The occurrence of root pathogens of vining pea was determined in field surveys in Sweden and Denmark from 1989 to 1994. The most serious yield-reducing root pathogen, Aphanomyces euteiches, was found in approximately one-third of the sampled fields in both Sweden and Denmark. In a few fields severely infested with this pathogen, there was a total crop failure. The most frequently isolated pathogens were Phoma medicaginis var. pinodella and Fusarium solani; the latter also was isolated from vascular tissue up to the seventh node level. Other pathogens isolated from roots were F. avenaceum, F. oxysporum, F. culmorum, Chalara elegans, Pythium irregulare, and Mycosphaerella pinodes. In greenhouse pathogenicity tests, A. euteiches caused the most severe root damage and plant death of pea, followed by F. avenaceum and P. irregulare. There was an inverse relationship between field disease severity index and yield for fields infested with A. euteiches.

The influences of the arbuscular mycorrhizal fungus Glomus intraradices on the culturable aerobic-heterotrophic bacterial communities in the rhizosphere and hyphosphere of cucumber plants (Cucumis satvius) were investigated. Mycorrhizal and non-mycorrhizal plants were grown in compartmentalised growth units, each with a root compartment and two lateral root-free compartments. Samples representing rhizosphere, root-free soil, root-free sand and washed sand extract were collected 52 days after sowing from treatments both with and without mycorrhiza. No significant difference in total bacterial number was observed between the mycorrhizal and non-mycorrhizal treatment. Fourteen hundred bacterial colonies were isolated and identified by fatty acid methyl ester analysis using the Sherlock system (MIDI Inc.), 87 species within 48 genera were identified with a similarity index >0.30. Pseudomonas, Arthrobacter and Burkholderia were the genera most frequently encountered. Large differences in bacterial community structure were observed between rhizosphere soil, root-free soil/sand and washed sand extract, whereas major differences between mycorrhizal and non-mycorrhizal treatments were observed for a few bacterial species only. Isolates identified as Paenibacillus spp. were more frequently found in the mycorrhizal treatment and especially in the washed sand extract with mycelium of G. intraradices, indicating that bacteria within this genus may live in close association with mycelium of these fungi.

No abstract

Until now, molecular and biochemical methods have only been used to show whether or not Plasmodiophora brassicae is present in plant or soil samples but not to what extent. Here, in planta quantification of P. brassicae by whole-cell fatty acid (WCFA) measurements and real-time polymerase chain reaction (PCR) was evaluated. Arachidonic acid (ARA, 20:4) was the most abundant fatty acid in resting spores and was only found in infected roots, which indicates a potential of ARA as a biomarker for P. brassicae. A real-time PCR assay was developed using primers designed from the internal transcribed spacer region of the ribosomal DNA. Using these primers, it was possible to detect P. brassicae in infected roots 10 days after germination of plants sown in infested soil. A bioassay showed that the amounts of ARA found by WCFA analysis and the DNA found by real-time PCR in infected plants were well correlated. These measurements also correlated with the soil spore content and the assessed disease incidence and disease severity scores. Therefore, we conclude that WCFA analysis and real-time PCR are good tools for P. brassicae quantification that can be applied to basic studies of the pathogen and in resistance screens.

No abstract