The behaviour of potato mop-top virus in soil, and evidence for its transmission by Spongospora subterranea (Wallr.) Lagerh.

Potato mop-top virus (PMTV) was best detected in field soils by air-drying them for more than a week before remoistening and growing seedlings of Nicotiana tabacum or N. debneyi for a 6–10 week period. Infection of N. tabacum was assessed by inoculating sap from roots and shoots to Chenopodium amaranticolor. Similar inoculations from N. debneyi were far less convenient for detecting PMTV than recording leaf symptoms, but slightly more efficient. Air-dry soil retained PMTV infectivity for 9 months, when passed through a 50 μ sieve or when diluted with 10³ but not 10⁴ parts of steamed soil. Tobacco seedlings were not infected when their roots were steeped in PMTV-containing tobacco sap. Infective soils contained Spongospora subterranea, spore balls of which resisted air-drying for more than a year and passed a 50 μ sieve. Roots of susceptible seedlings were infected with PMTV when exposed to spore balls of S. subterranea taken from powdery scabs on PMTV-infected potato tubers, or to suspensions obtained by steeping, in nutrient solution, roots infected with virus-carrying cultures of S. subterranea. Plants in several families were hosts of S. subterranea, but probabilities of infection when exposed to spore balls differed greatly between families and only species of Solanaceae were good hosts. The ten species infected with PMTV when grown in infective soil or when exposed to spore balls of S. subterranea taken from PMTV-infected potato tubers are all members of this family. PMTV seems to be carried internally in S. subterranea spore balls and survived in them for at least a year. PMTV was transmitted by S. subterranea to Arran Pilot potato, causing yellow blotches in some leaves and spraing in many tubers. However, when newly infected with PMTV in the field, not all Arran Pilot tubers developed spraing. Also, although many spraing-affected or symptomless but PMTV-infected tubers carried PMTV-containing spore balls of S. subterranea, powdery scabs were rarely found near the centres of the rings of primary spraing. PMTV became established in virus-free soil when PMTV-infected tubers carrying S. subterranea were planted as ‘seed’ but not when virus-free tubers bearing powdery scabs were used. 5. subterranea seems the main, and possibly the only, vector of PMTV in the soils examined. S. subterranea did not transmit potato aucuba mosaic virus from potato to N. debneyi or Capsicum annuum.     

Detection,distribution and control of Potato mop-top virus,a soil-borne virus,in northern Europe

Potato mop-top virus (PMTV; genus Pomovirus; family Virgaviridae) is transmitted by the soil-borne Spongospora subterranea f.sp. subterranea, a protoctist that causes powdery scab on potato. PMTV is distributed widely in the potato growing areas in South and North America, Japan and northwestern Europe. This article reviews the current knowledge on detection, distribution and control of PMTV with focus on the Baltic Sea region. Since the 1980s, PMTV has caused great economic losses to potato production in the Nordic countries (Norway, Sweden, Denmark and Finland), but its occurrence in other countries of the Baltic Sea region remained unknown. To fill this knowledge gap, harmonised sampling and virus detection procedures including bioassays and serological and molecular methods were employed by 21 research institutions to detect PMTV in potato tubers and soil samples in 2005–2008. Potato growing areas were widely contaminated with PMTV in the Nordic countries. Only the main seed potato production area in northern Sweden and the High Grade seed potato production zone in Finland were negative for PMTV. Intensive and systematic surveys in Poland in 2004–2008 found no evidence of PMTV, except a single PMTV-infected tuber detected in 2008. Surveys in the Baltic countries (Lithuania, Latvia and Estonia) and northwestern Russia (Leningrad province) were negative for PMTV, except infection of minitubers in a screenhouse in Latvia in 2005. Varying percentages of tubers expressing spraing symptoms in Sweden, Norway, Denmark and Poland were infected with Tobacco rattle virus, and bioassays indicated similar results for Russia. Incidence of symptomless infections with PMTV was high in tubers of many potato cultivars. Here, we discuss the contrasting patterns of distribution of PMTV in the Baltic Sea region, factors playing a role in dispersal and establishment of PMTV in new fields and means for controlling PMTV and its spread to new areas. We emphasise the use of the current virus-specific methods for the detection of PMTV in symptomless potato tubers and the high risks of disseminating PMTV to new fields and areas in viruliferous resting spores of S. subterranea in the soil adhering to seed tubers. PMTV-resistant potato cultivars will provide the only sustainable means for preventing yield losses in the infested fields and the prospects of resistance breeding are summarised.     

Acquisition and transmission of potato mop-to furovirus by a culture of Spongospora subterranea f.sp. subterranea derived from a single cystosorus

Potato mop-top virus (PMTV) was detected by ELISA in primary zoospores from four out of six isolates of Spongospora subterranea f.sp. subterranea. One virus-free isolate (N) of S. subterranea was used to acquire PMTV from potato roots and to transmit the virus to healthy plants. A mono-fungal culture of S. subterranea (isolate N) was derived by infecting tomato plant roots with a single cystosorus. The culture was used successfully to acquire PMTV from the roots of infected Nicotiana debneyi plants that had been manually inoculated with virus isolates, and subsequently to transmit the virus to healthy bait plants. These experiments confirm that S. subterranea is a vector of PMTV. Two PMTV isolates that had been maintained by manual inoculation for 19 and 21 passages were also acquired and transmitted by the fungus culture.     

Ecological studies on potato mop-top virus in Scotland

Plants with symptoms of potato mop-top virus (PMTV) occurred in many commercial seed stocks of Arran Pilot and Red Craig's Royal potato in Scotland, but their incidence rarely exceeded 5%. In nuclear stocks of seed potatoes, most varieties examined in 1967 and 1968 were infected at one or more locality, but infected plants did not occur in all clones or at all stages of propagation of any one variety. infection of nuclear stocks resulted both from propagation on virus-infested land and from unwitting selection of infected plants to start new clones. PMTV was detected in farm soils ranging from light sands to heavy loams, in five Scottish counties. Soil was infested throughout the ploughed layer but the severity of infestation varied greatly within any one field; some sites of former potato clamps were heavily infested. PMTV was detected in field soil 12 years after potatoes were grown. In glasshouse tests many British crop and wild plants were colonized by Spongospora subterranea. Within some families all species tested were moderate to good hosts. (Solanaceae, Chenopodiaceae and Cruciferae), in others, species differed greatly in susceptibility (Compositae and Umbelliferae), and in a few, species were poor hosts or were not infected (Caryophyllaceae and Gramineae). Of the British crop and weed species that were moderate to good zoosporangial hosts of S. subterranea, only Solanum nigrum, potato, spinach and sugar beet were hosts of vector-borne PMTV. Potato probably survives between potato crops mainly in the resting spores of S. subterranea. PMTV was probably first brought to Europe with potatoes from South or Central America.     

Resting Spore Dormancy and Infectivity Characteristics of the Potato Powdery Scab Pathogen Spongospora subterranea

The soil‐borne potato pathogen Spongospora subterranea persists in soil as sporosori, which are aggregates of resting spores. Resting spores may germinate in the presence of plant or environmental stimuli, but direct evidence for resting spore dormancy is limited. A soilless tomato bait plant bioassay and microscopic examination were used to examine features of S. subterranea resting spore dormancy and infectivity. Dried sporosori inocula prepared from tuber lesions and root galls were infective after both short‐ and long‐term storage (1 week to 5 years for tuber lesions and 1 week to 1 year for root galls) with both young and mature root galls inocula showing infectivity. This demonstrated that a proportion of all S. subterranea resting spores regardless of maturity exhibit characteristics of stimuli‐responsive dormancy, germinating under the stimulatory conditions of the bait host plant bioassay. However, evidence for constitutive dormancy within the resting spore population was also provided as incubation of sporosorus inoculum in a germination‐stimulating environment did not fully exhaust germination potential even after 2.4 years. We conclude that S. subterranea sporosori contain both exogenous (stimuli‐responsive) and constitutively dormant resting spores, which enables successful host infection by germination in response to plant stimuli and long‐term persistence in the soil.     

THE PATTERN OF FIELD INFECTION OF POTATO BY THE BEET RINGSPOT STRAIN OF TOMATO BLACK-RING, A SOIL-BORNE VIRUS

The potential importance of the beet ringspot strain of tomato black-ring, a soil-borne virus, was assessed by growing stocks of Kerr's Pink potato for 1 year on infested land and subsequently on uninfested land. The incidence of infection in two stocks was 39 and 8% in the first year on uninfested land, and 29 and 5% after 2 years.
The virus was usually restricted to the roots of plants in the first year of infection, but a few plants showed black rings and spots in their leaves. In the second year, 20–55% of the plants grown from tubers set by symptomless, but infected, mother plants were infected: many of these showed leaf necrosis, others had stunted shoots, and cupped and distorted leaves; some were symptomless although systemically infected. In the third and fourth years, most of the progeny from plants which had symptoms or which were symptomless but systemically infected, contained the virus: nearly all such infected plants were stunted and distorted or were symptomless. Infection decreased the weight of tubers produced by plants with severe necrotic spotting but not the yield of plants with less necrosis. The number and weight of tubers per plant were decreased by 15 and 20% respectively, in symptomless systemically infected plants, and by 20 and 30% in stunted plants.     

Factors affecting the development of spraing in potato tubers infected with potato mop-top virus

A larger proportion of tubers of Arran Pilot potato growing at the surface of soil infested with potato mop-top virus (PMTV) showed spraing symptoms (brown rings) at harvest than of tubers from below the surface. Infected tubers with or without spraing developed a spraing ring when stored in darkness, first for 1–2 wk at 18 d?C and then for 1–2 wk at any constant temperature between 5 and 13 d?C. Only a faint surface ring developed when either of these periods was decreased to 1 day; 4-day periods were needed to induce distinct symptoms. Internal tuber symptoms developed more slowly than surface symptoms, and their formation was favoured by cutting the tubers in half. Additional pigmented surface rings were produced outside the first ring by successive cycles of treatment at 18 and 9d?. Spraing did not develop when the first stage of treatment was at 22–25d?, when the tubers were kept first at 10d? and then at 5d?, when the treatment at 5–13d? preceded that at 18d?, or when the tubers were kept at constant temperatures ranging from 5 to 25d?. When tubers of six potato varieties were grown in PMTV-infested soil and then stored at temperatures designed to induce symptoms, the varieties known to be the most susceptible in the field were those which had the greatest tendency to develop spraing during storage. When infected tubers were exposed to light, typical spraing symptoms were not induced, but greening of the tuber surface was much delayed in localized ring-shaped areas, so that pale weals appeared. Spraing symptoms were produced, in favourable conditions, by the reaction of cells at the periphery of the PMTV-invaded zone. Internal spraing did not prevent PMTV invading tissue outside the brown arcs; its rate of spread was about 10 μm/h at 14–18d?.     

Host range and some properties of potato mop-top virus

Potato mop-top virus (PMTV) was transmitted by inoculation of sap to twenty-six species in the Solanaceae or Chenopodiaceae and to Tetragonia expansa; species in eleven other plant families were not infected. The virus was cultured in inoculated leaves of Nicotiana tabacum cv. Xanthi-nc or in N. debneyi. Diagnostic local lesions were produced in Chenopodium amaranticolor. In winter, ten solanaceous species were slowly invaded systemically but the first leaves infected were those immediately above inoculated leaves. When transmitted to Arran Pilot potato by the vector Spongospora subterranea, PMTV induced all the main types of shoot and tuber symptoms found in naturally infected plants. Isolates of PMTV from different sources differed considerably in virulence. PMTV-containing tobacco sap lost infectivity when heated for 10 min at 80 °C, diluted to 10^-4, or stored at 20 °C for 14 weeks. Infectivity was partially stabilized by 0·02% sodium azide. When sap was centrifuged for 10 min at 8000 g, infectivity was mainly in the sediment. Infective sap contained straight rod-shaped particles about 20 nm wide, with lengths up to 900 nm and crossbands at intervals of 2·5 nm. Many of the particles were aggregated side-to-side, and the ends of most seemed damaged. The slight infectivity of phenol-treated leaf extracts was abolished by pancreatic ribonuclease. The present cryptogram of PMTV is R/*:*/*:E/E:S/Fu.     

The Role of Infective Plant Debris, and its Concentration in Soil, in the Ecology of Tomato Mosaic Tobamovirus—a Non-vectored Plant Virus

Only a few plants in a crop are generally thought to become infected by abiotic soil transmission. In glasshouse experiments we have induced almost total infection in tomatoes growing in soil with infective debris, but levels of infection are dependent upon certain conditions. We found that as the inoculum concentration decreased (i) a greater percentage of the infections were either latent (symptomless) or restricted to roots and (ii) infection levels decreased. Thus, apparent infection (based on symptoms) of 0. 10% was in reality 0 60–70% based on enzyme-linked immunosorbent assay and immunoelectron microscopy testing of roots and leaves. This occurred whether seedlings or seed were planted into infected mix. Under conditions in which minimal root damage was caused (planting seed) or roots were mechanically inoculated only once, almost all systemically infected plants were symptomless.     

Multiplex real-time PCR (TaqMan) assay for the simultaneous detection and discrimination of potato powdery and common scab diseases and pathogens

Aims: To develop a multiplex real‐time PCR assay using TaqMan probes for the simultaneous detection and discrimination of potato powdery scab and common scab, two potato tuber diseases with similar symptoms, and the causal pathogens Spongospora subterranea and plant pathogenic Streptomyces spp. Methods and Results: Real‐time PCR primers and a probe for S. subterranea were designed based on the DNA sequence of the ribosomal RNA ITS2 region. Primers and a probe for pathogenic Streptomyces were designed based on the DNA sequence of the txtAB genes. The two sets of primer pairs and probes were used in a single real‐time PCR assay. The multiplex real‐time PCR assay was confirmed to be specific for S. subterranea and pathogenic Streptomyces. The assay detected DNA quantities of 100 fg for each of the two pathogens and linear responses and high correlation coefficients between the amount of DNA and C_t values for each pathogen were achieved. The presence of two sets of primer pairs and probes and of plant extracts did not alter the sensitivity and efficiency of multiplex PCR amplification. Using the PCR assay, we could discriminate between powdery scab and common scab tubers with similar symptoms. Common scab and powdery scab were detected in some tubers with no visible symptoms. Mixed infections of common scab and powdery scab on single tubers were also revealed. Conclusions: This multiplex real‐time PCR assay is a rapid, cost efficient, specific and sensitive tool for the simultaneous detection and discrimination of the two pathogens on infected potato tubers when visual symptoms are inconclusive or not present. Significance and Impact of the Study: Accurate and quick identification and discrimination of the cause of scab diseases on potatoes will provide critical information to potato growers and researchers for disease management. This is important because management strategies for common and powdery scab diseases are very different.     

Susceptibility of potato tubers to infection by Phytophthora infestans

Phytophthora infestans infects King Edward potato tubers more readily through inoculated eyes than through lenticels, but more lenticels than eyes became infected when whole tubers were sprayed with inoculum. The resistance of lenticels but not of eyes increased as tubers aged. The spores did not infect through intact periderm. The likelihood of tubers on plants grown in pots becoming infected by sporangial suspension poured on to the soil increased the nearer the tubers were to the soil surface, the stem, or the side of the pot. Naturally infected tubers, and those sprayed with sporangial suspension, had most eyes infected at the rose end, and most lenticels infected on the middle region of the tuber. Of naturally infected tubers, on which the site of infection could be identified, most were infected through eyes at the rose end.     

Efficacy of potent antagonistic yeast Wickerhamiella versatilis against soft rot disease of potato caused by Pectobacterium carotovorum subsp. carotovorum

Abstract

In this study, an antagonistic yeast isolate, Wickerhamiella versatilis was considered as a promising biocontrol agent against Pectobacterium carotovorum subsp. Carotovorum (Pcc) the causal agent of soft rot disease of potato. Antagonistic yeast inhibited the growth of Pcc in vitro, and reducing the soft rot severity of infected potato tubers (cv. Diamant) under greenhouse conditions. Consequently, cellulase and pectinase hydrolytic activities in infected potato tubers with yeast?+?Pcc were decreased compared with infected tubers with Pcc. The histological characterization of treated potato tubers with antagonistic yeast W. versatilis using scanning electron microscope showed the accumulation of extracellular substances that may induce plant resistant and protects potato tubers from hydrolysis and damages. This study may introduce the possibility of using the antagonistic yeast isolate, as a biocontrol agent against soft rot of potato tubers.     

二级水灌溉对土壤及马铃薯块茎中大肠菌群数量的影响

采用多管发酵法检测二级水灌溉对马铃薯种植土壤中大肠菌群数量,结果表明:充分灌溉后的土壤中大肠菌群的数量均高于分根交替灌的土壤;地下滴灌的土壤中大肠菌群数量高于沟灌土壤的大肠菌群数量;二级水经加氯消毒后,土壤中大肠菌群数量明显下降。同样采用多管发酵法检测马铃薯块茎,结果发现:采用加氯消毒后的二级水灌溉的小区E和F中生长的马铃薯,其块茎组织中大肠菌群数量低于其他小区,采用地下滴灌灌溉的小区I和J所种植出的马铃薯块茎中大肠菌群的数量高于采用沟灌灌溉的小区K和L。结果表明:马铃薯块茎中大肠菌群的数量与其生境中大肠菌群的数量有密切关系。     

Increased sensitivity of detection of plant viruses obtained by using a fluorogenic substrate in enzyme-linked immunosorbent assay

The fluorogenic substrate 4-methylumbelliferyl phosphate (MUP) of alkaline phosphatase was compared with the chromogenic substrate p-nitrophenyl phosphate (NPP) in tests for plant viruses by enzyme-linked immunosorbent assay (ELISA). In tests on leaf extracts of squash infected with prune dwarf virus, Chenopodium quinoa and apple infected with apple mosaic virus (ApMV), and potato infected with potato leafroll virus (PLRV), MUP increased sensitivity 2–16 times, the smallest and greatest increases being obtained with ApMV (in apple) and PLRV respectively. In similar tests on 21 dormant PLRV-infected potato tubers, sensitivity was increased 2–4 times with 13 tubers, but the two substrates gave the same detection end-points with eight tubers. When individual seeds of potato plants infected with the Andean potato calico strain of tobacco ringspot virus were tested, the virus was detected in virtually all seeds by MUP-ELISA, but detection by NPP-ELISA was inefficient unless absorbance values were measured after overnight incubation at 4 °C, instead of after 2 h at room temperature. In tests on Myzus persicae carrying PLRV and Sitobion avenae carrying barley yellow dwarf virus (BYDV), both viruses were consistently detected in a greater proportion of individual aphids by MUP-ELISA than NPP-ELISA irrespective of whether incubation was for 2 h at room temperature or overnight at 4 °C. The effeciency of detection of virus in single viruliferous aphids by MUP-ELISA was not decreased by grouping with one or four non-viruliferous aphids but was decreased (PLRV) or greatly decreased (BYDV) by grouping with nine. MUP-ELISA and transmission tests to Physalis floridana seedlings (2–3 day inoculation access periods) both detected PLRV in most individual M. persicae, but the results obtained with the two methods did not correlate completely. In similar tests for BYDV in individual S. avenae, virtually all aphids transmitted BYDV to oat seedlings during a 3-day inoculation access period but it was subsequently detected by MUP-ELISA in less than half of them. By contrast, MUP-ELISA detected PLRV in most viruliferous M. persicae even after they had fed for 3 days on Chinese cabbage, a non-host for this virus.     

The influence of potato harvesting and grading machinery on contact spread of Phoma exigua var. foveata on tubers

The incidence of wounds infected by Phoma exigua var. foveata was increased if freshly damaged tubers (recipients) were shaken in a bag with diseased tubers (donors) to simulate the tuber-to-tuber contact that occurs during potato handling. An increase in the number of gangrene rots on damage points also occurred if the recipient tubers were wounded after contact with the diseased tubers, rather than before, and when the donor tubers were heavily infested with P. exigua var. foveata but were free of gangrene lesions. Increasing the proportion of donor to recipient tubers increased the percentage of infected wounds on recipients. Increased incidences of infection in recipient tubers also occurred after they had been passed over an elevator digger when it was lifting stocks of tubers heavily infested with P. exigua var. foveata. When spores of an E +ve isolate of P. exigua var. foveata were sprayed onto the webs of manned potato harvesters, tubers harvested immediately after developed gangrene rots from many of which the E +ve isolate was cultured. An E +ve isolate was also used to demonstrate the transfer of P. exigua var. foveata inoculum from tubers onto soil on riddles of a potato grader and from these soil-coated surfaces onto other tubers during grading.     

Effects of skin spot (Oospora pustulans) on potatoes

King Edward and Majestic seed potatoes selected as ‘clean’ (macroscopically symptomless), moderate and severe according to the extent of skin spot were planted in field experiments at Rothamsted between 1964 and 1968. Usually crops from ‘clean’ and moderately infected seed did not differ detectably in growth or yield. Plants from severely infected seed tubers emerged more slowly, had fewer stems and yielded less (King Edward 20 %, Majestic 13 %). Seed infection also affected tuber size distribution; severely infected seed of King Edward yielded almost 4 tons/acre less of 1 1/4-2 1/4 in tubers and Majestic, 1 ton/acre less of these and 2 tons/acre less 2 1/4-3 1/4 in tubers. However, the total yield from diseased seed stocks was only slightly less (King Edward, o-6 ton/acre and Majestic o-8 ton/acre) than the yields from the ‘clean’ tubers selected from them. Seed severely infected by Oospora pustulans often increased infection of the progeny tubers, and usually decreased their infection by Rhizoctonia solani and sometimes by Helmintho-sporium solani. Another series of experiments compared King Edward seed tubers classified according to the number of live eyes showing in March. Seed with one, two, three and more live eyes yielded equally. About half the tubers without live eyes in March eventually produced plants, but late, with few stems and giving only half the yield of seed with three or more live eyes. Surprisingly, the progeny tubers from seed without live eyes were least infected by O. pustulans, R. solani and H. solani. Progenies of King Edward and Majestic seed from a common source grown on seven widely separated farms were infected more in 1963 than in 1964, but in each year infection differed widely between farms. Often where O. pustulans was common, R. solani was scarce and vice versa. By contrast, when King Edward stocks very differently infected by O. pustulans were grown at Rothamsted their progenies were almost uniformly infected by O. pustulans and R. solani.     

Survival of the causal agents of 'early-dying disease' (Verticillium wilt) of potatoes

Although the isolation of Verticillium albo-atrum and V. dahliae from soil and dried moribund stems following infection of a potato crop proved extremely difficult, both fungi were equally capable of overwintering in these substrates and of inducing disease in a subsequently planted susceptible crop. In the absence of a susceptible crop some weed species became colonized. The two species, however, appeared to differ in their capacity for survival both beneath a monocotyledonous crop and within the potato tubers. Colonization of the roots of wheat, barley, oats, rye and maize was observed with V. dahliae but not with V. albo-atrum. The latter appeared to be capable of prolonged survival in the tubers, whereas V. dahliae did not remain viable in storage over winter. Consequently only tubers infected with V. albo-atrum produced infected plants. The presence of the fungi within the tubers affected neither dormancy nor the initial development of the sprouts. Some correlation was noted between tuber size, the percentage of tubers infected, the distribution of V. albo-atrum within the tubers and the development of disease in plants subsequently grown from these tubers.     

The effect of soil- and tuber-borne inoculum on the incidence of potato gangrene

Under optimum growing conditions neither tuber- nor soil-borne Phoma exigua var. foveata inoculum appreciably affected stand or yield of the subsequent potato crop. Seed tubers with gangrene rots caused high levels of stem and tuber symptoms when planted in var. foveata contaminated or uncontaminated land; contaminated seed tubers with no rots also produced progeny with a high gangrene potential. Sufficient soil-borne inoculum was carried over in land that produced a gangrene affected crop in the previous year to override the effect of tuber disinfection. Effective gangrene control was achieved by a combination of tuber disinfection shortly after harvest over successive years with a 1 in 5 yr potato crop rotation. Gangrene rots usually developed through injuries to the tuber periderm, rots in other tubers being associated with pustules of powdery scab (Spon-gospora subterranea).