Purified cultures were prepared on solidified nutritive agar which was then used to infect sterilised oats. Once inoculated, these oats were placed in sterile water and incubated under light conditions at room temperature to stimulate the production of sporangia (the capsules within which the motile spores grow). Release of the zoospores was initiated following a short period of cold shock. The released zoospores were collected using a syringe. The procedure is described in detail in the SI Section S2 (Phytophthora cactorum zoospore production) and the Video 1 shows the release process of zoospores. The zoospore preparations were discarded after 8 hr.