Population dynamics of metalaxyl‐sensitive and metalaxyl‐resistant isolates of Phytophthora infestans in untreated crops of potato

Each of six pairs of metalaxyl‐sensitive (MS) and metalaxyl‐resistant (MR) field isolates of Phytophthora infestans was inoculated as a mixture of 90% MS sporangia with 10% MR sporangia onto fungicide‐free potato plants (cv. Alpha) in six walk‐in plastic tunnels in each of 2 years. In both experiments, late blight killed the crops in 26‐31 days. In all tunnels, the frequency of MR sporangia increased during the logarithmic phase of the epidemic to 70‐85% of the population. Later, however, the frequency of M R sporangia decreased to 7‐38% in six tunnels but continued to increase to 94‐100% in the other six. The increased frequency of MR during the logarithmic phase was attributed to the shorter latent period and larger lesion size of MR compared to MS isolates and to the unlimited availability of uninfected host tissue. The later decrease in the frequency of MR in six tunnels was attributed to the lower sporulation capacity and the shorter infectious period of MR compared to MS isolates, and to the limitation of uninfected host tissue. The data showed that the population dynamics of P. infestans was dependent not only on fitness components of isolates, but also on the availability of uninfected host tissue.