Control of potato powdery scab caused by Spongospora subterranea by foliage cover and soil application of chemicals under field conditions with naturally infested soil

Powdery scab (PS), caused by Spongospora subterranea, reduces the quality and marketability of potatoes worldwide. Disease symptoms include lesions on the tuber surface and root galling, which may lead to yield losses. In the current study we report a sustainable approach to reduce PS by manipulating soil temperature during tuber initiation. Plant cover with nonwoven fabric significantly reduced PS on tubers by 54%–69% in 2017 and 84%–93% in 2019, compared to the control, and root galling by 96% in 2019, due to an increased average minimum and maximum soil temperature of 1.8 and 4.2 °C in respective years. Additional preplanting soil treatments were also evaluated in naturally infested soil. In 2017, disease incidence and severity were significantly reduced using 2.5 or 5 L a.i./ha fluazinam in broadcast application or in-furrow, and by 0.375 or 0.75 L a.i./ha flusulphamide applied in-furrow. In 2019, disease incidence and severity were significantly reduced by the broadcast application of fluazinam, and 75 kg/ha calcium cyanamide, but the latter had a negative impact on yield. Soil fumigation with metam sodium resulted in a 98% reduction in PS. Root galling was significantly reduced by calcium cyanamide, metam sodium, and fluazinam in the 2019 trial only. Foliar application of resistance-inducing phosphonates combined with fluazinam application had no additive effect on PS incidence and severity. Integrated approaches such as tolerant cultivars, soil testing, preplanting fungicide application, and sustainable means of control such as foliage cover for a short period may be implemented in order to control the disease and minimize damage.     

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...     

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.     

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.     

Detection and characterization of Streptomyces causing potato common scab in Western Europe

A PCR-based diagnostic method was developed for direct detection from tuber lesions of pathogenic Streptomyces causing common scab of potato. Primers were designed to amplify a fragment of the txtAB ( txtA and txtB ) genes, which are pathogenicity determinants in the main pathogenic Streptomyces species. The method was evaluated on 84 naturally infected potato samples, comprising 19 potato cultivars, harvested in the years from 2000 to 2004 in the Netherlands, the UK, France, Germany and Spain. Pathogenic Streptomyces in tuber lesions were detected by PCR in 70 samples and were also successfully isolated from these 70 samples. All pathogenic isolates showed the basic general phenotypic traits of the S. scabiei phenetic cluster. RFLP analysis of amplified rRNA sequences, together with carbon source utilization and repetitive BOX profiles, allowed most isolates to be assigned to S. europaeiscabiei , which emerged as the main cause of potato common scab in Western Europe.     

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).     

Parasitic weeds of the Orobanchaceae family and their natural hosts in Jordan

A field survey of the Orobanchaceae family members and their hosts in Jordan was carried out from 2003 to 2007. The intensity of parasite infection on different hosts and the severity of the infestation were evaluated. The results showed the presence of seven species of Orobanche and three species of Cistanche . The Orobanche species were found parasitizing 86 plant species belonging to 24 botanical families. Most of the species attacked by Orobanche were from the Compositae (20 species), Solanaceae (11 species), Leguminosae (nine species), Umbelliferae (seven species), Cruciferae (seven species), Cucurbitaceae (four species), Labiatae (four species), and Rosaceae (four species) families. Other families were represented by one-to-three species. Cistanche attacked 20 species of forage wild shrubs, fruit trees, and forest trees of seven families, mostly belonging to the Chenopodiaceae (seven species) and Leguminosae (three species) families. Previously unreported hosts for both genera include: Amygdalus communis , Olea europaea , and Quercus coccifera , which were parasitized by Orobanche palaestina ; A. communis , O. europaea , Prunus armeniaca , and Prunus persica , which were parasitzed by Orobanche cernua ; O. europaea and A. communis , which were parasitzed by Orobanche schultzii ; Haloxylon persicum , which was parasitzed by Cistanche lutea ; Punica granatum , Alhagi maurorum , Casuarina equisetifolia , Centaurea postii , and Prosopis farcta , which were parasitzed by Cistanche tubulosa ; and Achillea spp., Anabasis syriaca , H. persicum , Haloxylon salicornicum, Suaeda spp., and Zilla spinosa , which were parasitzed by Cistanche salsa . Certain Orobanche species were completely destructive to the cultivated crops. The results indicated the high potential of both parasitic genera to spread and to attack new hosts, while the threat they impose to agriculture in Jordan will probably result from poor management and deficiences in farmers' training.     

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.     

Distribution and characterization of Streptomyces species causing potato common scab in Germany

Field‐grown potatoes showing scab infections were sampled in two successive years and analysed for prevailing Streptomyces strains. In 2008 and 2009, 293 Streptomyces isolates were collected in Germany and analysed for morphology, pathogenicity and strain type. Isolates varied in mycelium colour, sporulation and pigmentation. Based on their morphology, no clear differentiation of species was possible. At the genetic level, sampled isolates, as well as a number of type strains from culture collections, were characterized by PCR using 16S rRNA‐specific primers and PCR‐RFLP of the 16S–23S internal transcribed spacer (ITS) region with Hpy99I. Using this fingerprinting approach, Streptomyces species could be differentiated genotypically. The data from this study show that diversity among scab‐causing species in Germany is much higher than previously thought. Isolates belonged to various Streptomyces spp. previously associated with common scab. This is apparently the first report of pathogenic strains of S. europaeiscabiei, S. stelliscabiei, S. acidiscabiei, S. turgidiscabiei and S. bottropensis within Germany. Streptomyces europaeiscabiei was the predominant species found. Other scab‐causing species were identified, but their local distribution was uneven. For most of the isolates, the presence of the txtAB gene was demonstrated, indicating pathogenicity. This analysis is one of the first reports to examine the distribution of common scab‐causing species in Germany.     

Size, shape and intensity of aggregation of take-all disease during natural epidemics in second wheat crops

Point pattern analysis (fitting of the beta-binomial distribution and binary form of power law) was used to describe the spatial pattern of natural take-all epidemics (caused by Gaeumannomyces graminis var. tritici ) on a second consecutive crop of winter wheat in plots under different cropping practices that could have an impact on the quantity and spatial distribution of primary inoculum, and on the spread of the disease. The spatial pattern of take-all was aggregated in 48% of the datasets when disease incidence was assessed at the plant level and in 83% when it was assessed at the root level. Clusters of diseased roots were in general less than 1 m in diameter for crown roots and 1–1·5 m for seminal roots; when present, clusters of diseased plants were 2–2·5 m in diameter. Anisotropy of the spatial pattern was detected and could be linked to soil cultivation. Clusters did not increase in size over the cropping season, but increased spatial heterogeneity of the disease level was observed, corresponding to local disease amplification within clusters. The relative influences of autonomous spread and inoculum dispersal on the size and shape of clusters are discussed.     

Detection and Quantification of Spongospora subterranea f. sp. subterranea in Soils and on Tubers Using Specific PCR Primers

PCR-based methods were developed for the detection and quantification of the potato pathogen Spongospora subterranea f. sp. subterranea (S. subterranea) in peel, tuber washings and soil. A partial sequence was obtained for S. subterranea ribosomal DNA and specific PCR primers (Sps1 and Sps2) were chosen from the internal transcribed spacer regions. These primers amplified a 391bp product from S. subterranea DNA but did not amplify DNA from potato or a range of soil-borne microbes, including related species. Diluted S. subterranea DNA was detected at a concentration equivalent to 25×10^–5 cystosori or 1 zoospore per PCR. Amplification was detected from peel and washings of infected and apparently healthy tubers, but not from peel of Scottish classified seed potatoes or axenically micropropagated potatoes. A rapid method for extracting S. subterranea DNA from soils was developed. This yielded DNA pure enough for PCR within 3h and facilitated the detection of 1–5 cystosori per gram of soil. A PCR quantification technique was developed involving comparison of product ratios obtained after co-amplification of S. subterranea DNA along with an internal standard (competitor DNA fragment). This quantitative technique was also adapted for use in soil. PCR detection of S. subterranea in soil was considerably more sensitive than previously reported immunoassays and was quicker and easier than conventional bait plant bioassays. Such an assay could be useful for developing disease risk assessments for field soils and seed potato stocks and for future studies on the ecology and control of S. subterranea.     

西北地区马铃薯疮痂病病原菌鉴定及其生物学特性

为明确西北地区马铃薯疮痂病病原菌的种类和生物学特性,分别采用常规组织分离法和土壤混悬液分离法从宁夏、陕西和甘肃3个省区采集的29份疮痂病发病薯块和8份发病地块土壤中进行病原菌分离,并利用形态特征、生理生化特性和16S rDNA序列分析对病原菌进行鉴定。结果表明,从发病薯块和发病土壤中共分离到50株链霉菌Streptomyces spp.,通过回接法验证获得6株马铃薯疮痂病致病菌株。6株致病菌株的培养特性和形态特征差别较大;其中菌株G4-1、G9和SYN13不能以果糖和木糖为单一碳源,菌株SYNT3不能以棉子糖为单一碳源;除菌株NLG4-1外,其余5株菌株均能在络氨酸琼脂培养基上产生黑色素。经16S rDNA序列分析,菌株G4-1、G9与疮痂病链霉菌S. scabiei的相似率分别达99.47%和99.34%,菌株NLG4-1、SYNT3与S. enissocaesilis的相似率分别达97.90%和98.18%,菌株GBH2与加利利链霉菌S. galilaeus的相似率达99.93%,菌株SYN13与S. turgidiscabies的相似率达97.56%,表明西北地区马铃薯疮痂病病原菌至少存在4个种。     

Factors affecting the incidence and severity of Spongospora subterranea infection and galling in potato roots

The incidence and severity of root infection and root galling caused by Spongospora subterranea were assessed in potato plants (cv. Estima) grown under controlled environmental conditions. The effects of temperature, soil type, soil moisture regime and soil inoculum level on infection and root gall development were determined by molecular and visual methods at two plant growth stages. Root gall severity was scored at harvest, after which DNA was extracted from the roots and quantified in a real-time polymerase chain reaction (PCR) assay specific for S. subterranea . Root galling was severe at 17°C, with a disease score of 3·1 on a 0–4 scale, low (0·6) at 12°C, and did not occur at 9°C. The level of inoculum in soil, in the form of artificially added sporosori, had no effect on the incidence and severity of visual symptoms, with 21%, 41% and 33% incidence observed at 5, 15 and 50 sporosori g⁻¹ soil, respectively. Incidence of infection, as detected by the real-time PCR assay, was greater with increasing soil inoculum concentrations, ranging from 48% at 5 sporosori g⁻¹ to 59% (15 sporosori g⁻¹) and 73% (50 sporosori g⁻¹) of plants infected at maturity, but this effect was not statistically significant. No correlation was found between the occurrence of galls on roots and powdery scab on tubers of the same plants.     

Studies of infection withxanthomonas campestris pv.vesicatoria,causal agent of bacterial scab of pepper in Israel

Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial scab of pepper, was isolated in several regions in Israel. When artificial inoculation was practiced, pathogen growth was enhanced by high temperatures (30-36°C), and an inoculum concentration of 10^p6 colony-forming units (CFU) per ml was optimal for symptoms to develop on plants. Pre-inoculation treatments such as wounding the leaves by rubbing them with carborundum powder or spraying them with diluted wax solvents, markedly increased disease severity, but were not essential. Pre-inoculation conditioning at two different relative humidity levels (R.H. = 100% or R.H. < 40%) did not affect disease severity. Young leaves were more severely affected following infection than older leaves. Disease severity was similar with several isolates ofX. campestris pv. vesicatoria.     

The escalating threat of Rhizoctonia cerealis, the causal agent of sharp eyespot in wheat

Rhizoctonia cerealis, the causal agent of sharp eyespot on wheat, was not considered to be an important pathogen for many years. Recently, the disease has become endemic in many countries except for South America. The disease has created a new threat to world wheat production because the damage of wheat sharp eyespot has become increasingly severe. In this paper, previous studies on this pathogen, including the disease geographical distribution, pathogen identification, life cycle, symptoms, favourable environmental conditions, effects on wheat yield and control strategy, are reviewed. Such information will be helpful in management of sharp eyespot.     

Trichoderma as a biostimulator and biocontrol agent against Fusarium in the production of cereal crops: Opportunities and possibilities

Despite technological advances in global agriculture in recent years, the problem of pathogenic fungi in the production of cereal crops continues to be an issue. Currently, the high variability of weather factors that are considered unusual for specific locations affect the growth and physiology of pathogens attacking cereal crops. One of the most common plant protection methods is the use of synthetic pesticides; however, there is growing controversy over this approach due to the build-up of pesticides in the environment and the presence of their residues in food. The purpose of this literature review is to explore the current state of knowledge regarding the potential of using Trichoderma species as a biostimulator and for the biological protection of cereal crops against pathogenic fungi. Trichoderma fungi—through mycoparasitism, antibiosis and competition for space and nutrients—help to inhibit the growth of pathogens and have a positive impact on the growth of plants, including their root system, which is considered a desirable effect during drought episodes. It has also been demonstrated that Trichoderma fungi can convert Fusarium toxins into new metabolites that can be of lower toxicity. However, the highly limited number of in vivo studies investigating the use of these fungi for biocontrol in cereal crops remains an obstacle to the commercialization of Trichoderma fungi. It appears that the determination of their effectiveness in the biocontrol of cereal crops under variable weather and climate conditions presents a considerable challenge.     

Phytophthora andina sp. nov., a newly identified heterothallic pathogen of solanaceous hosts in the Andean highlands

A blight disease on fruits and foliage of wild and cultivated Solanum spp. was found to be associated with a new species of Phytophthora. The proposed novel species is named Phytophthora andina Adler & Flier, sp. nov. based on morphological characteristics, pathogenicity assays, mitochondrial DNA haplotyping, AFLP fingerprinting and nuclear and mitochondrial DNA sequence analyses. Isolates of P. andina (n = 48) from the Andean highland tropics of Ecuador were collected from 1995 to 2006. Phytophothora andina is closely related to P. infestans and has semipapillate, ellipsoidal sporangia borne on sympodially branched sporangiophores. It is heterothallic and produces amphigynous antheridia. The species consists of several clonal lineages, including the EC‐2 and EC‐3 RFLP lineages, which were described previously as P. infestans. Approximately 75% of isolates react as compatibility type A2 when paired with an A1 compatibility type isolate of P. infestans. However, when A2 isolates from the Anarrhichomenum section of Solanum were paired in all combinations, viable oospores were obtained in several crosses, suggesting that there is a unique compatibility interaction in P. andina that is complementary to that described in P. infestans. Nuclear and mitochondrial sequence analysis supported the species designation of P. andina. This newly identified heterothallic pathogen shares a common ancestor with P. infestans and may have arisen from hybridization events with sister taxa in the Andes.     

Studies on amoebae and cysts associated with the isolation of Spongospora subterranea f.sp. subterranea in vitro

New evidence is presented to support the contention that the amoeba/cyst colonies isolated from surface-sterilized Spongospora subterranea f.sp. subterranea -infected potato tubers and spore balls have a saprophytic phase but are contaminants and not S. subterranea. Amoebae isolated from infected tissues and spore balls formed colonies associated with bacteria on 1% water agar at 18°C and encysted after 5–7 days. These cysts were morphologically distinct from the resting spores of S. subterranea and were formed singly or in a layer, unlike the spore ball (cystosorus) of S. subterranea . Amoebae, cysts and mixtures of amoebae and cysts in primary, secondary and tertiary subcultures failed to infect tomato roots. PCR amplification of DNA from amoebae, cysts and spore balls using the S. subterranea -specific primer pair SsF/R generated a 434-bp product from S. subterranea spore balls only and not from amoebae or cysts. When an amoeba/cyst-specific primer pair AmF/R was designed and used for PCR amplification, a single 411-bp product was generated from DNA of amoebae and cysts, but not from DNA of S. subterranea spore balls. These results are discussed in relation to earlier reports claiming the successful isolation of S. subterranea and other plasmodiophorids in vitro .     

黑龙江省部分地区马铃薯疮痂病菌种类及致病性鉴定

为明确黑龙江省马铃薯疮痂病病原菌的种类及其特征,2012-2013年从黑龙江省克山县、绥化市、哈尔滨市、杜尔伯特蒙古族自治县采集具有疮痂病病斑的马铃薯块茎,从中分离纯化病原菌,根据16SrRNA基因的差异采用分子手段对所分离的菌株进行种类和致病性鉴定,并对txtAB阳性菌株采用萝卜幼苗法或马铃薯致病性试验测定其致病性。共分离出74株菌株,鉴定出致病性菌株26株,其中Streptomyces scabies或S.europaeiscabiei 21株,S.turgidiscabies 3株和S.acidiscabies 2株。所有的致病性菌株中共有4种致病岛基因型,即nec1+/tomA+、nec1-/tomA+、nec1+/tomA-和nec1-/tomA。     

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。