Characterisation of Phytophthora nicotianae isolates causing root and stem rot of tobacco
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Studies were undertaken to evaluate physiological and morphological variation, pathogenicity and races of 19 P. nicotianae isolates collected during the period 1985-1996 from infected tobacco in Zimbabwe. The growth of isolates was examined at a range of temperatures and potassium chloride-mediated osmotic potentials. Sporangia, oospore, oogonium and chlamydospore diameters were measured. Pathogenicity of isolates was determined on seedlings of Nicotiana tabacum and Lycopersicorn esculentum in greenhouse experiments. Optimum temperatures for growth of isolates were 25 °C and 30 °C. All isolates failed to grow at 37 °C. Growth of most isolates, based on a micro-well technique, was maximum at -0.14 and -1.00 MPa, and decreased at reduced water potentials. There were significant differences in the growth of an isolate at various temperatures and water potentials (p= 0.05). Length/breadth ratios of sporangia were 1.06:1 to 1.64:1. Chlamydospore, oogonia and oospore mean diameters were 22.5-27.5 pm, 22.5-32.5 pm and 20.0-24.0 pm, respectively. Although all isolates attacked tobacco and tomato, there were significant differences (p=0.05) in their pathogenicity. Significant differences (p = 0.05) insusceptibility of tomato cultivars to isolates were also observed. Based on the results of the tomato assay, all isolates were designated race 0.