Development of Resistance to Hymexazol Among Pythium Species in Cucumber Greenhouses in Oman.

A study was conducted to characterize the common Pythium spp. in greenhouses in Oman and their level of resistance to hymexazol, a widely used fungicide in the country. Pythium isolates were obtained from soil samples, cocopeat bags, and cucumber roots collected from seven regions in the country. Identification of 80 Pythium isolates to the species level using sequences of the internal transcribed spacer region of the ribosomal RNA showed that they belong to four species: Pythium aphanidermatum (77 isolates), P. spinosum (1 isolate), P. myriotylum (1 isolate), and P. catenulatum (1 isolate). Investigating the aggressiveness of three Pythium spp. on cucumber showed that P. aphanidermatum, P. myriotylum, and P. spinosum are pathogenic. Phylogenetic analysis of P. aphanidermatum isolates showed that most of the isolates obtained from cocopeat clustered separately from isolates obtained from soil and roots. This may indicate a difference in the origin of the cocopeat isolates. Evaluating the resistance of 27 P. aphanidermatum isolates to hymexazol showed that most isolates were sensitive (0.9 to 31.2 mg liter-1) whereas one isolate was resistant (142.9 mg liter-1). This study is the first to report P. myriotylum and P. catenulatum in Oman. It is also the first to report the development of resistance to hymexazol among P. aphanidermatum populations from greenhouses. Growers should use integrated disease management strategies to avoid further development of resistance to hymexazol.

Species or Genotypes? Reassessment of Four Recently Described Species of the Ceratocystis Wilt Pathogen, Ceratocystis fimbriata, on Mangifera indica.

Ceratocystis wilt is among the most important diseases on mango (Mangifera indica) in Brazil, Oman, and Pakistan. The causal agent was originally identified in Brazil as Ceratocystis fimbriata, which is considered by some as a complex of many cryptic species, and four new species on mango trees were distinguished from C. fimbriata based on variation in internal transcribed spacer sequences. In the present study, phylogenetic analyses using DNA sequences of mating type genes, TEF-1α, and ß-tubulin failed to identify lineages corresponding to the four new species names. Further, mating experiments found that the mango isolates representing the new species were interfertile with each other and a tester strain from sweet potato (Ipomoea batatas), on which the name C. fimbriata is based, and there was little morphological variation among the mango isolates. Microsatellite markers found substantial differentiation among mango isolates at the regional and population levels, but certain microsatellite genotypes were commonly found in multiple populations, suggesting that these genotypes had been disseminated in infected nursery stock. The most common microsatellite genotypes corresponded to the four recently named species (C. manginecans, C. acaciivora, C. mangicola, and C. mangivora), which are considered synonyms of C. fimbriata. This study points to the potential problems of naming new species based on introduced genotypes of a pathogen, the value of an understanding of natural variation within and among populations, and the importance of phenotype in delimiting species.

Genetic analysis of 'Candidatus Phytoplasma aurantifolia' associated with witches' broom on acid lime trees.

"Candidatus Phytoplasma aurantifolia" is associated with witches' broom disease of lime in Oman and the UAE. A previous study showed that an infection by phytoplasma may not necessarily result in the physical appearance of witches' broom symptoms in some locations in Oman and the UAE. This study investigated whether phytoplasma strains belonging to "Ca. P. aurantifolia" (based on the 16S rRNA gene analysis) in locations where disease symptoms are expressed are different from phytoplasma in locations where disease symptoms are not expressed. About 21 phytoplasma strains (15 from areas and trees with disease symptoms and six from areas and trees without disease symptoms) were included in the analysis. The study utilized sequences of the imp and SAP11 genes to characterize the 21 strains. Phylogenetic analysis of both genes showed that the 21 strains are similar to each other and to reference strains in GenBank. The study shows that there is a low level of diversity among all phytoplasma strains. In addition, it shows that phytoplasma in places where witches' broom symptoms are not expressed are similar to phytoplasma in places where disease symptoms are expressed. This may suggest that disease expression is not linked to the presence of different phytoplasma strains, but may be due to other factors such as weather conditions.