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.     

Occurrence of Potato virus Y strain PVYNTN in foundation seed potatoes in Japan,and screening for symptoms in Japanese potato cultivars

In 2008 and 2009 seasons, a sudden increase in Potato virus Y (PVY) incidence was recorded in foundation seed potatoes in Hokkaido, northern Japan. This increase was obvious during the field inspection and the postharvest indexing. Molecular typing revealed that besides the previously reported strains of PVY^O and PVY^NA‐N, the most common strain identified was the recombinant PVY^NTN, with three characteristic recombinant junctions at the HC‐Pro, VPg and CP regions. No potato tuber necrotic ringspot disease (PTNRD) was observed in foundation seed potatoes in correlation with the presence of PVY^NTN. Moreover, an isolate with a typical PVY^NTN recombinant genome, namely Eu‐12Jp, did not induce PTNRD in 62 Japanese potato cultivars tested in both primarily and secondarily infected plants. Two cultivars carrying the extreme resistance gene Ry_chc were resistant to the infection with Eu‐12Jp, which presents potential sources of resistance to PVY^NTN. Eu‐12Jp induced systemic mottle in potato cultivars Desiree and King Edward carrying resistance genes Ny and Nc, respectively, but induced a hypersensitive reaction in potato cultivar Maris Bard, with the Nz hypothetical resistance gene typical of the PVY^Z strain group. Therefore, based on the genome structure and the reaction of the potato N resistance genes, Eu‐12Jp should be classified as PVY^Z‐NTN, as described for isolates from Idaho, USA recently. This is the first report of PVY^Z‐NTN in Japan and the sudden and increased occurrence of PVY^NTN/PVY^Z‐NTN represents a potential risk of PTNRD developing and increases the significance of PVY in Japan.     

In vitro evaluation of fungal isolates for their ability to influence leaf rheology,production of pseudothecia,and ascospores of Venturia inaequalis

As part of an ongoing research project on biological control of apple scab, this study presents a novel approach for the in vitro selection of potential antagonists of the saprophytic phase of Venturia inaequalis. A collection of forty-two fungal isolates were tested for their in vitro ability to degrade apple leaf tissue, inhibit pseudothecia, and ascospore production. The inhibition of ascospore production cannot always be linked reliably with leaf degradation or the evaluation of pseudothecia production. Consequently, ascospore production was retained as the most useful screening parameter. Six isolates proved to significantly reduce the ascospore production of Venturia inaequalis. Two were as effective as Athelia bombacina, a previously reported antagonist of pseudothecia formation and inhibited over 98% of the ascospore production. These new organisms are now available for future field tests. Future selections from a large collection of fungal and bacterial saprophytes can now be based on a reliable and simple in vitro screening method.     

Occurrence and survival of potato scab pathogens (Streptomyces species) on tuber lesions: quick diagnosis based on a PCR-based assay

A time-saving and cost-effective polymerase chain reaction (PCR)-based method was developed for species-specific detection of the scab pathogens ( Streptomyces scabies and S. turgidiscabies ) prevalent in potato ( Solanum tuberosum ) in northern Scandinavia. Species specificity of primers was verified using a collection of previously characterized Streptomyces strains isolated from potato scab lesions in Finland and Sweden. A total of 1245 scab lesions was tested from potato cvs Matilda and Sabina grown in the field in two geographic regions of Finland in 2000 and 2001. Freshly harvested or stored potato tubers were incubated at room temperature (18–21°C) under humid conditions for a few days. Bacterial growth was collected from scab lesions for DNA isolation and PCR. The two scab pathogens were detected in the same potato fields, tubers and scab lesions. The relative incidence of S. scabies was high in freshly harvested tubers but was much lower than that of S. turgidiscabies following storage. Both pathogens were seed-transmitted in Matilda and Sabina after 24 weeks of storage at 4°C.     

A 9-year monitoring study of diseases on potato seed tubers imported to Israel

Potato seed tubers are imported annually to Israel from northern Europe. Although the seed is registered as certified, a survey carried out over a 9-year period indicated that most lots were affected by latent or active bacterial and fungal infections. Latent infection byErwinia carotovora subsp.atroseptica, the causal agent of blackleg, at a level of 10³ cells/g peel, was present in 30% of the lots in most years. Black scurf caused byRhizoctonia solani was present in 20–70% of the imported lots, with a moderate to high level of infection in all years except 1985. In contrast, although many lots were affected by powdery scab, common scab, and Fusarium dry rot in most years, disease incidence within lots was generally low. The gangrene pathogen (Phoma exigua) was rarely detected. The survey findings are of marked importance, due to the extensive use of soil fumigation in Israeli agriculture.     

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

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

Dynamics of soilborne Rhizoctonia solani in the presence of Trichoderma harzianum: effects on stem canker, black scurf and progeny tubers of potato

Stem canker and black scurf are diseases of potato caused by the fungus Rhizoctonia solani . Spatiotemporal experimentation and empirical modelling were applied for the first time to investigate the effect of antagonistic Trichoderma harzianum on the dynamics of soilborne R. solani on individual potato plants. Trichoderma harzianum reduced the severity of symptoms, expressed as 'rhizoctonia stem lesion index' (RSI), during the first 7 days post-inoculation when the inoculum of R. solani was placed at certain distances (30–60 mm) from the host. For example, with inoculum at 40 mm from the host, RSI was 6 and 40 with and without T. harzianum , respectively. At later observation times, the antagonistic effect was overcome. Trichoderma harzianum reduced the severity of black scurf on progeny tubers. Furthermore, the mean number of progeny tubers per potato plant was reduced by the biocontrol treatment (means of 6·5 ± 1·1 and 9·9 ± 2·7 tubers per plant with and without T. harzianum , respectively), as was the proportion of small (0·1–20·0 g) tubers (48% and 66% with and without T. harzianum , respectively). Additionally, there were fewer malformed and green-coloured tubers in pots treated with T. harzianum than in those without T. harzianum .     

Screening for fungal antagonists of seed-borne Fusarium culmorum on wheat using in vivo tests

A screening system is described for identifying fungal antagonists from soil which are effective in controlling seed-borne Fusarium culmorum on wheat (T. aestivum L.). The procedure comprised three consecutive in vivo tests done under controlled conditions using three different plant growing media. Altogether 1701 fungal isolates from Finnish field soils were screened for biocontrol activity. 210 fungal isolates that had advanced to various stages in the screening system were also included in a field experiment. The isolates that were the most effective in controlling seedling blight under natural conditions belonged to the genus Gliocladium. The screening tests separated Gliocladium spp. rather clearly from the rest of test fungi and thus the overall agreement among the results was good. The assays run under controlled conditions did not efficiently predict which of the Gliocladium isolates would be best in protecting wheat seedlings under field conditions.     

Effect of haulm destruction supplemented by cutting off roots on the incidence of black scurf and skin damage,flexibility of harvest period and yield of seed potatoes in field experiments

In field experiments, supplementing chemical haulm destruction (CHD) with cutting off roots resulted in a lower incidence of black scurf and skin damage (ripping off the skin) at harvest date than CHD alone. The lower susceptibility to skin damage at harvest allowed harvesting to beging on an earlier date, when only a few sclerotia ofRhizoctonia solani had developed. Furthermore, black scurf often developed more slowly after haulm destruction if roots had been severed and this enabled harvesting to be postponed.At harvest, gross yield was highest if roots had not been cut through prior to CHD: the extra weight existed merely of water. Weight loss during storage, however, as well as grading losses resulting from black scurf were greater after CHD alone. This resulted in an equal or even lower net yield after CHD alone than after CHD supplemented by root severing. The favourable effects of supplementing CHD with cutting off roots almost equaled those of the mechanical removal, often called haulm pulling or plant pulling. Factors that may affect the development of black scurf are discussed.Samenvatting Pootaardappelen raakten minder bezet met lakschurft en werden minder ontveld bij de oogst na looftrekken of na wortelsnijden plus doodspuiten dan na loofklappen plus spuiten. Al vanaf de derde dag na trekken of wortelsnijden plus doodspuiten was de mate van ontvelling zo gering, dat met het oogsten kon worden begonnen. Op zo'n vroeg tijdstip na loofdoding konden knollen worden geoogst met nog weinig lakschurft, want de stimulering van lakschurft werd pas zichtbaar vanaf 7 à 10 dagen na loofdoding. Lakschurft ontwikkelde zich het traagst na looftrekken. Ook wortelsnijden bij het doodspuiten gaf meestal een tragere ontwikkeling van lakschurft dan klappen plus spuiten. Daardoor konden na wortelsnijden plus doodspuiten en vooral na looftrekken op nog latere tijdstippen knollen met weinig lakschurft worden geoogst.Na klappen plus spuiten was het bruto gewicht bij de oogst het hoogst. Dat meergewicht bleek louter uit water te bestaan en het verloop in knolvochtgehalte leek erop te wijzen dat de wortels nog gedurende een week na doodspuiten blijven functioneren. Dat gaf wel knollen met een hoger vochtgehalte die meer ontvelden en meer vocht en dus gewicht verloren. Dit groter gewichtsverlies bij bewaren en de hogere leesverliezen door lakschurft deden de meeropbrengst teniet: zo werd het netto knolgewicht na klappen plus spuiten, al naar gelang het moment van oogsten, meestal gelijk aan, of lager dan het netto knolgewicht na het trekken of na wortelsnijden plus doospuiten. In dit artikel worden de factoren besproken, die mogelijk van invloed zijn op de ontwikkeling van lakschurft.     

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

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

Effect of growth medium, Striga seed burial distance and depth on efficacy of Fusarium isolates to control Striga hermonthica in Burkina Faso

Striga hermonthica is a destructive parasite of cereal crops in the semi‐arid tropical zone. Two greenhouse experiments were conducted at Kamboinsé, Burkina Faso, to investigate the effect of inoculum substrate and location of Striga seeds on the ability of 14 indigenous Fusarium isolates to control the parasite. In Expt 1, Fusarium isolates reduced emerged Striga number, Striga vigour and dry biomass. As a result, sorghum dry biomass and grain yield were enhanced. Inoculum substrate did not influence the ability of Fusarium isolates to control Striga. In Expt 2, Fusarium isolates, substrate and their interaction significantly influenced germination of Striga seeds at both 35 and 50 days after sowing. Isolates grown on compost were more effective at reducing germination of Striga seeds than those grown on chopped sorghum straw. The per cent germination of seeds 50 days after sowing, buried at 5 cm depth, was significantly lower than that of seeds buried at 10 cm. At 10 cm depth, Fusarium isolates still reduced Striga seed germination with respect to the control; horizontal planting distance, 5 or 10 cm from sorghum hills, had no effect.     

Further observations on the use of fungicides to control Peronosclerospora sacchari (T. Miyake) Shirai and K. Hara in sugarcane in Papua New Guinea

Abstract

Many biologists perceive organisms as constantly evolving and therefore consider the host plant ranges of biological control agents as labile. Host plant ranges are thus likely to undergo adaptive change should environmental conditions change, for example following successful biological control. As a consequence, the introduction of biological control agents against weeds is considered by many to be an inherently unsafe practice with non‐target plants at risk of attack. However, despite the introduction of over 600 insect species from one geographic region to another for biological weed control during this century, there are relatively few documented cases of changes in host plant range. Purported instances are discussed in relation to behavioural and genetic concepts. It is concluded that apparent additions to the host range can, in all of the cases examined, be explained in terms of established behavioural concepts of pre‐adaptation, threshold change resulting from host deprivation, and effects of experience (learning). The inappropriateness of the often‐used term host shift’ to describe these cases is demonstrated, and it is concluded that evidence from biological weed control contradicts some aspects of ecological and evolutionary theory.     

The role of the seed tuber in determining partial resistance to potato blackleg caused byErwinia spp.

In 1991 and 1992, 12 potato cultivars were screened at two locations for resistance to blackleg, after vacuum infiltration of the seed withErwinia carotovora subsp.atroseptica orE. chrysanthemi. Cultivar differences for resistance toE.c. subsp.Atroseptica andE. chrysanthemi were found which were consistent over locations and years. Seed tubers of the same cultivars were also screened for resistance to bothErwinia spp. by using a tuber slice inoculation method. Correlation coefficients for comparisons between resistance to blackleg in the field and tuber tissue resistance under aerobic or anaerobic conditions were not significant. This could partly be explained by drastic changes in relative tuber tissue resistance of the cultivars within a 5 weeks period after planting in the field. Presprouting of seed tubers in diffuse daylight had a less pronounced effect on relative tuber tissue resistance than planting in the field. Monitoring the process of mother tuber decay during the growing season of 1993 after vacuum infiltration withE.c. subsp.atroseptica andE. chrysanthemi revealed that cultivars differed in the extent to which these bacteria enhanced the process of mother tuber decay. These differences partly explained the cultivar differences for resistance to blackleg in the field.Abbreviations Eca Erwinia carotovora subsp.atroseptica - Ech Erwinia chrysanthemi - NOP Noordoostpolder - Wag Wageningen     

Assessing disease severity: accuracy and reliability of rater estimates in relation to number of diagrams in a standard area diagram set

Error in estimates of plant disease severity occur and standard area diagrams (SADs) help improve accuracy and reliability. The effects of diagram number in SADs are unknown. The objective of this study was to compare estimates of pecan scab severity made without SADs, and using three‐, five‐, seven‐ or 10‐diagram SADs. Disease severity was estimated to the nearest percent (NPE), or classified to the closest reference diagram value using a scale. Twelve raters assessed 20 images of scabbed pecan valves with and without the SADs using NPEs and the scale method (values were converted to midpoints prior to analysis). Increases in diagram number using NPEs did not necessarily result in more accurate or reliable estimates. Inter‐rater reliability was positively correlated with number of diagrams using NPEs (r = 0·3288 (P < 0·0001)) or a scale (r = 0·2803 (P < 0·0001)). The least accurate estimates improved the most using SADs with NPEs, but the gain did not relate to number of diagrams; as few as three diagrams reduced error of inaccurate estimates as much as five, seven or 10 diagrams. When used as a scale, only estimates made with 10‐diagram SADs had similar accuracy and inter‐rater reliability to estimates made using SADs with NPEs. Maximum disease severity and the disease severity range for the pathosystem and the SADs are important factors and will probably influence resulting accuracy and reliability. The ramifications of diagram numbers in SADs and assessment methods on accuracy and reliability of disease estimates are discussed.     

Effect of temperature on herbicidal properties of MTB-951, a mycoherbicide to control Echinochloa crus-galli L.

MTB-951 is a potential mycoherbicide using a fungal plant pathogen ( Drechslera monoceras ) isolated from native Echinochloa species in Japan. Conidia of this pathogen were used as the active ingredient and the effect of temperature on its properties was examined in a laboratory. The optimum temperature for conidium germination was broad between 15 and 35°C and no difference in the germination ratio existed within this temperature range. Mycelial growth was the fastest at 25°C but was slower outside of the range 15–35°C. When leaves of Echinochloa crus-galli L. were immersed in deionized water containing the conidia, cellular electrolyte leakage from the leaves was observed. However, no electrolyte leakage occurred from leaves of rice ( Oryza sativa L.) under the same conditions. The occurrence of electrolyte leakage from E. crus-galli was dependent on the temperature and was the highest at 25°C. Herbicidal activity of MTB-951 against E. crus-galli was high between 25°C and 30°C, but decreased at 35°C in a glass vessel. In the present study, it was revealed that the physiological and herbicidal properties of MTB-951 depended on the temperature.     

Sensitivity of Phytophthora infestans to mandipropamid and the effect of enforced selection pressure in the field

Sensitivity to the new carboxylic acid amide (CAA) fungicide mandipropamid (MPD) in Phytophthora infestans was measured for isolates collected between 1989 and 2002 in Israel prior to the commercial use of MPD (baseline sensitivity, 44 isolates), and from MPD-treated (25 isolates) and untreated fields (215 isolates) in nine European countries and Israel between 2001 and 2005. All isolates were sensitive to MPD, with EC₅₀ values ranging between 0·02 and 2·98  µ g mL⁻¹. Plastic-tunnel (UK), shade-house (Israel) and field experiments (Israel) conducted during 2001–05 showed that enforced selection pressure, applied preventively or curatively, imposed by repeated sublethal (5  µ g mL⁻¹) or excessive (500–1000  µ g mL⁻¹) doses of MPD on mixed isolates of P. infestans produced no isolates resistant to the compound. The results of this study indicate that the probability of a buildup of resistant sub-populations of P. infestans to mandipropamid in the field is low.     

Rat Damage to Coconuts in Fiji Part 1 Assessment of Damage

Abstract

The amount of rat damage to immature coconuts was assessed by two methods. The first involved fortnightly recordings of all coconuts produced, including those rat damaged, at representative sites for up to three years. The second method attempted to utilise decay characteristics of rat damaged coconuts as a basis for projecting spot counts of damaged coconuts to long term estimates of damage.

Damage was concentrated on coconuts aged three to six months with most of the damage being inflicted by Rattus rattus. Damage varied considerably from month to month with seasonal trends in damage being apparent at only one site.

Rats clearly favoured particular palms in all plantations, resulting in over 75% of the damage being confined to 30% of the palms. Those selectively attacked were randomly distributed within the survey plots and did not produce significantly fewer harvestable coconuts than those palms seldom attacked.

Tall palms, i.e. over 15 m incurred less damage than those under 10 m.

Decay characteristics of rat damaged coconuts proved to be of limited value as a basis for projecting spot counts to long term losses, but counts of green damaged coconuts accurately reflected the previous 33–35 days damage.

Overall production losses (derived after adjustments for known palm compensation for damage) were relatively low; being 2.7, 2.0 and 1.3 nuts per palm per year during 1970, 1971 and 1972.     

Combining nonpathogenic strains of Fusarium oxysporum with sodium chloride to suppress fusarium crown rot of asparagus in replanted fields

Nonpathogenic strains of Fusarium oxysporum (NPFO) were tested alone, and in combination with rock salt (NaCl), for their ability to suppress fusarium crown and root rot of asparagus in replanted fields. Five NPFO strains were first screened with and without NaCl in the glasshouse using autoclaved asparagus soil mixed with potting mix. Asparagus roots were treated with NPFO suspensions or water, then inoculated 1 week later with pathogenic F. oxysporum ; then after 1 week pots received 100 mL 1% NaCl or water. After 12 weeks the NPFO strain CS-20 increased root weight when compared with controls. Rock salt did not affect root growth. Root lesions were reduced relative to the control by all NPFO strains and by NaCl applications, but there were no interactions between NPFO and NaCl. The NPFO strains CS-20, CWB 314 and CWB 318 were compared with no treatment in two 4-year replanted field studies in Hamden, and Windsor, CT, USA. Crowns were treated with NPFO at planting, and NaCl was broadcast at 280 kg ha⁻¹, and at 560 kg ha⁻¹ in later years. CWB 318 reduced disease at both sites and increased marketable yield in Hamden. Treatment with NaCl increased yield by 29% over controls in Windsor, but did not increase yield in Hamden. Recovered root isolates of F. oxysporum categorized by heterokaryon tests revealed that, in the glasshouse, CS-20 colonized roots better than other strains, while CWB 318 was the only strain found on roots from field plots after 4 years. Applications of NaCl did not hinder the suppressive ability of the nonpathogenic strains. The perennial nature of asparagus may require that nonpathogenic strains possess superior survival traits along with biocontrol traits.     

The Importance of Rhizosphere Interactions in the Biological Control of Plant Parasitic Nematodes—a Case Study using Verticillium chlamydosporium

Verticillium chlamydosporium has provided considerable control of root knot nematodes in a range of laboratory experiments and is one of the most promising natural enemies with potential as a biological control agent so far tested. From a detailed study of the biology and ecology of the fungus, it has been possible to identify the key factors which affect its efficacy and a strategy for its exploitation has been developed. The fungus survives in soil throughout a growing season and its concentration in soil may be increased by repeated applications. The rapid multiplication of root-knot nematodes on susceptible crops means that control must be extremely effective to prevent crop damage. If suitable methods of production and application can be developed, V. chlamydosporium, in conjunction with other methods such as the rotation of poor hosts, may provide adequate control, but the reliability of such approaches needs extensive testing. Although genetic manipulation offers the possibility of enhancing the ability of the fungus to kill nematodes, such an approach will require much research and, in the short term, the development of the fungus will rely on the exploitation of carefully selected wild types.