Pseudomonas CMR12a was previously selected as an efficient biocontrol strain producing phenazines and cyclic lipopeptides (CLPs). In this study, biocontrol capacity of Pseudomonas CMR12a against Rhizoctonia root rot of bean and the involvement of phenazines and CLPs in this ability were tested. Two different anastomosis groups (AGs) of Rhizoctonia solani, the intermediately aggressive AG 2-2 and the highly aggressive AG 4 HGI, were included in growth-chamber experiments with bean plants. The wild-type strain CMR12a dramatically reduced disease severity caused by both R. solani AGs. A CLP-deficient and a phenazine-deficient mutant of CMR12a still protected bean plants, albeit to a lesser extent compared with the wild type. Two mutants deficient in both phenazine and CLP production completely lost their biocontrol activity. Disease-suppressive capacity of CMR12a decreased after washing bacteria before application to soil and thereby removing metabolites produced during growth on plate. In addition, microscopic observations revealed pronounced branching of hyphal tips of both R. solani AGs in the presence of CMR12a. More branched and denser mycelium was also observed for the phenazine-deficient mutant; however, neither the CLP-deficient mutant nor the mutants deficient in both CLPs and phenazines influenced hyphal growth. Together, results demonstrate the involvement of phenazines and CLPs during Pseudomonas CMR12a-mediated biocontrol of Rhizoctonia root rot of bean.

Aims:  To examine the effect of previously described bacterial antagonists on the viability of Verticillium microsclerotia in vitro and to elucidate the possible modes of action of bacterial strains in the suppression of Verticillium microsclerotia viability. Methods and Results:  A microplate assay was developed to test the suppressive effect of well‐defined Pseudomonas spp. on the viability of Verticillium microsclerotia in vitro. Experiments using phenazine‐ and biosurfactant‐deficient mutants indicated that biosurfactants and phenazine‐1‐carboxylic acid play a role in the suppression of microsclerotia viability by Pseudomonas spp. In addition, microsclerotia colonization tests revealed that Pseudomonas spp. are able to colonize the surface of the microsclerotia, but not the inner matrix. Growth response curves showed that the population levels of Pseudomonas spp. increased when they were in the vicinity of Verticillium microsclerotia, indicating that Pseudomonas spp. may utilize nutrients from the microsclerotia for their growth. Conclusions:  Pseudomonas spp. seem to be good candidates for Verticilllium microsclerotia biocontrol. Biosurfactant production is one of the main mechanisms involved in their mode of action. Significance and Impact of the Study:  This line of work may contribute to a better understanding of biological control agents and their working mechanisms.

Sixty-two isolates of Rhizoctonia spp. were collected from Belgian cauliflower fields during 2005 and 2006. The majority of the isolates (60 out of 62) had multinucleate cells and were identified as Rhizoctonia solani . Characterization of anastomosis groups (AGs) was performed using pectic zymograms, PCR-RFLP and sequencing of the rDNA-ITS region. The most prevalent AG was AG 2-1 (55% of isolates), followed by AG 2-1 subset Nt (11%), AG 1-1C (8%), AG 5 (8%), AG 4 HGII (6%), AG 3 (5%) and AG 1-1B (3%). Pathogenic potential towards different vegetable crops and towards maize was determined. Damage to cauliflower and endive was caused by different AGs, with the isolates aggressive towards cauliflower belonging to AG 2-1, AG 2-1 subset Nt, AG 4 HGII, AG 1-1C, AG 1-1B and AG 2-2, and those aggressive towards endive belonging to AG 1-1B, AG 1-1C, AG 2-1 subset Nt, AG 2-2, AG 4 HGII and AG 5. The most aggressive isolates towards bean belonged to AG 2-1 subset Nt and AG 2-2, for lettuce to AG 1-1B and AG 2-1, on carrot to AG 4 HGII and towards maize to AG 2-2. Within the isolates of AG 2-1, variability was observed in PCR-RFLP pattern and in aggressiveness towards several crops, indicating this subgroup to be heterogeneous. This is the first study concerning the occurrence of R. solani AGs causing wirestem in Belgian cauliflower fields and the first report of aggressive isolates of AG 1-1C, AG 2-1 subset Nt and AG 4 HGII associated with cauliflower.

Background: The soil borne fungus Rhizoctonia is one of the most important plant pathogenic fungi, with a wide host range and worldwide distribution. In cauliflower (Brassica oleracea var. botrytis), several anastomosis groups (AGs) including both multinucleate R. solani and binucleate Rhizoctonia species have been identified showing different levels of aggressiveness. The infection and colonization process of Rhizoctonia during pathogenic interactions is well described. In contrast, insights into processes during interactions with weak aggressive or non-pathogenic isolates are limited. In this study the interaction of cauliflower with seven R. solani AGs and one binucleate Rhizoctonia AG differing in aggressiveness, was compared. Using microscopic and histopathological techniques, the early steps of the infection process, the colonization process and several host responses were studied.

Basal rot is a common disease in lettuce greenhouses. A 3-year study on the diversity of pathogens associated with basal rot in Belgium was carried out. A total of 150 isolates were collected originating from 56 greenhouses. Four pathogens appeared to be involved. Rhizoctonia solani was found to be the causal agent at 23 locations, Sclerotinia spp. at 14, Botrytis cinerea at 17 and Pythium spp. at seven. The isolates of R. solani were further characterised to anastomosis groups and subgroups using morphological characteristics, pectic zymogram and PCR-RFLP. Five anastomosis groups could be distinguished: AG1-1B, AG4 HGI, AG10, AG2-1, AG2-1 Nt and AG3, with isolates of AG4 HGI and AG1-1B being the most prevalent and the most aggressive. Sclerotinia sclerotiorum was found at 13 locations, while S. minor was found at only one location. Based on ITS-sequencing Pythium isolates were assigned to three different species. At 20A degrees C, isolates of all pathogens were able to cause lesions on detached lettuce leaves, except isolates of R. solani AG3 and AG2-1 Nt. A correlation could be found between the occurrence of the pathogens and the growing season. Botrytis cinerea was the most common pathogen in winter, whereas R. solani was most frequently isolated in summer. Sclerotinia spp. and Pythium spp. were isolated in spring, summer and autumn. The information obtained in this study will be most useful in the development of an alternative control strategy for causal agents of basal rot