Effect of temperature on the transmission of Potato mop-top virus from seed tuber and by its vector, Spongospora subterranea

Potato mop-top virus (PMTV) causes disease in both the growing plant and tubers (spraing) of potato and is transmitted by the plasmodiophorid Spongospora subterranea , the cause of powdery scab. The effect of temperature during plant growth on the transmission of PMTV from infected seed tubers and from infested growing media was investigated in a series of glasshouse experiments. Symptoms developed on foliage of plants derived from infected seed tubers but none developed when PMTV was transmitted by S. subterranea in soil. The incidence of foliar symptoms was greatest on plants grown at 12°C, less at 16°C, few at 20°C and absent at 24°C. The transmission of PMTV from infected seed tubers was not significantly affected by temperatures between 12 and 24°C, but when the virus was transmitted by S. subterranea , minimal tuber infection occurred at 24°C and no differences were recorded at temperatures between 12 and 20°C. The incidence of powdery scab on tubers was greatest at 12 and 16°C and very low at 20 and 24°C. However, the incidence and severity of root galling caused by S. subterranea , was greatest at 20 and very low at 24°C. The incidence of powdery scab was greater on tubers of plants derived from infected seed tubers grown in a fluctuating temperature regime of 12 h at 20°C followed by 24 h at 12°C than on those grown at a constant 20°C, whereas the incidence of tuber infection by PMTV and spraing was similar for both regimes. This demonstrates that infection of roots can occur at a higher temperature than that for powdery scab on tubers and that this root infection can enable the transmission of PMTV into the potato plant.     

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.     

Necrotic diseases caused by viruses in Swedish potato tubers

Potato tuber necrosis in the form of spraing symptoms is caused by infection with Potato mop‐top virus (PMTV) or Tobacco rattle virus (TRV); spraing has become more important in the Swedish potato crop production. In this study, the presence in Sweden of three potato‐infecting viruses associated with necrotic symptoms in tubers was demonstrated: PMTV, TRV and Tobacco necrosis virus (TNV). This study shows that both PMTV and TRV are frequent in Swedish potato fields. PMTV was found in all Swedish counties, except the two most northern ones. TRV was present at several locations in southern and up to the central parts of Sweden. Both viruses were found further north than observed in earlier surveys. PMTV and TRV were analysed in tubers with spraing symptoms and it was not possible to decide visually if the symptoms were caused by either PMTV or TRV. Furthermore, a high occurrence of symptomless tuber infections was observed for several potato cultivars. A unique observation from this study was a mixed infection of both PMTV and TRV in tubers of cv. Berber, where no visual differences on symptom development were detectable for these tubers compared to single infections. During the survey, tubers of cv. Melody were found to display necrotic symptoms in the skin that are characteristic for the ABC disease. This suggested infection by TNV, which also could be confirmed.     

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).     

Molecular and biological characterization of Potato mop‐top virus (PMTV,Pomovirus) isolates from the potato‐growing regions of Colombia

Potato mop‐top virus (PMTV) causes necrotic flecks inside and on tubers in temperate countries. In South America, these symptoms have not been observed, although the presence of the virus has been confirmed in the Andes and in Central America. To characterize PMTV isolates from the Andes, soil samples were taken from the main potato‐producing regions in Colombia and virus was recovered by planting Nicotiana benthamiana as bait plants. The complete genomes of five isolates were sequenced and three of the isolates were inoculated to four different indicator plants. Based on sequence comparisons, three types of RNA‐CP (RNA2) and RNA‐TGB (RNA3) were found. The isolates from the centre of the country (CO3 and CO4) were similar to isolates from Europe. The genomes of CO1, CO2 and CO5 differ from other PMTV isolates, placing them in a separate clade in phylogenetic trees. The three Colombian isolates (CO1, CO2 and CO5) only induced slightly different symptoms in the indicator plants. However, the isolate from the northwest of the country (CO1) induced stronger symptoms in N. benthamiana including severe stunting. A correlation between the genotype of the isolates and the symptoms they induced on indicator plants was not found.     

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.     

Novel in vivo use of a polyvalent Streptomyces phage to disinfest Streptomyces scabies-infected seed potatoes

A highly virulent and polyvalent Streptomyces phage was isolated from a potato field near Albany, Western Australia. The efficacy of the isolated phage to disinfest seed potato tubers artificially inoculated with a common scab-causing streptomycete was evaluated. The phage suspension was prepared in a mini-bioreactor. Diseased potatoes were bathed in a phage suspension (1 × 10⁹ plaque-forming units per mL) for 24 h. The suspension was constantly circulated within a novel 25 L phage bath by means of an air-sparging pipe driven from an air compressor. Phage-treated scab-affected seed potatoes planted into free-draining polystyrene boxes containing steam-pasteurized field soil produced tuber progeny with significantly ( P  < 0·05) reduced levels of surface lesions of scab (1·2%) compared with tubers harvested from nonphage-treated tubers (23%). The number of scab lesions was also significantly reduced ( P  < 0·05) by phage treatment of mother tubers. No significant differences were recorded in weight, size or number of harvested tubers from phage-treated or nontreated mother tubers. This is the first in vivo study that has used Streptomyces phage to significantly disinfest seed potatoes of Streptomyces scabies and thereby reduce contamination of soil from seed-tuber-borne inoculum and reduce infection of daughter tubers.     

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.     

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。     

Tuber transmission of ‘Candidatus Liberibacter solanacearum’ and its association with zebra chip on potato in New Zealand

Zebra chip, an emerging disease of potatoes, has recently been associated with ‘Candidatus Liberibacter solanacearum’ in New Zealand. The phloem-limited bacterium is known to be vectored by the tomato potato psyllid (Bactericera cockerelli). In this study, the role of tuber transmission in the spread of Ca. L. solanacearum was investigated by re-planting potato tubers infected with Ca. L. solanacearum in the absence of the psyllid. Nested PCR demonstrated that Ca. L. solanacearum could be transmitted from the mother tubers both to the foliage of growing plants and to progeny tubers, resulting in symptomatic and asymptomatic plants. Of 62 Ca. L. solanacearum-infected tubers four did not sprout symptomatic of zebra chip. A further two plants developed foliar symptoms associated with zebra chip during the growing season and died prematurely. Fifty-six of the infected tubers produced asymptomatic plants, although Ca. L. solanacearum was detected in the foliage of 39 of them indicative of transmission into asymptomatic progeny plants. At harvest, Ca. L. solanacearum was found in the daughter tubers of only five of the 39 asymptomatic plants, and only one of these plants was found to have zebra chip symptoms in the daughter tubers. Our results show that tuber transmission of Ca. L. solanacearum could play a role in the life cycle of this pathogen, providing a source for acquisition by Bactericera cockerelli and for movement of the pathogen to other regions of New Zealand via transport of seed tubers.     

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.     

Prevalence of fungal and fungus‐like diseases on potato seed tubers imported from Europe to Jordan

A total of 109 samples of potato seed tubers imported to Jordan from France, Netherlands and Denmark during the 2007/2008 growing seasons were surveyed for 10 different diseases caused by fungi and fungus‐like organisms to determine the prevalence of different tuberborne diseases and to evaluate the efficacy of the current visual examination procedures in detecting different potato diseases. The results demonstrated that most potato seed tuber lots imported to Jordan were infected with one or more of the following potato pests: Colletotrichum coccodes (black dot) (66.0%), Rhizoctonia solani (black scurf) (42.4%), Helminthosporium solani (silver scurf) (92.6%), and Spongospora subterranea (powdery scab) (13.8%). Just over a quarter (25.7%) of all lots examined in this study were found to exceed the acceptance limits for one or more potato diseases according to the Jordanian standards. This study suggests that the current visual examination procedure is not adequate to detect all tuberborne diseases of potato and to estimate their incidence and severity precisely.     

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.     

Improved immunological detection of S<Emphasis Type="Italic">pongospora subterranea</Emphasis>

The genus Spongospora has two members which are important pathogens of vegetables, S. subterranea f.sp. subterranea (Sss) and S. subterranea f.sp. nasturtii (Ssn). The close taxonomic relationship of these formae speciales is based on similar cystosori morphology. The potato disease powdery scab, caused by Sss, is difficult to control. The key control measure is avoidance, aimed at planting clean seed in clean soil. For the development of routine tests for the presence of the pathogen on tubers and in soil, a monoclonal antibody (MAb) was developed using Sss cystosori as immunogen. It detected less than one Sss cystosorus and recognised Sss material from many parts of the world. No cross-reactions with other Plasmodiophoromycetes including Plasmodiophora brassicae, Polymyxa betae, Polymyxa graminis and different Streptomyces species causing common and netted scab of potatoes were observed. A novel tuber sample test method was developed using a kitchen peeling machine. This detected two tubers with one powdery scab lesion each in a sample including eighteen uninfected tubers. When soil samples spiked with cystosori were tested with the MAb, different Sss infestation levels could be discriminated. Ssn cystosori gave absorbance values in ELISA as high as Sss cystosori, whereas fresh crook roots of watercress containing Ssnzoosporangia and plasmodia or mud from an Ssninfected watercress bed gave low absorbance values or no reaction. The potential of these findings for the development of a disease control management are discussed.     

Investigations on potato mop-top furovirus in Sweden1

Potato mop-top furovirus (PMTV) was isolated for the first time in Sweden in 1985. During 1986 and 1987, soil samples from potato fields, mainly in the south-west of Sweden, were tested for PMTV. The virus was detected in 49 out of 305 samples tested. PMTV was also found to be present in soil collected from potato riddles during potato grading in early spring. The soil samples were tested by using Nicotiana clevelandii and N. debneyi as bait plants in a glasshouse. Seven potato cultivars were grown in a PMTV-infested field. PMTV symptoms in the tubers, i.e. external and/or internal brown rings and arcs (spraing), were observed in cvs Evergood, Provita, Saturna and Ukama but not in cvs Bellona, Bintje or King Edward. Very few, or no, PMTV symptoms were found in tubers of susceptible cultivars when examined 2-3 weeks after lifting. In later examinations spraing appeared and increased. The increase was greater in tubers stored at 9°C than in those at 4°C, and in cut than in uncut tubers.     

我国马铃薯疮痂病及其防治研究进展

马铃薯疮痂病(potato common scab)是由放线菌目链霉菌属的链霉菌Streptomyces spp.引起的土传兼种传病害,广泛分布于世界各马铃薯种植区,不仅影响马铃薯的外观品质和销售价格,严重时还会导致马铃薯出苗延迟甚至引起幼苗死亡,造成产量下降,给马铃薯产业带来巨大的经济损失,已经成为全球危害马铃薯生产的第四大病害。2015年我国确立马铃薯主粮化战略,推动了马铃薯产业的发展。近年来,随着种植区域和规模不断扩大,马铃薯疮痂病在我国很多省(自治区)有不同程度的发生,并有逐年扩大和加重的趋势,严重影响商品薯、加工原料薯和种薯的生产,成为制约我国马铃薯生产的主要病害。本文对马铃薯疮痂病症状、发病因素、传播规律、致病机理、分类方法以及我国马铃薯疮痂病发生情况、种类及分布进行归纳,并对马铃薯疮痂病防治措施进行总结,以期为我国马铃薯疮痂病的研究和防治奠定理论基础。     

Powdery scab resistance in Solanum tuberosum: an assessment of cultivar × environment effect

Powdery scab of potato caused by Spongospora subterranea is one of the main disease problems in many potato production regions of the world. However, no efficient and economically sound control method is currently available. Host resistance will be a key component of the integrated management of powdery scab, but there are discrepancies in published powdery scab resistance ratings of cultivars between countries. In order to identify the main factors causing such discrepancies, 10 reference cultivars thought to have a range of susceptibility to powdery scab and potato mop‐top virus were cropped over 4 years in four to six locations across Europe and disease levels on roots and tubers were assessed using standardized scoring scales. Soil contamination was tested using real‐time PCR and ELISA. The cultivars performed as expected according to previous characterization, with one exception. No relationship was found between tuber and root susceptibility. Assessment of powdery scab symptoms 1 month before harvest gave results comparable to those assessed 2 months after harvest. Neither real‐time PCR nor ELISA soil test results were closely related to disease index data. The field trial results indicate that different scoring methods are the main factor for the discrepancy in resistance ratings, and that environmental conditions and/or soil inoculum level play a minor role. Furthermore, there was either no difference between the pathogen populations in each location or the resistance of most of the cultivars is polygenic.     

马铃薯粉痂病综合防治技术初探

近年来,粉痂病已成为云南省马铃薯生产上的重要病害之一,但目前还没有有效防治粉痂病的药剂和措施。本试验于2005年对云南省马铃薯粉痂病的综合防治技术进行了大田试验和温室盆栽试验。结果表明:在大田试验和温室盆栽试验中,品种会-2对马铃薯粉痂病均表现较高的抗性,在播种穴中施用适量的豆饼进行处理均能较好地防止粉痂病的发生,其他处理的防治效果则不明显。