Abstract
Attenuated stock cultures of Phytophthora infestans resemble some variants arising from single zoospore propagation in cultural and pathogenic characters. To investigate this similarity a mass culture and four single zoospore derivatives were successively transferred on to potato tubers. No changes were observed in ability to infect tubers or rate of growth for the parent isolate, one highly aggressive zoospore culture or one non-aggressive zoospore culture. The other two zoospore cultures showed a marked improvement in these two components of aggressiveness during the training programme. A similar result was obtained with tubers from three potato varieties. The response of the single zoospore cultures to serial transfer is compared with that of attenuated stock cultures, and it is suggested that the processes of attenuation in culture and rejuvenation on host tissue may be explained by changes in the selection pressures acting upon the variable cytoplasmic system of the fungus.
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