zuzu said:From a 2009 article by Gregg Lowery of Vintage Gardens:
"We are indebted to a scientific study done at the University of California at Berkeley in recent years. It showed that while green leaves can harbor fungal spores, once the leaves die, the fungi die on them.
Thanks, I now found that article. He doesn't, unfortunately, give the scientific reference (nor, I think, does he specifically refer to black spot unless I missed that). I'd really like to see the scientific study referred to, but I've so far been unable to find anything or even anything that agrees with the statement.
I have found several scientific studies that show black spot winters over on dead fallen leaves. This article: Interactions of four pathotypes of Diplocarpon rosae with species and hybrids of Rosa from the journal Plant Pathology -2009, says:
"During the rose growing season, conidia of D. rosae are dispersed by rain splash and animal vectors that include insects and arachnids. The mycelium spreads between the epidermis and cuticle of the upper leaf surface and forms haustoria in the cells of the epidermis and mesophyll. The fungus overwinters on stem tissues and in dead leaves (Cook, 1981). In spring, new infections are initiated from asexual conidia produced in acervuli and sexually produced ascospores formed in apothecia but the sexual stages have rarely been found (Knight & Wheeler, 1977)." (Cook 1981 says: "When overwintered at Wisley, Diplocarpon rosae from Silwood Park alone and not the local strain produced apothecia on fallen rose leaves."
An article on black spot in Descriptions of Fungi and Bacteria 1976 said: "TRANSMISSION: Mainly by splash dispersed conidia formed in acervuli on infected leaves on the host or after they have been cast. A microconidial state (spermagonia) with a similar dispersal mechanism may occur on fallen leaves in the spring and autumn. Conidia lose viability rapidly, few surviving more than one month (24, 508). Overwintering is by saprophytic mycelium in cast foliage or infected stem tissues. The perfect state has been reported from Britain, North America and the USSR (51, 2590 and IMI 185129), where it is formed on infected cast foliage in the spring. Ascospores are forcibly ejected; they do not appear to be essential for the survival of the pathogen."
Even if we assume that it overwinters as saprophytic mycelium in cast foliage, that's still a reason to pick up the dead leaves even if existing spores did not survive that long. As I said, I'd really like to see the study he's referring to or something else that supports it, still skeptical
. Do you have any other references?