Your experiment would involve different test groups of varying density levels in otherwise controlled environments.
Work Step by Step
You would need to create a set of test populations under all similar circumstances except for the overall population density of the plant. You would then have to observe for signs of fungal infection for whatever time period was relevant for this infection—let’s say a season. Your test population densities should range from lower than observed in the wild to higher than is observed in the wild. You would theoretically want to conduct this experiment over several seasons to guarantee that any results observed in one season are not a fluke.