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Abstract 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. Results For 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. Conclusion The 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.

Sixty isolates of Rhizoctonia spp. were obtained from Cuban bean fields during the period 2004–2007. Isolates were characterized with different techniques, including nuclei staining, pectic zymogram, PCR–RFLP analysis of the rDNA–ITS region and sequencing of the rDNA–ITS region. The majority of the isolates were identified as multinucleate Rhizoctonia solani isolates, representing two different anastomosis groups (AGs), AG 2-2 WB and AG 4 HGI; the remaining isolates were binucleate Rhizoctonia isolates and belonged to AG F and AG A. AG 4 HGI isolates were equally distributed in all soil types; AG 2-2 isolates were more frequently isolated from cambisols, whereas AG F isolates were related to calcisols. Pathogenicity experiments in vitro and in the greenhouse, revealed that binucleate isolates only caused root rot, whereas R. solani isolates were able to cause root rot and hypocotyl rot. Furthermore, differences in virulence level were observed between R. solani and binucleate isolates and among different AGs. Isolates of R. solani AG 4 HGI and R. solani AG 2-2 WB were the most aggressive, binucleate isolates of AG F were intermediate aggressive, whereas a binucleate isolate of AG A was weakly aggressive. In contrast with other reports about R. solani in bean, web blight symptoms were never observed during this study. [ABSTRACT FROM AUTHOR]