Van der Beek, J. G., Poleij, L. M., Zijlstra, C., Janssen, R., and Janssen, G. J. W. 1998 The root-knot nematode species Meloidogyne chitwoodi Golden et al. and M. hapla Chitwood can cause growth and yield reductions and quality loss in a large number of crops. In potato (Solanum tuberosum L.), these root-knot nematodes are able to reproduce on roots, causing yield reduction (21). M. fallax Karssen, which was described recently (22), is allopatric with M. chitwoodi in the Netherlands, and its host range is similar to that of M. chitwoodi. M. hapla causes little quality damage due to gall formation on the outside of tubers, in contrast to M. chitwoodi (7). Recent observations indicate that M. chitwoodi and M. fallax reproduce similarly on tubers, and both result in quality loss (J. G. van der Beek, unpublished data). In addition to yield reduction and quality loss, an increase in the nematode population due to cultivation of a susceptible host makes these pathogens a potential threat to agriculture in Western Europe, because many crops in the current rotation schemes are good hosts (33). However, M. hapla does not reproduce on monocotyledonous crops, which may be used in rotation with dicotyledons as an effective control measure.The introduction of resistant potato cultivars also would improve the efficiency of crop rotation in reducing this threat. However, in current potato cultivars, no desirable level of resistance to M. chitwoodi and M. hapla has been identified (7,17).Thus far, intraspecific variation in M. chitwoodi has been described by the occurrence of three host races that can be distinguished on alfalfa cv. Thor (nonhost for race 1 and host for races 2 and 3), carrot cv. Red Cored Chantenay (host for race 1 and nonhost for races 2 and 3), and S. bulbocastanum Dun. SB22 (nonhost for races 1 and 2 and host for race 3) (26,27,36).Resistance to M. chitwoodi and M. fallax has been identified in accessions of wild tuber-bearing Solanum species, originating mainly from Central America, such as S. bulbocastanum and S. hougasii Corr. (5,6,18). Resistance to M. chitwoodi has been transferred from a resistant genotype of S. bulbocastanum to the gene pool of the cultivated potato by somatic hybridization (1,7), and a single gene has been localized on chromosome 11 that appears to control this resistance (8). Also, various levels of resistance to M. hapla have been reported (9,15,18). Recent investigations indicated the presence of variation in host responses to different M. hapla isolates on some genotypes of S. bulbocastanum, S. hougasii, S. chacoense Bitt., S. gourlayi, S. sparsipilum, and S. spegazzinii that were selected for resistance (19).In general, the efficacy of resistance depends largely on genetic variation in the virulence of the pathogen and factors involved in the durability of resistance. Often the presence of resistance is concluded from experiments with one or a few isolates. The current study focused on the variation in virulence within M. chitwoodi, M. fallax, and M. hapla by testing s...
To study virulence and aggressiveness in root-knot nematodes on cultivars of potato (Solanum tuberosum L., four isolates of Meloidogyne hapla Chitwood race A, one of M. hapla race B, three of Meloidogyne chitwoodi Golden et al., and two of Meloidogyne fallax Karssen were evaluated on 10 commercial potato cultivars under semisterile conditions in Petri dishes. Virulence and aggressiveness were assessed in terms of nematode reproduction by egg-mass index and reproduction factor, estimated by the number of egg masses and juveniles produced, respectively, divided by the number of juveniles inoculated. Significant interaction for both parameters was revealed between species of Meloidogyne and potato cultivars. Only M. hapla showed significant isolate-by-cultivar interaction, which was predominantly caused by the M. hapla race B isolate Hh. This indicates variation in virulence and suggests the occurrence of different genetic factors for virulence and resistance in M. hapla isolates and potato cultivars, respectively. Despite large differences, the observed levels of resistance were too low to be of practical meaning for breeding, with the exception of resistance to isolate Hh. No significant differences were obtained between isolates of M. chitwoodi and M. fallax or isolate-by-cultivar interaction, indicating neither variation in aggressiveness nor in virulence in the isolates used. A comparative greenhouse experiment gave comparable results for egg-mass index but contradicting results for reproduction factor, most likely because of differences in life cycle, which appeared to be shorter in M. fallax than in M. hapla and M. chitwoodi. The Petri-dish method proved to be accurate for virulence studies of root-knot nematodes on potato culitvars. Meloidogyne fallax was most aggressive on potato, followed by M. chitwoodi, M. hapla race A, and finally by M. hapla race B.Résumé : Afin d'étudier la virulence et l'agressivité des nématodes à nodosités racinaires sur des cultivars de Solanum tuberosum L., quatre isolats du Meloidogyne hapla Chitwood race A, un du M. hapla race B, trois du Meloidogyne chitwoodi Golden et al. et deux du Meloidogyne fallax Karssen ont été évalués sur 10 cultivars commerciaux de pomme de terre, en utilisant des conditions semi-stériles en plaques de Pétri. Les auteurs ont évalué la virulence et l'agressivité en termes de reproduction des nématodes, sur la base d'un index de masse des oeufs et d'un facteur de reproduction, estimés par les nombres de masses d'oeufs et de jeunes produits, divisés par le nombre de jeunes inoculés, respectivement. Des interactions significatives apparaissent pour les deux paramètres entre les espèces de Meloidogyne et les cultivars de pomme de terre. Seul le M. hapla montre une interaction isolat versus cultivar significative, qui résulte de façon prédominante de l'isolat Hh de la race B du M. hapla. Ceci indique une variation dans la virulence et suggère l'existence de différents facteurs génétiques pour la virulence et la résistance chez le M. hapla et les cultivars d...
The objective of the present study was to establish whether exposure to temperatures of 55-70 °C results in eradication of the pathogen Clavibacter sepedonicus (Cs) in colonised potato tissue, in order to evaluate the efficiency of composting for disinfection of Cs-infected potato waste. Pre-sprouted potato tubers were inoculated and planted to produce Cs-colonised stem and tuber material. After incubation in temperature-controlled water baths, the infected potato tissue was analysed for the presence of culturable and pathogenic Cs. Additional experiments were performed with Cs-colonised potato stem tissue crushed and deaerated, thus simulating macerated stem tissue in the compost heap. To enable a comparison with bacterial cells that are not enclosed by their natural organic matrix, temperature treatments were applied to non-infected stem tissue crushed and deaerated, and spiked with freshly prepared Cs-suspensions. Cs settled inside colonised potato tissue, as well as Cs present as Cs-suspensions supplemented to potato tissue, was eradicated by exposure to heat, even after a treatment of 1 h at 55 °C, with the exception of one case in which the pathogen present in intact stem material escaped a treatment of 6 h at 60 °C, indicating that incidentally stems may provide niches in which the pathogen is protected against heat.
The parasitoids Apoanagyrus lopezi De Santis and A. diversicornis (Howard) (Hymenoptera: Encyrtidae) have been introduced into Africa for the biological control of the cassava mealybug Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae). We have studied competition between these species to investigate if they can coexist. Here we report on the influence of the simultaneous presence of non-conspecific adult females on searching efficiency on patches. Wasps of either species foraged on discs of cassava leaf with mealybugs, while at the same time different numbers of non-conspecifics were also depleting the patch. Patch area per parasitoid and number of hosts available to each parasitoid were equal in all treatments.In both species, the presence of other foragers clearly affected several aspects of the parasitoids' behaviour. Patch residence time increased with the number of non-conspecifics in A. diversicornis. In both parasitoid species, the proportion of hosts left unparasitized after the patch visit decreased with increasing numbers of females on the patch. The proportions of super-and multiparasitism did not change with the number of females. Both species produced more offspring during a patch visit in the presence of more non-conspecifics. These behavioural changes did not, however, lead to a change in the offspring production rate on patches. A. diversicornis produced offspring at a rate three times that of A. lopezi when one A. lopezi and one A. diversicornis foraged simultaneously. This is the first report of an aspect of interspecific competition where A. diversicornis has an advantage over A. lopezi. Interference between adult females thus promotes coexistence of the two species on P. manihoti.
Pathogenic Escherichia coli strains are responsible for food-borne disease outbreaks upon consumption of fresh vegetables and fruits. The aim of this study was to establish the transmission route of E. coli strain 0611, as proxy for human pathogenic E. coli, via manure, soil and plant root zones to the above-soil plant compartments. The ecological behavior of the introduced strain was established by making use of a combination of cultivation-based and molecular targeted and untargeted approaches. Strain 0611 CFUs and specific molecular targets were detected in the root zones of lettuce and leek plants, even up to 272 days after planting in the case of leek plants. However, no strain 0611 colonies were detected in leek leaves, and only in one occasion a single colony was found in lettuce leaves. Therefore, it was concluded that transmission of E. coli via manure is not the principal contamination route to the edible parts of both plant species grown under field conditions in this study. Strain 0611 was shown to accumulate in root zones of both species and metagenomic reads of this strain were retrieved from the lettuce rhizosphere soil metagenome library at a level of Log 4.11 CFU per g dry soil.
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