Abstract Phytophthora species are plant pathogens that cause considerable damage to agrosystems and ecosystems, and have a major impact on the economy. Infection occurs when their biflagellate zoospores move and reach a root on which they aggregate. However, the communication between the plant and the zoospores and how this communication modifies the behavior of the swimming zoospores is not yet well characterized. Here we show that using a microfluidic device comprising a growing Arabidopsis thaliana root, we are able to study the kinetics of Phytophthora parasitica zoospores approaching the root and aggregating on a specific area, in real time. We show that the kinetics of zoospores is modified only below a distance of about 300 μ m from the center of aggregation, with a decrease in the speed coupled with an increase in the number of turns made. In addition, we show that the rate of aggregation is constant throughout the experiment, approximately one hour, and depends on the density of zoospores. The rate is consistent with a random encounter of zoospores with the root, indicating that no long range signal is evidenced in our set-up.