Zuzu and Lyn, that article I quoted is only one of several that say fallen leaves are a source of inoculum for the next year. The American Rose Society site says: "Make sure to clean up the beds completely of all leaves or stems to help keep the disease from wintering over." The Missouri Botanical Garden page for black spot says "Remove diseased leaves. As soon as diseased leaves are noticed, remove and dispose of them. Dead leaves on the ground should also be collected and destroyed. Compost only if this material will not be used back in rose beds. To minimize overwintering of the fungus, collect and remove all leaves from the ground in the fall, mulch with 2–3 inches of leaf mold or fine bark, and prune diseased canes before growth begins in spring."
I also checked on Google Scholar for scientific articles and found this one from the American Society for Horticultural Science dated 2008 in HortTechnology (Evaluation of Griffith Buck Roses for Resistance to Black Spot)
http://horttech.ashspublicatio... - it says "Black spot of roses, caused by the fungus Marssonina rosae (teleomorph = Diplocarpon rosae)........... M. rosae survives unfavorable periods on fallen leaves and diseased canes."
The American Phytopathological Society says "Remove fallen leaves and other infested material and prune out infected stems during the dormant season"
http://www.apsnet.org/publicat...
Perhaps I'm missing something, in which case I'd like to know
- might you have references?
