The initial incidence of Phytophthora fragariae infection of Merton Princess strawberries was not affected by different cultivation systems. Healthy plants planted in infested soil in Apr. and sampled at 35 d intervals showed symptoms of red core on 1 Sep. Disease severity increased with time and, when averaged over intervals of 70 d, the greatest increases occurred between the end of Jul. and the beginning of Oct. and between Dec. and Feb. Whereas the former was due to infection of plants in which the total numbers of new roots were low, the latter was due to an increase in the numbers of diseased roots. Most primary roots were produced in Oct. and early Nov. and 75% of the final total were formed by 15 Dec. Healthy Cambridge Favourite and Redgauntlet plants dug from the field at monthly intervals and challenged with zoospores, were less susceptible in May and Jun. than in other months. It is suggested that a fungicide applied in late Sep. or Oct. would protect the max. number of new roots and achieve the opt. effect for a single application.