Hi Tomcucolo: It depends on your type of soil, and how well-rotted your compost is. Well-rotted compost is neutral in pH, and dry & fluffy. Half-rotted compost is acidic & wet and great for fungal germination. It takes at least 1 year for compost to be well-rotted.

The type of soil matters, only 1/2 hour from here was my LAST house with acidic clay. I mulched with pine bark (pH 4, high in manganese) which encouraged fungal growth. I also piled up leaves (pH 5, wet & acidic) and all my roses had black spots badly in early spring.

In my current garden, I have ALKALINE clay (pH 7.7) and I don't mulch whatsoever. The rose park (15 min. away) with thousands of roses has zero mulch whatsoever. My roses are 100% clean in early spring & mid summer without spraying. I tested mulching with horse manure: clean, IF LIMED and dry & fluffy. There're bacteria in animal manure which suppress fungal growth. Recently the stable switched to wet straw for their horses, I saw mushroom growing for the 1st time, so I stopped getting horse manure.

Last year I tested mulching with alfalfa hay (NPK 3.5-1-2), it was great when the weather was dry, but it was messy when the weather was wet. Alfalfa hay is VERY dry as it ages, and is near neutral in pH. I never see mushroom growing in alfalfa hay as in other types of mulch: cypress, pine, and much less in dyed hardwood chips. Conclusion: it depends on how dry your compost is, and how alkaline it is to suppress fungal growth. Fungus doesn't like it DRY nor alkaline, that's why baking soda with pH 8.3 is effective for black spot.