Abstract: Phytophthora ramorum came to the Netherlands in 1993. Despite initially not seeming to pose a high risk, findings in California showed its potential destructive impact on ecosystems. A programme began in the Netherlands to eliminate P. ramorum from nurseries and surveys in the natural environment were held to obtain information to determine a strategy for dealing with the disease. About 1100 nurseries are inspected annually by NAK tuinbouw under the auspices of the Plant Protection Service and measures are taken according to EC directives. The percentage of infected nurseries decreased steadily during recent years, from 4% in 2002/2003 to 0.5% in 2004/2005. Surveys in the natural environment show that P. ramorum occurs on 2% of the sites with Rhododendron and therefore it was concluded that an elimination scenario is not realistic. A programme based on containment measures supported by an extension programme was put into place with its effects being monitored by the Plant Protection Service. 12 years of observing P. ramorum show that the risk for indigenous trees and shrubs in the Netherlands is very limited. Spread from infected rhododendrons to other potential hosts, even at heavily infected Rhododendron sites, hardly takes place although some infected Quercus rubra trees have been found. Recently several new Phytophthora species were found in natural environments in Europe and California, mainly as a result of intensive P. ramorum surveys. As well as P. ramorum, the Phytophthora spp. P.kernoviae, P. numerosa and P. pseudosyringae pose risks, indicating the need for a more general approach against Phytophthora diseases. As a result, a new protocol for detection and identification of Phytophthora spp. both as a group and individually is being developed and workers are asking whether these Phytophthora species could be managed together.
Abstract: The impact of colonization by the arbuscular mycorrhizal fungus (AMF)Glomus mosseae on tomato root necrosis caused by the soil-borne pathogen Phytophthora parasitica was investigated. Studies in situ permitted infection loci to be identified and the effects of the AMF on numbers to be elucidated. Effects were significant and, 7 and 16 days after inoculation with zoospores of the pathogen, roots of plants colonized by the AMF had 39% and 30%, respectively, fewer infection loci than those that were not. Concurrent studies of the rate of spread of necrosis within roots showed no changes caused by the AMF. At harvest, 26 days following inoculation with the pathogen, 61% of roots of noncolonized plants were necrotic compared with only 31% in AMF-colonized plants. It is concluded that effects on numbers of infection loci are one mechanism via which AMF achieve biocontrol of this pathogen in tomato. Measures of the effects of the AMF on root system architecture suggest that no significant changes occur and are thus not the reasons for the reduction in infection loci. The implications of these data for agricultural practice and biocontrol research are discussed.
Abstract: Actinomycetes enhance soil fertility and have antagonistic activity against wide range of plant root-pathogens. These microorganisms were isolated from agricultural soils of Kerman as pure cultures. Phytophthora drechsleri Tucker, causes gummosis and root rot of pistachio trees worldwide. From 130 Actinomycetes isolates, 12 inhibited growth of the pathogen of pistachio gummosis in culture plates and four of the most active isolates exhibited biological control of the pathogen under greenhouse conditions. When plants were grown in sterile soil mix and treated both with Actinomycetes and the pathogen, the number of healthy plants increased dramatically and the symptoms on diseased plants were less severe in comparison with seedlings treated with the pathogen alone. From the collected data it was well conclusive that in greenhouse tests, soil applications of Actinomycetes controlled causal agent of root rot of pistachio seedlings. Antifungal activity was of fungicidal type on the pathogen mycelia. From the stand point of biotechnological goals, the results indicate that the active isolates can be investigated for use as biofertilizers, biofungicides and use in future development of recombinant DNA in pistachio trees bearing elevated resistance to gummosis. Field trials of the active isolates are under investigation.
Abstract:Ralstonia solanacearum biovar 2, the causative agent of brown rot in potato, has been responsible for large crop losses in Northwest Europe during the last decade. Knowledge on the ecological behaviour of R. solanacearum and its antagonists is required to develop sound procedures for its control and eradication in infested fields. A polyphasic approach was used to study the invasion of plants by a selected R. solanacearum biovar 2 strain, denoted 1609, either or not in combination with the antagonistic strains Pseudomonas corrugata IDV1 and P. fluorescens UA5-40. Thus, this study combined plating (spread and drop plate methods), reporter gene technology (gfp mutants) and serological (imunofluorescence colony staining [IFC]) and molecular techniques (fluorescent in situ hybridization [FISH], PCR with R. solanacearum specific primers and PCR–DGGE on plant DNA extracts). The behaviour of R. solanacearum 1609 and the two control strains was studied in bulk and (tomato) rhizosphere soil and the rhizoplane and stems of tomato plants. The results showed that an interaction between the pathogen and the control strains at the root surface was likely. In particular, R. solanacearum 1609 CFU numbers were significantly reduced on tomato roots treated with P. corrugata IDV1(chr::gfp1) cells as compared to those on untreated roots. Concomitant with the presence of P. corrugata IDV1(chr::gfp1), plant invasion by the pathogen was hampered, but not abolished. PCR–DGGE analyses of the tomato rhizoplane supported the evidence for antagonistic activity against the pathogen; as only weak R. solanacearum 1609 specific bands were detected in profiles derived from mixed systems versus strong bands in profiles from systems containing only the pathogen. Using FISH, a difference in root colonization was demonstrated between the pathogen and one of the two antagonists, i.e. P. corrugata IDV1(chr::gfp1); R. solanacearum strain 1609 was clearly detected in the vascular cylinder of tomato plants, whereas strain IDV1 was absent. R. solanacearum 1609 cells were also detected in stems of plants that had developed in soils treated with this strain, even in cases in which disease symptoms were absent, indicating the occurrence of symptomless infection. In contrast, strain 1609 cells were not found in stems of several plants treated with either one of the two antagonists. The polyphasic analysis is valuable for testing antagonistic strains for approval as biocontrol agents in agricultural practice.
Abstract:Kahili ginger (Hedychium gardnerianum) is an invasive weed in tropical forests in Hawaii and elsewhere. Bacterial wilt caused by the ginger strain of Ralstonia (=Pseudomonas) solanacearum systemically infects edible ginger (Zingiber officinale) and ornamental gingers (Hedychium spp.), causing wilt in infected plants. The suitability of R. solanacearum as a biological control agent for kahili ginger was investigated by inoculating seedlings and rooted cuttings of native forest plants, ornamental ginger, and solanaceous species to confirm host specificity. Inoculation via stem injection or root wounding with a bacterial–water suspension was followed by observation for 8 weeks. Inoculations on H. gardnerianum were then carried out in ohia-lehua (Metrosideros polymorpha) wet forests of Hawaii Volcanoes National Park to determine the bacterium's efficacy in the field. No native forest or solanaceous species developed wilt or other symptoms during the study. The bacterium caused limited infection near the inoculation site on H. coronarium, Z. zerumbet, Heliconia latispatha, and Musa sapientum. However, infection did not become systemic in any of these species, and normal growth resumed following appearance of initial symptoms. All inoculated H. gardnerianum plants developed irreversible chlorosis and severe wilting 3–4 weeks following inoculation. Systemic infection also caused death and decay of rhizomes. Most plants were completely dead 16–20 weeks following inoculation. The destructiveness of the ginger strain of R. solanacearum to edible ginger has raised questions regarding its use for biological control. However, because locations of kahili ginger infestations are often remote, the risk of contaminating edible ginger plantings is unlikely. The ability of this bacterium to cause severe disease in H. gardnerianum in the field, together with its lack of virulence in other ginger species, contributes to its potential as a biological control agent.
Abstract: Bacterial wilt caused by Ralstonia solanacearum (Smith) has become a severe problem mainly on potato and tomato in Ethiopia and no effective control measure is available yet. To explore possibilities for the development of biological control for the disease, 118 rhizobacteria, most of them collected from Ethiopia, were screened against an Ethiopian R. solanacearum strain. On the basis of in vitro screening, six strains (RP87, B2G, APF1, APF2, APF3, and APF4) with good inhibitory effect were selected for in planta testing in a greenhouse. In the greenhouse, soil and tomato seedlings were treated with the antagonists and their effects studied. The study showed that APF1 and B2G strains significantly reduced disease incidence and increased weight of tomato plants. Area under disease progress curves (AUDPC) was reduced by 60% and 56% in plants inoculated with APF1 and B2G strains, respectively. Plant dry weight increase in plants inoculated with APF1 and B2G strains was 96% and 75%, respectively. APF1 was found to be the most beneficial strain in disease suppression and also growth promotion resulting in 63% dry weight increase compared to untreated control. The study revealed that APF1 and B2G strains are promising strains whose effectiveness under field conditions and their mode of action should be investigated.
Abstract: Bacterial wilt (Ralstonia solanacearum) of tomato, Lycopersicon esculentum, causes a considerable amount of damage to tomato in Southern China. Biological control is one of the more promising approaches to reduce the disease incidence and yield losses caused by this disease. Based on antagonistic activity against R. solanacearum and three soil-borne fungal pathogens as well as biocontrol efficacy in the greenhouse, two bacterial strains Xa6 (Acinetobacter sp.) and Xy3 (Enterobacter sp.) were selected out of fourteen candidates as potential biocontrol agents. In order to find a suitable antagonist inoculation method, we compared the methods of root-dipping with soil-drenching in the aspects including rhizocompetence, biocontrol efficacy, and effect of promoting plant growth under greenhouse conditions. The drenching treatment resulted in a higher biocontrol efficacy and plant-yield increase, and this method was also easier to operate in the field on a large scale. Field trials were conducted for further evaluation of these two antagonistic strains. In both greenhouse and field experiments, the strain Xy3 had a better control effect against bacterial wilt than Xa6 did, while Xa6 caused higher biomass or yield increases. As recorded on the 75th day after treatment in two field experiments, biocontrol efficacy of Xy3 was about 65% in both field trials, and the yield increases caused by Xa6 were 32.4 and 40.7%, respectively, in the two trials. This is the first report of an Acinetobacter sp. strain used as a BCA against Ralstonia wilt of tomato.
Abstract: Bacterial wilt (Ralstonia solanacearum) is one of the production constraints of potato (Solanum tuberosum). The intent of the study was to evaluate potential of bacterial antagonists to suppress bacterial wilt disease development and evaluate the role of the strains as plant growth-promoting rhizobacteria (PGPR) in potato. One hundred-twenty rhizosphere bacterial isolates were screened against virulent strain of Ralstonia solanacearum PPRC-Rs. After in vitro screening, six antagonistic strains (PFMRI, BS-DFS, PF9, PF20, BC, and BS-wly) with inhibition diameter >11 mm were selected and studied further in the greenhouse, in vivo. During in vivo study, the strains were evaluated for their effect in suppressing disease development in terms of area under disease progress curve (AUDPC) and increasing biomass (plant height and dry weight) of potato. Accordingly, PFMRI, BS-DFS, and PF9, significantly reduced AUDPC by 78.6, 66, and 64.3%, and wilt incidence by 82.7, 66.2, and 65.7%, respectively, compared to the control. During the sole application, the strains significantly (P < 0.0001) increased plant height by 35.6, 45.9, and 45%, and dry matter by 111, 130.4, and 129%, respectively compared to non-bacterized control. In the presence of the pathogen strain PFMRI, BS-DFS, and PF9 increased plant height by 66, 50, and 48.2%, and dry matter by 153.8, 96.8, and 92.5%, respectively compared to the pathogen treated control. Hence, the study shows that PFMRI, BS-DFS, and PF9 strains have potential use in potato bioprotection, as PGPR or in an integrated bacterial wilt management; whose effectiveness under a variety of field conditions should be investigated.
Abstract: Fungal endophytes isolated from healthy Theobroma cacao tissues were screened in vitro for antagonism against major pathogens of cacao. Of tested endophytic morphospecies, 40% (21/52), 65% (28/43) and 27% percent (4/15) showed in vitro antagonism against Moniliophthora roreri (frosty pod rot), Phytophthora palmivora (black pod rot) and Moniliophthora perniciosa (witches broom), respectively. The most common antagonistic mechanism was simple competition for substrate. Nonetheless, 13%, 21%, and 0% of tested morphospecies showed clear antibiosis against M. roreri, P. palmivora, and M. perniciosa respectively. One isolate of Trichoderma was observed to be parasitic on M. roreri. Endophyte species that were common in the host plants under natural conditions often are good colonizers and grow fast in vitro whereas antibiosis producers usually appear to be relatively rare in nature, tend to grow slowly in vitro, and often are not good colonizers. We suggest that there is an inherent general trade-off between fast growth (high colonization) and production of chemicals that produce antibiosis reactions. Finally, field trials assessing the effects of three endophytic fungi (Colletotrichum gloeosporioides, Clonostachys rosea and Botryosphaeria ribis) on pod loss due to M. roreri and Phytophthora spp. were conducted at four farms in Panama. Although the overall incidence of black pod rot was very low during the tests, treatment with C. gloeosporioides significantly decreased pod loss due to that disease. We observed no decrease in pod loss due to frosty pod rot, but treatment with C. rosea reduced the incidence of cacao pods with sporulating lesions of M. roreri by 10%. The observed reduction in pod loss due to Phytophthora spp., and sporulation by M. roreri, supports the potential of fungal endophytes as biological control agents. Further, these studies suggest that combined information from field censuses of endophytic fungi, in vitro studies, and greenhouse experiments can provide useful a priori criteria for identifying desirable attributes for potential biocontrol agents.
Abstract: In greenhouse experiments, three strains of plant growth-promoting rhizobacteria (PGPR), Serratia sp. J2, fluorescent pseudomonad J3, and Bacillus sp. BB11, were evaluated for biological control of bacterial wilt of tomato caused by Ralstonia solanacearum. A total of four field trials were conducted, each in a different location in Jiangsu and the Hebei provinces. Strains J2, J3, and BB11 provided disease control and increased yield. In trial one in Qixia (1999), disease was reduced 66.1, 73.6, and 68.4% by J2, J3, and BB11, respectively, compared to the control. Yield increases with bacteria in this trial ranged from 49.5 to 70.8%. In trial two in Huaian (2000), disease was reduced 78.1, 94.1, and 86.9% by J2, J3, and BB11, respectively. Yield increases ranged from 180 to 237%. In trial three in Handan (1999), biocontrol efficiencies of 71.3, 63.6, and 78.2% were achieved by J2, J3, and BB11, respectively. Yield increases ranged from 53.5 to 76.2%. In trial 4 in Handan (2000), disease was reduced 74.4, 75.1, and 81.9% by J2, J3, and BB11, respectively, compared to the control. Yield increases with these bacteria ranged from 46.3 to 78.5%. Additional tests were conducted with 1-year-old and 2-year-old formulations of each PGPR strain. Populations of PGPR in the 1-year-old formulations were nearly identical to freshly prepared PGPR formulations. Biological control efficacy was retained by the stored formulations of all three PGPR strains, with 1-year-old formulations providing 68.4–99.5% control and 2-year-old formulations providing 63.4–78.5% control. Yield increases with stored formulations ranged from 35.4 to 67.0%. We conclude that our method of formulating PGPR products provides stable formulations that retain biological control and plant growth-promoting activities.
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