Molecular identification and pathogenicity of Rhizoctonia solani and Pythium spp. associated with damping-off disease on baby leafy vegetables in Greece

In the last years, leafy vegetables cultivated as baby leaves have been established in the market and have attracted the interest of consumers throughout the world. During the growing seasons of 2019 and 2020, 97 isolates of Rhizoctonia solani and 112 isolates of Pythium spp. were obtained from baby leaf vegetables exhibited damping-off symptoms. Representative isolates of R. solani from each surveyed plant species were characterized using sequence analysis of the internal transcribed spacer (rDNA-ITS) region. Isolates were identified as belonging to four anastomosis groups (AGs): AG2-1, AG-IB, AG4-HGI and AG4-HGIII. AG4-HGI was the most prevalent group and phylogenetic analysis showed that the isolates were distinctly separated according to their AGs. Pathogenicity among the four AGs on 23 plant species varied considerably, from not susceptible to highly susceptible, while, in general, AGs did not exhibit host specificity. Furthermore, a total of 112 Pythium spp. isolates were obtained. The ITS region and the cytochrome oxidase II (coxII) gene were amplified, and three Pythium spp. were identified (P. ultimum, P. aphanidermatum and P. sylvaticum), which were used further for maximum-likelihood phylogenetic analysis. The pathogenicity of representative isolates was assessed in vitro and in vivo on 10 plant species. In general, all three tested Pythium spp. were virulent when used in vitro, while P. ultimum was the most virulent in vivo. This is the first comprehensive study aimed at determining the occurrence of specific R. solani AGs and Pythium spp. derived from baby leafy vegetables exhibiting damping-off symptoms in Greece.