Yes, phytoparasitic fungi can co-occur on the same host. They don't have to be competitive but can be reinforcing: If fungus A infects and weakens the plant, fungus B will find it easier to infect the plant as well.
Is there any quantitative work on competitive/facilitative interactions? Often it's quite qualitative/anecdotal unfortunately. Maybe I'll do something with Puccinia lagenophorae and Pustula senecionis next year bc it shouldn't be hard to show the ratio is/isn't significantly different to random.
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https://www.researchgate.net/publication/326977256_Contribution_to_the_ecology_of_Peronospora_violacea_Berk