Ecology of Hymexazol-Insensitive Pythium Species in Field Soils
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- Ali-Shtayeh, M., Salah, A.M. & Jamous, R.M. Mycopathologia (2003) 156: 333. doi:10.1023/B:MYCO.0000003561.47451.f7
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Soils from 100 irrigated fields (95 under vegetables, 5 under citrus) indifferent geographical locations in the West Bank (Palestinian Autonomous Territory) were surveyed forhymexazol-insensitive (HIS) Pythium species using the surface soil dilution plate (SSDP) method withthe VP3 medium amended with 50 mg/L hymexazol (HMI) (VP3H50), over a period of 12 months. HIS Pythiumspecies were isolated from 37% of the soils surveyed, with mean population levels ranging from4.3–1422 CFU g-1 dry weight. Eight HIS Pythium taxa were recovered on the VP3H50 medium, the mostabundant of which was P. vexans (found in 29% of field soils surveyed). Seasonal variations inpopulation levels of HIS Pythium species were studied in four fields over a period of 12 months. Significantseasonal variations in HIS population levels were detected in the four fields, with the highest population levelsof HIS Pythium spp. encountered in spring and the lowest population levels in winter in three of thefields surveyed. Effects of HMI on linear growth and colony morphology of 149 Pythium ssp. isolates wereexamined on CMA amended with HMI at five concentrations. Pythium vexans isolates responded differentlyfrom those of the other Pythium species. Isolates of this important pathogen were more insensitive to HMI athigh concentrations than the other main species tested. A large proportion of the P. ultimum isolateswas either insensitive or weakly sensitive to HMI. Furthermore, a few isolates of other Pythium specieswere insensitive to the fungicide at various concentrations. The colony morphology of P. vexans isolateswas not affected by HMI, whereas colonies of the other species showed sparse growth on the HMI amended mediumrelative to the control. The pathogenicity of P. vexans and P. ultimum isolates to cucumber seedlingswas examined in growth chambers. Insensitive isolates of both species were found to be more virulentdamping-off pathogens than the sensitive isolates. The present study demonstrates that HMI can not be usedeffectively in controlling Pythium spp. in soil inhabited with high densities of HIS Pythium spp. pathogens.