Joke Pannecoucque

Biosurfactants are involved in the biological control of Verticillium microsclerotia by Pseudomonas spp.

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.

Detection of rDNA ITS polymorphism in Rhizoctonia solani AG 2-1 isolates

The sequence variability of the ribosomal internal transcribed spacer regions ITS1 and ITS2, including the 5.8S gene, was investigated for Rhizoctonia solani isolates of anastomosis group (AG) 2-1. During PCR RFLP analysis of eight isolates, the restriction patterns of four isolates showed an excess of bands after restriction with the enzymes AvaII and/or HincII, which suggested the presence of more than one ITS region. By cloning the ITS region of six isolates sequence heterogeneity was detected in the isolates that showed an excess of bands in the PCR RFLP analysis; up to nine different ITS regions were identified within one isolate. The same level of diversity was found within the same isolate as among isolates. In the phylogenetic tree based on the rDNA ITS sequences of several AG 2-1 isolates, sequences derived from the same isolate did not form distinct clusters, questioning the relevance of further subdivision of heterogeneous AG 2-1 isolates based on the ITS region.

Characterisation of fungal pathogens causing basal rot of lettuce in Belgian greenhouses

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 20°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.

Interactions between cauliflower and Rhizoctonia

BackgroundThe 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.ResultsFor aggressive Rhizoctonia AGs (R. solani AG 1-1B, AG 1-1C, AG 2-1, AG 2-2 IIIb and AG 4 HGII), a higher developmental rate was detected for several steps of the infection process, including directed growth along anticlinal cell walls and formation of T-shaped branches, infection cushion formation and stomatal penetration. Weak or non-aggressive AGs (R. solani AG 5, AG 3 and binucleate Rhizoctonia AG K) required more time, notwithstanding all AGs were able to penetrate cauliflower hypocotyls. Histopathological observations indicated that Rhizoctonia AGs provoked differential host responses and pectin degradation. We demonstrated the pronounced deposition of phenolic compounds and callose against weak and non-aggressive AGs which resulted in a delay or complete block of the host colonization. Degradation of pectic compounds was observed for all pathogenic AGs, except for AG 2-2 IIIb. Ranking the AGs based on infection rate, level of induced host responses and pectin degradation revealed a strong correlation with the disease severity caused by the AGs.ConclusionThe differences in aggressiveness towards cauliflower observed among Rhizoctonia AGs correlated with the infection rate, induction of host defence responses and pectin breakdown. All Rhizoctonia AGs studied penetrated the plant tissue, indicating all constitutive barriers of cauliflower were defeated and differences in aggressiveness were caused by inducible defence responses, including cell wall fortifications with phenolic compounds and callose.

Interactions between cauliflower and Rhizoctonia anastomosis groups with different levels of aggressiveness

BackgroundThe 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.ResultsFor aggressive Rhizoctonia AGs (R. solani AG 1-1B, AG 1-1C, AG 2-1, AG 2-2 IIIb and AG 4 HGII), a higher developmental rate was detected for several steps of the infection process, including directed growth along anticlinal cell walls and formation of T-shaped branches, infection cushion formation and stomatal penetration. Weak or non-aggressive AGs (R. solani AG 5, AG 3 and binucleate Rhizoctonia AG K) required more time, notwithstanding all AGs were able to penetrate cauliflower hypocotyls. Histopathological observations indicated that Rhizoctonia AGs provoked differential host responses and pectin degradation. We demonstrated the pronounced deposition of phenolic compounds and callose against weak and non-aggressive AGs which resulted in a delay or complete block of the host colonization. Degradation of pectic compounds was observed for all pathogenic AGs, except for AG 2-2 IIIb. Ranking the AGs based on infection rate, level of induced host responses and pectin degradation revealed a strong correlation with the disease severity caused by the AGs.ConclusionThe differences in aggressiveness towards cauliflower observed among Rhizoctonia AGs correlated with the infection rate, induction of host defence responses and pectin breakdown. All Rhizoctonia AGs studied penetrated the plant tissue, indicating all constitutive barriers of cauliflower were defeated and differences in aggressiveness were caused by inducible defence responses, including cell wall fortifications with phenolic compounds and callose.

Comparison of image analysis and direct measurement of UPOV taxonomic characteristics for variety discrimination as determined over five growing seasons, using industrial chicory as a model crop

Tests for distinctness, uniformity and stability (DUS) are an essential component of variety registration and granting plant breeders’ rights. This study, performed on industrial chicory (Cichorium intybus L.), included (1) an evaluation of the discriminating power of the UPOV characteristics and (2) a comparison of the root shape between the UPOV-based characteristics and characteristics determined via image analysis. Five consecutive growing periods were studied to assess the long-term stability of the parameters. Lack of discriminating power of characteristics for examining distinctness may impede breeding progress by failing to discriminate novel candidate varieties. However, a balance needs to be set so that newly introduced characteristics or new evaluation methodologies are not too powerful as to damage the protection of registered varieties. The results showed great differences in the discriminating power of the UPOV characteristics. The elimination of some of these low-discriminating characteristics would probably result in savings, but would have no consequences on the decisions taken by DUS testing authorities. Image analysis-derived characteristics could either replace currently-used parameters or provide additional characteristics with good discriminating power, determined in an objective and standardized way. Here we discuss possible implications for variety registration.