Pathogenicity and genetic diversity of Pythium species associated with damping-off of cucumbers in Oman.

A total of 166 isolates of Pythium spp. were recovered from 282 soil and root samples collected from cucumber roots and rhizosphere soil of 10 Governorates of Oman, from July 2019 to July 2020. Based on ribosomal DNA (rDNA) internal transcribed spacer (ITS) regions, 4 species of Pythium were identified. Pythium aphanidermatum was the most abundant species (86.75 %), followed by P. spinosum (10.24 %), P. myriotylum (1.2 %) and P. splendens (0.6 %). Pathogenicity to cucumber of 34 isolates including P. aphanidermatum (26) P. spinosum (8), P. splendens (1) and P. myriotylum (1) was tested. P. aphanidermatum, P. spinosum and P. myriotylum developed symptoms of dampingoff with a range of 86.7 to 100%, while P. splendens was avirulent. Tolerance tests of 15 isolates of P. aphanidermatum to hymexazol (0, 10, 100 and 1000 µL L-1) revealed that this species could grow in the presence of up to 100 µL L-1hymexazol while growth was inhibited at 1000 µL L-1with P. aphanidermatum Br010 from Al Buraimi, exhibiting the highest level of tolerance followed by Br013. Phylogenetic analyses were conducted based on the sequences of internal transcribed spacer (ITS) and COI (cytochrome c oxidase subunit I) genes. Pythium species were separated into different clades based on similarities to ITS and COI. Amplified fragment length polymorphism fingerprinting among 93 strains of P. aphanidermatum produced 93 genotypic features and 781 polymorphic loci. Overall, genetic diversity of P. aphanidermatum populations in the country was low (H=0.1101). Analysis of molecular variance (Fst= 0.11435) revealed gene flow and a moderate level of genetic variability among P. aphanidermatum populations. It is concluded that P. aphanidermatum is the most prevalent species, mainly involved in damping-off disease of cucumbers in the greenhouses in Oman. Most of these isolates exhibited differences in susceptibility to fungicide and showed no resistance to hymexazol. [ABSTRACT FROM AUTHOR]