The summers here are hot and very humid (90's). Black spot is prevalent - runs rampant. Drought has been another problem to contend with - reflected in a higher water bill.
A couple of years ago, I read an article that horse manure helps reduce black spot. Last Fall I was conversing with someone from Palatine Roses. The discussion lead to the type of materials used to hill up roses for winter protection (topsoil, compost, mulch etc.). And than she mentioned - "and 1 shovelful of horse manure". Last Fall I added 1 shovelful of well rotted horse manure to each rose bush. This one time application appeared to result in much less blackspot than previous years.
I am impressed with the work of Dr. Rumen Conev at the Vineyard Research Center, however, I will continue to nurture my favorite roses utilizing the manure each Spring and Fall.
I have no answer as to why the manure seemed to help eliminate the blackspot. Did it effect that particular strain of fungi? Changed the pH? Strengthen the immune system of the plant to ward off the spores/infection?
