The yam nematode (Scutellonema bradys), a potential threat to potato (Solanum tuberosum) production in West Africa

Potato (Solanum tuberosum) production in Africa is rapidly expanding and becoming increasingly important. As its geographical production range broadens, so does its potential to host new pests and diseases. Following the discovery that potato can be affected by Scutellonema bradys, further studies were undertaken to assess its potential pathogenicity on potato under screenhouse and field conditions, and on marketed tubers. Potato plants inoculated with S. bradys produced tubers with substantial cracking and evident tuber rot, compared with tubers from uninoculated plants. Symptoms of nematode infection on tubers included a scaly appearance, surface cracking as well as deeper tissue cracks, distortions, and darkened surface patches. In most cases these patches were related to sub‐surface rot. Nematodes were recovered from the soil, roots and tubers of inoculated plants. Eight weeks after inoculation, the reproduction factor of the nematode was greatest (2·0) at the lowest inoculation rate assessed (1000 nematodes per 2·5‐L pot) and least (0·4) at the highest inoculation rate (5000 nematodes per pot). In the screenhouse, potato tuber weights were low and mostly unaffected by nematode inoculation rate, except at 5000 nematodes per pot. In the field, non‐inoculated plants yielded over nine times more tubers than plants inoculated with 2000 S. bradys. Low densities of S. bradys were also recovered from 10 of 15 (67%) samples collected from market stalls, indicating field infection. This study confirms that potato can host and be damaged by S. bradys, raising its prospect as a likely significant biotic constraint to the crop.     

腐烂茎线虫在马铃薯不同组织中的数量动态

腐烂茎线虫病是近年来我国马铃薯生产中的一种新病害, 了解腐烂茎线虫的侵染规律对该病害的预防和控制及对马铃薯产量和品质提高具有重要意义。本研究采用盆栽法, 在苗期接种腐烂茎线虫, 研究该线虫在马铃薯不同组织中的数量动态及危害程度。结果显示, 在发芽-幼苗期(播种0 ~ 75 d), 母薯先受到腐烂茎线虫侵染; 在发棵期(播种75 ~ 90 d), 母薯和匍匐茎均检测到腐烂茎线虫; 在结薯期到成熟期(播种90 ~ 105 d), 线虫开始危害新生块茎, 随着块茎膨大危害逐渐加重。在马铃薯成熟期, 症状表现最明显, 新生块茎中线虫数量最多, 可达767.67条/g。不同时期的接种试验表明, 在匍匐茎形成期(播种后70 d)接种, 马铃薯受害最为严重, 发病率达100%, 块茎中线虫密度最高, 达到99.32条/g, 且块茎鲜重最低, 为14.02 g/个。本研究结果可为马铃薯腐烂茎线虫的适时有效防治提供参考。     

Detection of the root lesion nematode Pratylenchus penetrans in potato tubers

The root lesion nematode Pratylenchus penetrans parasitizes a wide range of economically important crops, including potato (Solanum tuberosum). Damage by P. penetrans impacts not only the potato yield but can also reduce the tuber quality. Detailed information on tuber infection by P. penetrans is scarce for most cultivars and molecular detection of nematodes from infected tubers is needed. The objective of this study was to assess tuber symptomatology due to P. penetrans infection in 10 potato cultivars and to provide an accurate molecular methodology for nematode detection using tuber peels. Sprouts of certified potato seed from cultivars Agata, Agria, Camel, Désirée, Dirosso, Kennebec, Laura, Picasso, Royata, and Stemster were planted in 2 L pots, and soil was inoculated with 4 P. penetrans/g of soil. Sixty days after inoculation, tubers were harvested, inspected for lesions, and the number of nematodes/g of potato peel assessed. Observations of tubers with symptoms showed the presence of P. penetrans in superficial layers of peels around the lenticels and injured necrotic tissue. Different nematode stages were detected in tubers of all inoculated cultivars, varying from 4 to 46 nematodes/g of potato peel. Species-specific primers showed suitable sensitivity and reproducibility for the detection of P. penetrans in tuber potato peel samples. The molecular detection of P. penetrans directly from tuber peels can facilitate routine nematode inspections of potato seed tubers or cull potatoes for nematode detection, and prevent further dissemination of this species.     

Biological control of potato soft rot caused by Dickeya solani and the survival of bacterial antagonists under cold storage conditions

Dickeya and Pectobacterium are responsible for causing blackleg of plants and soft rot of tubers in storage and in the field, giving rise to losses in seed potato production. In an attempt to improve potato health, biocontrol activity of known and putative antagonists was screened using in vitro and in planta assays, followed by analysis of their persistence at various storage temperatures. Most antagonists had low survival on potato tuber surfaces at 4 °C. The population dynamics of the best low-temperature tolerant strain and also the most efficient antagonist, Serratia plymuthica A30, along with Dickeya solani as target pathogen, was studied with TaqMan real-time PCR throughout the storage period. Tubers of three potato cultivars were treated in the autumn with the antagonist and then inoculated with D. solani. Although the cell densities of both strains decreased during the storage period in inoculated tubers, the pathogen population was always lower in the presence of the antagonist. The treated tubers were planted in the field the following growing season to evaluate the efficiency of the bacterial antagonist for controlling disease incidence. The potato endophyte S. plymuthica A30 protected potato plants by reducing blackleg development on average by 58.5% and transmission to tuber progeny as latent infection by 47–75%. These results suggest that treatment of potato tubers with biocontrol agents after harvest can reduce the severity of soft rot disease during storage and affect the transmission of soft rot bacteria from mother tubers to progeny tubers during field cultivation.     

Contamination of potatoes by Erwinia carotovora during grading

Considerable tuber contamination by soft rot erwinias in rotting tubers can occur when grading potato stocks. Erwinia carotovora pv. carotovora , from a single rotting tuber, contaminated c. 100 kg of potatoes during mechanical grading, c. 50% being contaminated with 10⁴–10⁵ bacteria per tuber. Survival of the bacteria during storage was related to the degree of damage sustained by the tubers during grading which in turn depended on the model of oscillating riddle grader used and the cultivar of potato. Chemical disinfection of the grader immediately before use and of tubers immediately after grading reduced contamination.     

Potato diseases caused by soft rot erwinias: an overview of pathogenesis

Three soft rot erwinias, Erwinia carotovora ssp. carotovora , E. carotovora ssp. atroseptica and E. chrysanthemi are associated with potatoes causing tuber soft rot and blackleg (stem rot). Latent infection of tubers and stems is widespread. As opportunistic pathogens, the bacteria tend to cause disease when potato resistance is impaired. Pathogenesis or disease development in potato tubers and stems is discussed in terms of the interaction between pathogen, host and environment, microbial competition and recent findings on the molecular basis of pathogenicity. Emphasis is placed on the role of free water and anaerobiosis in weakening tuber resistance and in providing nutrient for erwinias to multiply. Blackleg symptoms are expressed when erwinias predominate in rotting mother tubers, invade the stems and multiply in xylem vessels under favourable weather conditions. Soft rot erwinias tend to out-compete other bacteria in tuber rots because of their ability to produce larger quantities of a wider range of cell wall-degrading enzymes. However, despite extensive studies on their induction, regulation and secretion, little is known about the precise role of the different enzymes in pathogenesis. The putative role of quorum-sensing regulation of these enzymes in disease development is evaluated. The role certain pathogenicity-related characters, including motility, adhesion, siderophores, detoxifying systems and the hrp gene complex, common to most bacteria including symbionts and saprophytes, could play in latent and active infections is also discussed.     

Oospore Formation by Phytophthora infestans in Potato Tubers

ABSTRACT The ability of Phytophthora infestans, the causal agent of potato and tomato late blight, to produce oospores in potato tuber tissue was studied in the field and under laboratory conditions. In 1998 and 2000 field experiments, the canopy of potato cvs. Alpha and Mondial, respectively, were coinoculated with A1 + A2 sporangia of the fungus, and the infected tubers collected at harvest were examined for the presence of oospores. In 1998, only 2 of 90 infected tubers had oospores, whereas none of the 90 tubers examined in 2000 had any oospores. In the latter experiment, infected tubers kept in storage up to 12 weeks after harvest had no oospores. Artificial co-inoculations of whole tubers with A1 + A2 sporangia resulted only rarely in the formation of oospores inside the tubers. Co-inoculations of potato tuber discs taken from dormant tubers 0 to 16 weeks after harvest failed to support any oospore production, whereas discs taken from sprouting tubers of >/=18 weeks after harvest allowed oospores to form. Tuber discs showed enhanced oospore formation when treated before inoculation with either sugars, amino acids, casein hydrolysate, beta-sitosterol, or chloroethylphosphonic acid. In contrast, reducing airflow into the petri dishes where potato tuber discs were incubated reduced the number of oospores produced. The number of oospores produced in tuber tissue was lower compared with that in leaf tissue regardless of the origin of isolates used. The data show that the ability of Phytophthora infestans to produce oospores in potato tuber tissue is very limited and increases with tuber aging.     

Fusarium graminearum as a dry rot pathogen of potato in the USA: prevalence,comparison of host isolate aggressiveness and factors affecting aetiology

A 2004–2005 survey of potatoes from stores in the north‐central potato‐producing region of the USA showed that the predominant causes of dry rot were Fusarium graminearum and Fusarium sambucinum. Isolates of F. graminearum originally isolated from potato tubers with dry rot (n = 15), wheat kernels with scab (n = 15), and sugarbeet tap roots with decay (n = 5) were tested for aggressiveness to potato tubers. There were no significant differences in aggressiveness among isolates of F. graminearum, regardless of original host, as measured by their ability to cause dry rot. These findings may have implications for survival of F. graminearum inoculum since potatoes, wheat and sugarbeets are frequently used in crop rotation in the region. Fusarium graminearum required larger wounds for infection of potato tubers than F. sambucinum. Plug‐removal injury, simulating a stolon‐removal injury, resulted in equal incidence of dry rot caused by the two Fusarium species, whereas abrasion and bruising injury were sufficient for infection and dry rot development by F. sambucinum, but not F. graminearum. A change in harvest practices from vine‐killing prior to harvest to mechanical vine‐killing on the day of harvest may be a factor affecting the onset of dry rot caused by F. graminearum, since this process often causes large wounds at the stem end of the tubers when the stolon is forcibly removed.     

Distribution of polygalacturonase, total phenolic substances, polyphenol oxidase and peroxidase in rot zones in sweet potato

Botryodiplodia theobromae causes a spreading rot of sweet potato, while Botrytis cinerea, Phoma exigua var. foveata and Fusarium solani var. coeruleum cause rots of limited extent. Cladosporium cucumerinum does not rot sweet potato tuber tissue. Whichever of these fungi is inoculated into tubers, a zone at the margin of the lesion can be clearly distinguished, from slightly affected but uninfected tissue on one side and infected rotted tissue on the other, by its high content of phenolic substances and polyphenol oxidase and peroxidase activities. Polygalacturonase is present in this zone for spreading B. theobromae lesions but not for the limited lesions caused by the other fungi. Total content of phenolic substances is generally greater in and around limited lesions.     

Dry rot of potatoes caused by Gliocladium roseum

The pathogenicity of Gliocladium roseum, isolated in a previous study as the sole organism from potato tubers with typical Fusarium dry-rot lesions, was investigated. Although usually considered to be a saprophyte, G. roseum was pathogenic to potato tubers (cv. Up-to-date) after artificial inoculation. It caused raised dry-rot lesions from which it was consistently reisolated. This is the first report that dry rot of potato tubers is caused by G. roseum .     

QTLs for potato tuber resistance to Dickeya solani are located on chromosomes II and IV

The goal of this research was to identify quantitative trait loci (QTLs) for potato tuber resistance to the soil- and seedborne bacterium Dickeya solani and for tuber starch content, to study the relationship between these traits. A resistant diploid hybrid of potato, DG 00-270, was crossed with a susceptible hybrid, DG 08-305, to generate the F₁ mapping population. Tubers that were wound-inoculated with bacteria were evaluated for disease severity, expressed as the mean weight of rotted tubers, and disease incidence, measured as the proportion of rotten tubers. Diversity array technology (DArTseq) was used for genetic map construction and QTL analysis. The most prominent QTLs for disease severity and incidence were identified in overlapping regions on potato chromosome IV and explained 22.4% and 22.9% of the phenotypic variance, respectively. The second QTL for disease severity was mapped to chromosome II and explained 16.5% of the variance. QTLs for starch content were detected on chromosomes III, V, VI, VII, VIII, IX, XI, and XII in regions different from the QTLs for soft rot resistance. Two strong and reproducible QTLs for resistance to D. solani on potato chromosomes IV and II might be useful for further study of candidate genes and marker development in potato breeding programmes. The relationship between tuber resistance to bacteria and the starch content in potato tubers was not confirmed by QTL mapping, which makes the selection of genotypes highly resistant to soft rot with a desirable starch content feasible.     

The Detection of Latent Ring Rot (Corynebacterium sepedonicum [Spiek. et Kotth.] Skapt. et Burks.) in Potato Stocks1

Very low numbers of Corynebacterium sepedonicum (Spiek. et Kotth.) Skapt et Burkh. in potato tubers can be detected by inoculating the bacterial fraction separated from cores (including the vascular tissue) taken from the heel end of tubers into young eggplants (Solanum melanogena). One tuber with simulated latent ring rot infection could be detected readily when mixed with 99 uninfected tubers.     

马铃薯镰刀菌干腐病研究进展及防控手段

马铃薯是世界第四大粮食作物。马铃薯干腐病是一种重要的马铃薯收获后真菌病害,在我国各马铃薯产区内广泛发生,导致马铃薯块茎的商品薯率大幅下降,对马铃薯的产量造成严重经济损失。本文针对马铃薯干腐病的症状、发生与危害、病原菌种类以及综合防控手段进行综述,以期为病害的研究及防治提供参考。     

Tropical cover crops for the management of the yam nematode,Scutellonema bradys

Yam tuber dry rot caused by Scutellonema bradys results in the reduction of tuber quality. Studies were conducted on the effect of intercropping cover crops with yams on nematode damage to tubers. Eleven cover crops were intercropped with yams in pot screen house and field experiments. Plants were inoculated with S. bradys in water suspension in pots and by incorporating infected yam peels into soil around plant roots in field experiments with un-inoculated plants serving as control. The experiments were factorial with six replications per treatment in pot and four in field experiments. In pot experiments, populations of S. bradys recovered from tubers and nematode damage were lower in yam tubers intercropped with Tagetes erecta, Aeschynomene histrix, Stylosanthes guianensis, Mucuna pruriens and Pueraria phaseoloides compared to where no cover crops were intercropped with yams. Conversely, S. bradys populations from pots with Crotalaria ochroleuca, Crotalaria juncea, Centrosema pubescens, Cajanus cajan and Lablab purpureus were similar to Vigna unguiculata, a known host of S. bradys. The use of A. histrix, T. erecta and S. guianensis, M. pruriens and P. phaseoloides offers promise for inclusion in yam-based cropping systems as a measure for S. bradys management in yam production.     

Methods of testing potato cultivars for resistance to soft rot of tubers caused by Erwinia carotovora subsp. atroseptica

The resistance of eight potato cultivars to tuber soft rot caused by E. carotovora subsp. atroseptica was assessed in 2 years using three different test methods. Similar cultivar resistance rankings were obtained for any one method within a year and between years for two methods (single site, infectivity titration), but not for the third (vacuum infiltration). However, the ranking of cultivars differed for the three methods. Ranking was not affected by inoculating the cortex or the more susceptible medullary tissue, or by assessing rotting in terms of infection frequency or lesion size, but it was affected by oxygen concentration during incubation. Differences among cultivars were greater when inoculated tubers were incubated anaerobically than when incubated with 5% oxygen. There was no relationship between the relative susceptibilities of cultivars to tuber soft rot in storage in January/February and those of mother tubers after planting.     

Effects of systemic infections with Tobacco rattle virus on agronomic and quality traits of a range of potato cultivars

Infection of several commercially important potato cultivars with Tobacco rattle virus (TRV) can result in systemic infection with M-type virus, which persists through generations of vegetative propagation, although spraing symptoms in the tubers rarely develop. In field trials of nine such cultivars it was shown that TRV infection can delay plant emergence and retard subsequent growth, reduce tuber yield and mean tuber weight, increase tuber numbers and the proportion of small tubers, increase the incidence of growth cracks and misshapen tubers, diminish dry matter content, and exacerbate after-cooking blackening. Each cultivar was affected differently, showing different combinations and degrees of these effects. Virus concentration in leaf extracts from individual plants was also highly variable. These, and other potato cultivars similarly susceptible to TRV, are grown on large areas of UK and European potato land. The implications of growing such cultivars for the seed, ware and processing sectors of the potato industry are discussed.     

硫酸铜防治马铃薯软腐病及其对薯块有关酶类的影响

 用0.05% CuSO₄·5H₂O浸泡马铃薯薯块半小时,几乎完全防治了贮藏期的细菌性软腐病。田间试验中,叶面喷洒0.1% CuSO₄·5H₂O能显著减轻黑胫病的发生。叶面喷铜后叶片,茎杆和子薯的含铜量提高,子薯对伤口接种和皮孔接种的抗性增强。叶面喷铜对子薯的多酚氧化酶、过氧化物酶活性没有明显影响。生长期内喷25次和10次铜使薯块组织的苯丙氨酸(PAL)活性升高。用CuSO₄·5H₂O浸泡薯块或向薯块的酶抽提液中直接加入Cu²⁺,对多酚氧化酶和过氧化物酶活性均影响不大,而PAL活性显著降低。文中讨论了Cu²⁺在薯块抗性中的作用。     

Detection of Erwinia carotovora subsp. atroseptica and Erwinia chrysanthemi in potato tubers using polymerase chain reaction

Soft rot erwiniae are a group of notorious plant pathogens for which currently available detection methods are inadequate. Based on the polymerase chain reaction, specific and sensitive detection of Erwinia carotovora subsp. atroseptica and E. chrysanthemi in potato tubers has been achieved. The composition of the PCR primers used in two specific detection systems is based on identification of the consensus of sequences of metalloprotease-coding genes present in soft rot erwiniae. Bacterial DNA was extracted from the potato tuber matrix by differential centrifugation in order to avoid interference of potato-derived compounds with the performance of the PCR assay. The PCR assay jjerformed with the E. carotovora subsp. atroseptica specific primer set was found to be capable of distinguishing E. carotovora subsp. atroseptica from all other Erwinia species and the closely related subspecies E. carotovora subsp. carotovora. With the E. chrysanthemi specific primer set, agarose gel electrophoresis is required for unequivocal differentiation between E. chrysanthemi and other erwiniae. Combined with the efficient extraction procedure, the assay allowed specific detection of less than 10³ culturable erwiniae per tuber. The specificity and sensitivity of the assay were not reduced in the presence of a 100-fold excess of DNA from both related and unrelated bacteria. This PCR-based method for detection of erwiniae in potato tubers provides a relatively fast and sensitive alternative to routinely applied serological methods.     

Resistance to thiabendazole in Fusarium species isolated from potato tubers affected by dry rot

Fusarium species obtained from stored potato tubers affected with dry rot were grown on agar containing thiabendazole. All 40 isolates of F. coeruleum and 60 isolates of F. avenaceum tested were sensitive to the fungicide, but 68% of the 85 isolates of F. sulphureum and one isolate of F. culmorum were classified as resistant. When isolates were made from dry rots on tubers that had been treated with thiabendazole during loading into store, all 81 isolates of F. sulphureum were resistant, whereas all the isolates of F. coeruleum (25), F. avenaceum (4) and Phoma foveata (10) were sensitive. Resistance was not found in five isolates of Cylindrocarpon destructans. All the Fusarium spp. were sensitive to imazalil and were pathogenic when inoculated into potato tubers. Resistant and sensitive isolates of F. sulphureum caused rots of similar size.     

Evaluation of a diagnostic microarray for the detection of major bacterial pathogens of potato from tuber samples

Potato can be infected with many bacterial pathogens, the detection of which is necessary in seed certification. In this study, a diagnostic microarray previously tested for specificity of probes for detecting the potato bacteria causing blackleg and soft rot (Pectobacterium atrosepticum, Pectobacterium carotovorum, and Dickeya spp.), ring rot (Clavibacter. michiganensis subsp. sepedonicus), scab (Streptomyces scabies and Streptomyces turgidiscabies) and brown rot (Ralstonia solanacearum) from pure culture was evaluated for analytical sensitivity when testing directly from tuber samples. The microarray readily detected all the bacterial species when 100 ng of the target bacterial DNA from pure culture was mixed with DNA from soil microbes and potato. However, detection was inconsistent when total DNA isolated directly from infected tubers or enriched bacterial culture was used. While the high specificity of the probes could be confirmed from the results of the DNA cocktail experiment used as a control, the study demonstrated that the level of analytical sensitivity of the microarray under the tested condition was not sufficient to detect bacteria directly from tubers. Therefore, in addition to the cost and organizational complexities, the low analytical sensitivity and limited reproducibility of the microarray are constraints for establishing the platform for routine detection of potato bacterial pathogens from tuber samples.