The ability of oospores of P. infestans to form in the leaves and stems of pot-grown potato plants when the Al and A2 strains originate from different infector plants within a plot was tested in semi-controlled conditions inside a polythene tunnel. Oospores were found in 9 out of 64 potato plants in the experiment, with the Al and A2 isolates found intermixed on 17 plants. The resultant blighted plant material was left to decompose from October to April after which the detritus was assayed for any surviving P. infestans propagules. Sexual recombinants were detected from three different potato plants from the experiment with a total of 6 recombinant isolates recovered. The effect of the carbamate fungicide propamocarbHCl on oospore formation in leaf disks, in pot-grown potato plants and field-grown potato plants was also assessed. PropamocarbHCl was found to inhibit the fom1ation of oospores at concentrations where asexual reproduction was not affected. In pot-grown potatoes the fom1ation of oospores was found to be dependant on temperature, with oospores formed at l 8°C, but not at 5 or 24°C. PropamocarbHCl was found to have no effect on the gennination of oospores in vitro at concentrations ranging from Oto 500μg/mL. The inheritance of sensitivity to propamocarbHCl was also studied. The level of spornlation and the latent period of 15 progeny from each of two crosses of P. infestans was tested in the presence of propamocarbHCl on 5 different potato cultivars (King Edward, Home Guard, Maris Piper, Tina and Stormont Enterprise) differing in their horizontal resistance to late-blight. It was determined that offspring that are more tolerant to propamocarbHCl than parental isolates could be generated through sexual reproduction, although this was expressed differently on different potato cultivars. Some coITelation was found between latent period of offspring in the absence of propamocarbHCI and their propamocarbHCl sensitivity.