The host range ofSpongospora subterranea f. sp.subterranea in the United States

This study was performed to determine the host range ofSpongospora subterranea f. sp.subterranea on common crops and weeds in the northeastern United States. Seedlings of the plants were grown in nutrient solutions and inoculated with spore balls ofS. subterranea. The roots were microscopically examined for the presence of plasmodia or zoosporangia 14 days after inoculation. The plants were then transferred to a greenhouse and grown in a soilless mix. The roots were examined for the presence of symptoms and spore balls after 4 months. Of 26 species within 10 families from monocotyledons and dicotyledons tested, 16 species were found to be susceptible toS. subterranea. Twelve species were newly recorded hosts forS. subterranea. Gall symptoms were observed on the roots of six species and spore balls were found on three species. Evidence is presented for the first time that galls and spore balls ofS. subterranea might form on non-Solanaceous species. This investigation is important for the cultural management of potato powdery scab disease because there currently are no effective controls.     

Detection and quantification ofSpongospora subterranea f. sp.subterranea by PCR in host tissue and naturally infested soils

A polymerase chain reaction (PCR) assay using primers SsF and SsR designed from the internal transcribed spacer (ITS) regions ofSpongospora subterranea f. sp.subterranea was developed for the specific identification and quantification ofS. subterranea. These primers amplified a 434 bp product from DNA ofS. subterranea spore balls, but not from DNA of healthy potato, common scab tuber, and taxonomically related plasmodiophorids. This PCR assay was successfully used for the detection ofS. subterranea in naturally infected symptomatic and asymptomatic potato tubers.Spongospora subterranea in other infected symptomless host plants was detected by PCR. The PCR assay was modified with improved soil DNA extraction methods to detectS. subterranea in soil. The assay was sensitive, and one spore ball per gram of soil could be detected. Following the design of a heterologous competitor DNA template from the sequence of λDNA, a competitive PCR assay for the quantification ofS. subterranea in soil was developed and provided accurate quantification in the range of 1 to 10⁴ spore balls per 0.25 g of soil. In a preliminary survey of naturally infested field soil samples, spore ball concentrations were estimated to vary from ca 0 to 3600 spore balls per 0.25-g soil sample by this competitive PCR assay. The spore ball levels were compared with the powdery scab disease incidence of potatoes in these fields, and a correlationship between spore ball levels and subsequent disease incidence was found. The PCR assays developed in this investigation can be routinely used to detect and quantifyS. subterranea in diseased plant tissue, asymptomatic plant tissue, and infested soil.     

Potato powdery scab, caused bySpongospora subterranea f. sp.subterranea, in Costa Rica

Summary Seven samples of potato powdery scab like-lesions were used to determine and confirm the occurrence ofSpongospora subterranea f.sp.subterranea in Costa Rica. Light microscope observations of spore balls and zoosporangia in tomato bait plants identified the organism asS. subterranea. Positive DAS-ELISA test results from all seven samples and PCR specific amplification products from five samples confirmed its identity. The geographical location of the samples source plantations suggests wide distribution of the plant pathogen across Costa Rica main potato growing areas. Different lesion types may indicate host-pathogen or environment-pathogen interactions. The symptoms differences and PCR amplification results among specific primer pairs suggest that some degree of genetic variation among theS. subterranea samples may exist; however, further testing is required.     

Alternative hosts for potato mop-top virus, genus Pomovirus and its vectorSpongospora subterranea f.sp. subterranea

Summary Seventeen weed species common in the potato fields in Denmark were grown in a hydroponic system infested with viruliferous zoospores ofSpongospora subterranea f.sp.subterranea carrying potato mop-top virus (PMTV). The plants were examined for infection with PMTV andS.s.s. using DAS-ELISA and based on visible symptoms. Only two weed species were found to be infected with PMTV,Chenopodium album andSolanum nigrum, whereas 13 became infected withS.s.s. C. album was infected with PMTV by mechanical inoculation were the infection became systemically and the leaves showed necrotic spots.S. nigrum became local infected with PMTV by mechanical inoculation. By inoculation with viruliferousS.s.s. the roots ofS. nigrum became infected but in three weeks PMTV had not spread into the top of the plants.Nicotiana benthamiana was used as a control susceptible toS.s.s. and PMTV.     

Reaction of potato varieties toPotato mop-top virus infection in the andes

The unexpected detection ofPotato mop-top virus (PMTV) in potatoes growing at several locations in the U.S.A. and Canada in 2002 has led to the realization that this soil-borne virus may be widespread throughout the potato-producing regions of both countries. A lack of information concerning the response of U.S. cultivars to PMTV infection caused us to examine the effects of PMTV on field-grown plants at three locations in the Peruvian Andes where the virus and its fungal vector,Spongospora subterranea, are endemic. All 21 U.S. cultivars tested proved susceptible to PMTV infection, especially at La Victoria where the overall incidence of both PMTV and powdery scab was high. Symptom expression in the foliage proved to be an unreliable indicator of virus infection, and thus, more reliable serological (NCM-ELISA) and nucleic acid-based techniques (NASH and RT-PCR) were used to monitor tuber tissue for the presence of PMTV. As much as 25% of field-grown tubers of cvs Kennebec, Monona, and Norland were infected with PMTV, but no correlation between virus infection and either the incidence or severity of powdery scab for any variety was apparent. Although PMTV infection did not result in the appearance of spraing or other internal necrotic reactions resembling those reported from many North American and European locations, a disproportionate proportion of infected tubers did exhibit a reticulate surface cracking. In hydroponic culture, the proportion of cracked tubers increased dramatically when the nutrient solution was seeded with virus-infestedS. subterranea. Comparatively few tubers of cvs Monona and Russet Burbank exhibited this surface cracking, suggesting that these two varieties may be somewhat tolerant to PMTV infection.     

Chloropicrin Soil Fumigation Reduces <Emphasis Type="Italic">Spongospora subterranea</Emphasis> Soil Inoculum Levels but Does Not Control Powdery Scab Disease on Roots and Tubers of Potato

The effect of chloropicrin fumigation on the soil populations of Spongospora subterranea and the development of powdery scab, formation of root galls and tuber yield was investigated in seven field trials conducted in Minnesota and North Dakota. Sixteen potato cultivars, with different levels of susceptibility to disease on roots and tubers, were planted in plots treated with chloropicrin at rates ranging from zero to 201.8 kg a.i. ha^?1. The amount of S. subterranea DNA in soil was determined using qPCR. Bioassays were conducted to further assess the effect of chloropicrin fumigation on root colonization by S. subterranea in two potato cultivars with contrasting disease susceptibility. In the field, chloropicrin applied at rates between 70.1 to 201.8 kg a.i. ha^?1 significantly decreased S. subterranea initial inoculum in soil but increased the amount of disease observed on roots and tubers of susceptible cultivars. The effect of increasing disease was confirmed in controlled conditions experiments. Although the amount of S. subterranea DNA in roots of bioassay plants increased with increasing chloropicrin rates, it remained similar among potato cultivars. Chloropicrin fumigation significantly increased tuber yield which in cultivars such as Shepody and Umatilla Russet were associated with the amount root galls (r = 0.30; P < 0.03). Results of these studies contradict earlier reports on the use of chloropicrin fumigation for the control of powdery scab. Factors other than inoculum level, such as environmental conditions that affect inoculum efficiency and host susceptibility, may be significant contributors to the development of powdery scab and root gall formation.     

Genetic variation and phylogeny ofSpongospora subterranea f.sp.subterranea based on ribosomal DNA sequence analysis

The nuclear rDNA regions of the two internal transcribed spacers (ITS1 and ITS2) and 5.8S rRNA gene from 52 field isolates ofSpongospora subterranea f.sp.subterranea obtained from the British Isles and North America were polymerase chain reaction-amplified, sequenced, and assessed for genetic variation. Two genetically distinct groups (I and II) were identified based on the ITS sequence diversity among the isolates, representing 34.6% and 65.4% of the isolates, respectively. British Isles isolates occurred in groups I and II, whereas North American isolates belonged only to group II. The British Isles groups ofS. subterranea were associated with particular potato cultivars. The full-length small-subunit rRNA gene ofS. subterranea was sequenced and analyzed by both neighbor-joining and parsimony methods to clarify the taxonomic position of this pathogen. The results of phylogenetic analysis showed thatS. subterranea grouped together with other species of plasmodiphorids, and this group clustered with the phylum Cercozoa, an assemblage of filose and reticulose amoebae and phylogenetically related zooflagellates. The recognition of the existence of different genetic groups withinS. subterranea will be important for the design of plant-breeding programs and in testing for plant resistance.     

Resistance toSpongospora subterranea in tuber-bearing and non tuber-bearingSolanum spp.

Summary The incidence of infection withSpongospora subterranea was studied in the non tuber-bearingSolanum brevidens andS. etuberosum and the tuber-bearingS. acaule, S. sucrense andS. tuberosum cvs Olympia and Pito. Shoot cuttings were grown in soil naturally infested withS. subterranea, or the roots were inoculated with a zoospore suspension. Logit models were used to analyse the data. The incidence ofS. subterranea was higher in plants inoculated with zoospore suspension than in those grown in infested soil (odds ratio (OR) 10.65). Ageing the inoculum reduced the incidence of infection in the plants (OR 0.30) without altering the interspecific differences. The ORs of infection (compared to cv. Olympia) were 0.07, 0.29, 0.60 and 2.88 forS. acaule, S. sucrense, S. brevidens andS. etuberosum. OnlyS. acaule was significantly more resistant to infection than cv. Olympia. No infection was detected in cv. Pito.     

A Greenhouse Assay for Powdery Scab (<Emphasis Type="Italic">Spongospora subterranea</Emphasis> f. sp. <Emphasis Type="Italic">subterranea</Emphasis>) Resistance in Potato

The plasmodiophorid pathogen Spongospora subterranea f. sp. subterranea causes powdery scab disease of potatoes (Solanum tuberosum L.), which severely affects tuber quality. Adequate chemical control of the disease is lacking, so breeding resistant potato cultivars is important. Little is known about the mechanisms or genetics of host resistance, as the epidemiology of the pathogen makes research on the disease difficult. An environmentally controlled disease screen that allows the accurate and repeatable assessment of diseased tubers (rather than a ranking system) is required to study host resistance in more detail. A high-throughput greenhouse disease assay is described that has been used to assess varying levels of tuber disease resistance of potato genotypes for three consecutive Southern Hemisphere growing seasons (2001–2002, 2002–2003, 2003–2004). This assay allows the evaluation of symptoms on tubers to verify resistance, rather than inferring the tuber infection on the basis of root galling or zoosporangial root infection. The assay was used to determine the level of resistance of genotypes of both known and unknown resistance status. The assay clearly distinguished the susceptible and resistant standards, and the results were highly correlated over the three seasons. The most efficient ways of assessing tuber disease susceptibility of genotypes are discussed. The consistency of the results and the ability to screen many genotypes confirms the value of the assay as a tool for early-generation selection in a plant breeding programme.     

Relationship Between Tuber Storage Proteins and Tuber Powdery Scab Resistance in Potato

The potato genotypes (Solanum tuberosum L.) with russet tuber skin are generally resistant to powdery scab (Spongospora subterranea f.sp. subterranea or Sss). Lipoxygenase (LOX; EC 1.13.11.12) and patatin are two key storage proteins that are known to offer resistance to several diseases and insects. The objective of this study was to find out the relationship of these proteins in stored tubers with potato tuber powdery scab resistance, especially in russet skinned potatoes. An evaluation of potato germplasm with different tuber characteristics in a greenhouse environment over several years (2006–’11) suggests that russet skinned tuber genotypes (Mesa Russet, Centennial Russet and Russet Nugget) with negligible tuber disease severity index (DSI) and 100 % marketability were resistant to powdery scab. Higher physiological levels of LOX protein (on a dry weight basis) were negatively correlated with tuber DSI and positively correlated with tuber russet skin. Tuber total protein and patatin-lipase levels did not have a significant relationship with tuber powdery scab resistance. The proposed role of LOX protein in suberin- and/or non-suberin-mediated mechanisms of powdery scab resistance in russet skinned tubers are discussed here. The physiological levels of LOX protein can be considered as a useful marker for powdery scab resistance in potato breeding programs.     

Evaluation of some North and South American potato breeding lines for resistance toPotato mop-top virus in Sweden

The necrotic symptoms (spraing) in tubers caused byPotato mop-top pomovirus (PMTV) have become a severe problem for potato production in Scandinavia. PMTV is transmitted by the powdery scab pathogen (Spongospora subterranea) and no methods for control are currently available. Two breeding lines (NY99 and NY103) from the Cornell University potato breeding program, twotuberosum ×acaule lines (TA3.8.3.3 and TA3.5.3.7) from the International Potato Center (CIP), and one line (IvP35) ofSolarium phureja were tested for resistance to PMTV in field trials in Sweden in 1998 and 1999. In both years, NY99 had a low incidence of PMTV infection in tubers, and the infected tubers showed a low accumulation of PMTV, indicating that NY99 is a promising new source of resistance to PMTV for breeding programs. Furthermore, it may be suitable for cultivation in Scandinavia, which, however, needs to be evaluated in more extensive field trials.     

Occurrence of Skin Blemish Diseases (Scab and Scurf) in Norwegian Potato Production

A survey of the prevalence of skin blemish diseases in potatoes after the growing seasons of 2008 and 2009 was carried out on 247 potato lots representing different cultivars and production regions in Norway. The results showed the presence of silver scurf (Helminthosporium solani) in all lots. Skin spot (Polyscytalum pustulans) and black scurf (Rhizoctonia solani) were found in 80% of the lots, and black dot (Colletotrichum coccodes) and common scab caused by Streptomyces spp. were present in 50–70%. Also, powdery scab (Spongospora subterranea) occurred in 65–80% of the lots, and root-lesion nematodes (Pratylenchus spp.) were detected in 60% of the sub-samples that exhibited symptoms of common scab.     

Assessing the resistance of potatoes to powdery scab (Spongospora subterranea (Wallr.) Lagerh.)

Summary The reaction of nine cultivars to infection bySpongospora subterranea (powdery scab) was studied in two successive years of trials in naturally infested field soil and in artificially-inoculated field soil contained in brick-sided beds. The incidence of infection was recorded on a 1–9 scale, with complete absence of the disease scoring 9. The correlation coefficient (r) between scores from bed and field was 0.84 (1985) and 0.81 (1986). The field test gave satisfactory discrimination between resistant and susceptible cultivars in only one year, but the bed test gave good results in both years and is suggested as a small-scale alternative to field trials for this disease.     

Soilless production of potato minitubers using a hydroponic technique

Summary Potato minituber production is a classic way to multiply or acclimatize in vitro material before its use in the open field. This production is generally performed in a substrate of vegetable origin (based on peat) but it involves the risk of an infection by different pathogens present in the substrate, such asStreptomyces scabies (common scab),Spongospora subterranea (powdery scab), orErwinia spp. (bacterial soft rot). Another drawback associated with this system of production is its relatively high cost due to the initial cost of the in vitro material and the low multiplication rates. To solve these two problems, the Research Unit of Libramont (CRAGx, Libramont, Belgium) is developing a soilless production technique using clay balls as substrate, a recycled and disinfected nutritive solution as source of nutrients, and microtubers as in vitro starting material. With this technique (depending on variety), we have shown that a multiplication rate of 8 to 13 can be obtained, giving 470 to 760 minitubers per m² instead of a multiplication rate of 4 to 8 as observed in the controls (230 to 470 minitubers per m²). Multiplication rate may be improved even more with an adjustment of the composition of the nutrient solution. The quality of microtubers produced is excellent: no infections were observed and the physiological behaviour of these tubers is not different from the ones produced in the open field.     

Review: Powdery Scab of Potato—Increased Knowledge of Pathogen Biology and Disease Epidemiology for Effective Disease Management

The importance of the potato tuber disease powdery scab, caused by the zoosporic pathogen Spongospora subterranea f. sp. subterranea, has increased worldwide, and the disease is one of the most important problems facing potato production in some regions. This soilborne pathogen produces many resting spores which can remain dormant for long periods, are highly resistant to environmental stresses and can spread the disease on seed potatoes and in contaminated soil. The enigmatic nature of this organism exacerbates the development of effective powdery scab control methods. Substantial knowledge has been gained in the last decade on the biology of the pathogen and the epidemiology of the disease, but no single effective control measure is, or is likely to be, available. An integrated approach to powdery scab management is the aim, with host resistance as a substantial and sustainable component. Further research on the epidemiology of powdery scab and population genetics of the pathogen is urgently required. All stakeholders involved in the potato industry must become aware that solution of the powdery scab problem is likely to be a long-term goal. When resistant cultivars with all the other characteristics demanded for marketing high-quality potatoes are available, and when disease risk from the pathogen on seed and/or in soil can be accurately determined, then the mission to develop effective powdery scab control will become achievable.     

基于环介导等温扩增技术检测江苏省中部地区小麦赤霉病病菌的种群组成

为明确江苏省中部地区小麦赤霉病病菌的种群组成,于2017年在江苏省镇江市、扬州市和南京市采集小麦赤霉病样本151份,采用可特异性识别亚细亚镰孢、禾谷镰孢、黄色镰孢、藤仓镰孢、燕麦镰孢、木贼镰孢和拟轮枝镰孢的环介导等温扩增（loop-mediated isothermal amplification,LAMP）检测技术检测了样本中该7种镰孢菌的分布情况。结果表明,从151份样本中共检测出7种镰孢菌中的6种。其中,亚细亚镰孢为优势种,检出率达100%;藤仓镰孢和禾谷镰孢为次优势种,检出率分别为10%和6%;黄色镰孢、燕麦镰孢和拟轮枝镰孢检出率依次为2.6%、2.0%和0.6%。小麦赤霉病存在复合侵染现象,从单份发病样本中最多检测到3种致病菌。LAMP法可用于小麦赤霉病病菌与种群组成的快速检测。     

Role of seed tubers in the spread of plant pathogenicStreptomyces and initiating potato common scab disease

The influence of tuber-borne pathogenicStreptomyces on potato common scab incidence in daughter tubers was examined under micro-plot conditions. Visually healthy tubers, surface-disinfested healthy tubers, and tubers with 25% scab coverage were planted in pasteurized soil. At 30, 57, and 93 days after planting, soil and belowground plant parts were sampled from various zones around the mother tuber. The total population of actinomyctes was determined by plating on a semiselective medium, and colonies of pathogenicStreptomyces were identified using polymerase chain reaction (PCR) targeting thetxtA gene. At day 30, pathogenicStreptomyces populations were below detectable levels, whereas by day 93, measurable levels were present on the belowground plant parts and in soil adjacent to scabby mother tubers at 10⁴ to 10⁵ CFU g^?1 soil. Incidence of scab in the progeny tubers was 89% at harvest. Progeny tubers produced by visually healthy seed tubers had an incidence of 60% scab and substantial populations of pathogenicStreptomyces were present in the zone near the mother tuber (10⁴ CFU g^?1 soil). Plots planted with surface disinfested seed tubers had a very lowStreptomyces population in the tuber zone and yielded 100% marketable tubers. A perfect correlation (r = 1.00) was found between the population densities of pathogenicStreptomyces in the root zone and daughter tuber disease incidence indicating that measurement of such populations in the field can serve as an excellent predictor of scab disease. After harvest, populations of pathogenicStreptomyces (10⁶ to 10⁷ CFU g^?1 lesion tissue) remained high on the mother tubers.     

Photoperiod regulation of development and growth in bambara groundnut (Vigna subterranea)

The influence of constant photoperiods of 10, 12, 14 and 16 h on development and growth in two bambara groundnut genotypes (Vigna subterranea (L.) Verdc., syn. Voandzeia subterranea (L.) Thouars) was studied in a greenhouse experiment in the Netherlands. Data on dry matter accumulation were collected by sequential harvesting. Photoperiod influenced the onset of flowering in one genotype (‘Ankpa 4’) and the onset of podding in both (‘Tiga Nicuru’ and ‘Ankpa 4’). Under 14- and 16-h photoperiods plants of ‘Ankpa 4’ produced no pods. Photoperiod did not influence total aboveground dry matter production per plant in ‘Ankpa 4’ and had only a slight effect on ‘Tiga Nicuru’. Photoperiod indirectly affected dry matter partitioning via its influence on development: in both genotypes assimilate distribution changed after the photoperiod-induced onset of podding. In addition, a direct influence of photoperiod on partitioning was observed. Firstly, just after the onset of flowering (40 DAS), ‘Tiga Nicuru’ plants under 10- and 12-h photoperiods had accumulated more dry matter as leaf blades and less as stem material than plants under 14- and 16-h photoperiods. Secondly, for ‘Ankpa 4’ the increase in pod dry weight per plant under the 10-h photoperiod was nearly double the increase under the 12-h photoperiod. This difference was associated with a smaller number of developing pods under the 12-h photoperiod. Photoperiod apparently strongly affects the number of developing sinks and, as a consequence, the total sink-strength of the plant, irrespective of the numerous ovaries present on plants of all treatments (including plants of ‘Ankpa 4’ under 14- and 16-h photoperiods).     

Effects of Different Soil Treatments on the Development of <Emphasis Type="Italic">Spongospora subterranea</Emphasis> f. sp. <Emphasis Type="Italic">subterranea</Emphasis> in Potato Roots and Tubers in the Greenhouse

Powdery scab caused by Spongospora subterranea f. sp. subterranea (Sss) causes extensive losses in potato production systems globally. Two pot experiments were established in the greenhouse in summer 2013 and winter 2014 to evaluate the effectiveness of different soil chemicals, fumigant, amendments and biological control agents (BCAs) against Sss in the rhizospheric soil, potato roots and tubers. The study used visual assessment methods to assess the effect of treatments on root galling and zoosporangia production, and qPCR to measure Sss concentration in the soil and in the potato roots and tubers. All six soil treatments, namely metam sodium, fluazinam, ZincMax, calcium cyanamide, Biocult and a combination of Bacillus subtilis and Trichoderma asperellum recorded significantly (P < 0.05) lower numbers of zoosporangia in the roots compared to the untreated control. The same effect was observed on the concentration of Sss DNA in the roots at tuber initiation. A more diverse picture was obtained when root gall scores at tuber initiation and Sss DNA in the rhizospheric soil at tuber initiation and harvesting were compared. Significant differences (P < 0.05) were also noted in disease severity, disease incidence, and tuber yield between metam sodium, fluazinam, ZincMax, calcium cyanamide and the untreated control. Calcium cyanamide gave the highest tuber yield. The study demonstrated the potential of soil treatments such as metam sodium, fluazinam, ZincMax and calcium cyanamide in managing Sss in potatoes by reducing the pathogen both in the rhizospheric soil and the roots of the potato plant.     

Disease resistance inSolanum phureja and diploid and tetraploidS. tuberosum × S. phureja hybrids

Summary DiploidSolanum phureja, diploid and tetraploidS. tuberosum × S. phureja hybrid, and tetraploidS. tuberosum backcross clones were assessed for resistance to commercially important diseases. There was a reduction in the expression ofS. phureja characteristics with increasing dosage ofS. tuberosum genetic material. The generally high resistance of theS. phureja group to common scab, potato leafroll virus and potato virus Y decreased, while resistance to gangrene, foliage and tuber blight tended to increase. Resistance genes were probably well dispersed throughout theS. phureja genotype so that many were lost on hybridisation, butS. phureja could be a useful source of scab and virus resistance in the production ofS. tuberosum cultivars.