Recently a member of our rose society called me and invited me to come to his rose garden and discuss a problem that he was having with black spot, the most common fungus affecting roses in Indiana. As a consulting rosarian, I was most happy to oblige. I thought my discovery was interesting and worthy of writing an article for the newsletter. This article is not meant, in any way, to criticize the horticultural efforts of the member, as he was doing a very good job of tending to his roses.

Upon arrival at his residence, Paul showed me around his garden. He is growing about 25 roses half of which are on the north side of his house and the rest are along a board privacy fence on the south side of his yard. Paul has a regular fungicidal spray program. He applies the fungicide with a hose end type sprayer. There was little or no black spot on the roses located on the north side of his home. However, the roses on the south side along the board fence had quite a bit of black spot. Paul had removed the infected leaves from the plants and generally kept the beds tidy.

Because of a crab apple tree, located about 35 feet from the fence bed, the roses did not get direct sunlight until about 1:00 PM. The roses were planted far enough apart, but because of the fence they did not always get good drying winds. Since the black spot fungus requires the spore to be immersed in free standing water for at least 7 hours before it can germinate, growing roses in a area that gets morning sun and good air circulation are very important in controlling the black spot fungus.

Paul was spraying his roses regularly, and that would ordinarily overcome the poor location of his rose beds. I thought the poor air circulation around the roses was essentially the root cause of Paul’s black spot problem. As I was discussing the solution to the problem with Paul, we were standing under the nearby crab apple tree.

I looked down and noticed that many leaves had fallen off of the crab apples tree; the leaves were yellow and had black spots.

In an instant I knew why Paul was having a problem. The tree was infected with black spot which is exactly the same fungus that affects roses. Upon closer inspection I noticed many of the crab apple leaves in the rose beds. They had simply fallen or were blown into the beds by the wind. With such a massive amount of black spot spore available to infect the roses, it would be an almost impossible task to keep the plants disease free. I jokingly told Paul to cut the tree down as it would give the roses more light and would probably solve his black spot problem. Seriously, I told Paul that he had to keep up a good spray program and make every effort to keep the infected crab apple leaves out of his rose bed.

For your information, the black spot fungus is very host specific and it only affects members of the botanical family which includes roses. Some members of the rose family are: crab apples, of course, shadblow and serviceberry trees, hawthorns, pears, quinces, apples, peaches, plums, cherry laurels, and white beams. This is a very large family of plants. Just remember, if you have your rose garden located around these trees be on the lookout for leave infected with the black spot fungus and take action accordingly. Thanks Paul for bringing this problem to my attention. Your concern taught me something and now I have been able to pass it on to our members and, hopefully, they will also have learned something.