streptomycete community to achieve plant disease control.     

Evaluation of the <Emphasis Type="BoldItalic">Sss</Emphasis> AgriStrip rapid diagnostic test for the detection of <Emphasis Type="BoldItalic">Spongospora subterranea</Emphasis> on potato tubers

Spongospora subterranea, f.sp. subterranea (Sss), which causes powdery scab, is mainly spread through infected seed tubers and survives in contaminated soil for many years. The visual assessment of tuber lots by inspectors carries the risk of misidentification due to the difficulty of distinguishing lesions caused by either Sss or Streptomyces spp.. To avoid this, the “Sss AgriStrip”, a rapid and lab-independent test tool based on a lateral flow immunoassay has been developed, and we assessed its accuracy and sensitivity for detecting Sss. The Sss AgriStrip performed as well as other lab-based identification methods. The Sss AgriStrip, microscopy, ELISA, PCR, and real-time PCR techniques identified infection with S. subterranea in all tubers with typical powdery scab lesions. When lots with tubers showing a mixture of typical and atypical (suspicious) symptoms were tested, the presence of S. subterranea was confirmed in all lesions by all methods. The DNA content was generally lower in atypical than in typical lesions. Diverse and suspicious symptoms, which were difficult to assign to either powdery or common scab, tested negative with Sss AgriStrip and the other methods. This was despite microscopic observation of sporosori-like structures in some samples. Isolation and molecular identification confirmed that these lesions were mostly caused by Streptomyces spp. The Sss AgriStrip is as sensitive as DAS-ELISA with a detection limit between 1 and 10 sporosori per ml buffer. It is ideal for rapid and selective detection of Sss on farms and border inspection points to prevent spread of the pathogen.     

Identification of differentially expressed genes in tolerant and susceptible potato cultivars in response to Spongospora subterranea f. sp. subterranea tuber infection

Powdery scab caused by Spongospora subterranea f. sp. subterranea (Sss) has recently become one of the most devastating potato diseases of economic importance in South Africa. The use of resistant cultivars has long been considered the most effective and sustainable strategy to manage the pathogen. However, little is known about the molecular mechanisms underlying resistance of potato tubers to Sss. Using RNA-sequencing (RNA-seq), 2058 differentially expressed genes (DEGs) were identified from two potato cultivars (tolerant and susceptible) in response to Sss infection. Analysis of the expression patterns of 10 selected defence-response genes was carried out at two different stages of tuber growth using RT-qPCR to validate the RNA-seq data. Several defence-related genes showed contrasting expression patterns between the tolerant and susceptible cultivars, including marker genes involved in the salicylic acid hormonal response pathway (StMRNA, StUDP and StWRKY6). Induction of six defence-related genes (StWRKY6, StTOSB, StSN2, StLOX, StUDP and StSN1) persisted until harvest of the tubers, while three other genes (StNBS, StMRNA and StPRF) were highly up-regulated during the initial stages of disease development. The results of this preliminary study suggest that the tolerant potato cultivar employs quantitative resistance and salicylic acid pathway hormonal responses against tuber infection by Sss. The identified genes have the potential to be used in the development of molecular markers for selection of powdery scab resistant potato lines in marker-assisted breeding programmes.     

Wisselbouw van aardappelrassen als bestrijdingsmiddel tegen schurft

Summary Four potato varieties were grown in a frame filled with soil, heavily infected with scab. For six successive years each variety was grown on the same plot. After these six years two potato varieties, Bintje and Eigenheimer, were cultivated on all plots. The attack and type of scab varied according to the susceptibility of the potatoes grown during the preceding period. Where Jubel was cultivated for six years the attack was least, where Bintje had been grown successively much superficial scab occurred, in the Eigenheimer-plot many deep scab lesions were found. Rotation with potato varieties resistant to scab may have good results if the disease occurs on later grown susceptible potato varieties. These results agree with the facts found by different authors (1, 5, 7, 8) viz. that different types of scab on the same potato are due to special races of Actinomyces, that the susceptibility of the different potato varieties to these Actinomycesraces is not the same and that the virulence of the scab organisms is increased through development on the living growing potato. In verband met de nog weinig uitgewerkte systematiek van de Actinomyceten is geen verschil tussen rassen en soorten van Actinomyces gemaakt en wordt in dit artikel alleen van rassen gesproken.     

The effect of physiological age of potato tubers on female production of potato cyst nematodes (Globodera pallida Pa 2/3)

Fourteen cultivars generally regarded as uniformly susceptible toGlobodera pallida Pa 2/3 were assessed in plastic closed containers for degree of susceptibility and also for root and sprouting characteristics. The material was assessed by scoring degree of sprouting before planting and number of cysts and root growth at the end of experiment at intervals from the time of initial dormancy break through to prolific sprouting. Using covariate analyses, it was found that the degree of root development, particularly in the earlier assessments, significantly influenced the degree of cyst development and so the perceived levels of susceptibility/resistance. Having taken account of root growth characters, significant differences between the cultivars with regard to nematode multiplication were still evident. The implications of the results are discussed in relation to potato breeding and nematological studies.     

Linkage of Vfa4 in Malus × domestica and Malus floribunda with Vf resistance to the apple scab pathogen Venturia inaequalis

The Vf locus from Malus floribunda clone 821 is an important source of resistance to apple scab disease caused by Venturia inaequalis , and has been introduced into numerous cultivars of domesticated apple, Malus  ×  domestica . Cloning of the putative Vf locus has revealed that it contains several receptor-like Vf candidate genes. In order to determine which of these genes is most closely linked to Vf resistance, primers were designed based on conserved regions in the Vf candidate genes adjacent to a variable portion of the leucine-rich repeat (LRR) domain, to yield PCR product length polymorphisms. PCR products were obtained from 31 cultivars of M.  ×  domestica , of which 19 are known to contain Vf resistance, and from 10 selections of M. floribunda . PCR products corresponding in size to Vfa1 and Vfa2 were found in all the plants tested. However, a PCR product with 100% predicted amino acid identity to Vfa4 was found only in M.  × domestica cultivars known to have Vf scab resistance. This PCR product was also found in most, but not all, selections of M. floribunda tested, including the original source of Vf resistance, M. floribunda 821. The PCR product matching Vfa4 appears to be the most closely linked to Vf resistance and should be a valuable tool for monitoring Vf inheritance in apple.