It’s that time of the year that we begin to see the symptoms of disease on various garden plants. One of the most common problems that I am seeing at this time is black spot on roses.
The black spot on roses are caused by the fungus Diplocarpon rosae and appear on the upper surface of the leaves that are usually surrounded by extensive yellowing. When the symptoms are heavy, these yellow and black areas tend to coalesce and eventually the leaf drops.
If left unchecked, the disease can eventually weaken the plant which leads to fewer blooms. It also opens the rose up to other stresses that can lead to further damage of the plant.
The fungus overwinters on canes and leaves that stay in the garden. In the spring, when conditions are right, spores are splashed to new leaves and stems. These then become infected.
Ideal environmental conditions for infection are leaf wetness, humidity, and temperatures above 75. Since we live in Ohio and tend to see these conditions every growing season, it’s very likely that if you have a rose that is susceptible to black spot, it will be infected.
In order to manage diseases, one needs to understand the disease triangle. This is a very important concept taught in the plant pathology world and to this day has served me well in the understanding of disease management.
There are three components to the disease triangle: a pathogen, a host plant and the right environmental conditions.
For black spot on roses, the susceptible (more on this below) rose plant, the pathogen is the fungus mentioned above and the environmental conditions are also mentioned above. When all of these are present in the landscape, the disease black spot on roses will occur.
One of the steps to manage a disease is to manipulate one of the three components of the triangle. So let’s take a look at each of them in this example of black spot on roses.
The first is the host plant. I mentioned a susceptible plant. Some roses are more susceptible to this disease than others. There are also varieties that are resistant to black spot, therefore, they won’t get the disease.
The next leg of the triangle is the environment. While it’s impossible to control natural weather conditions, there are a few things you can do to manipulate the environment around a plant.
For instance, make sure you have good air circulation around plants and that they aren’t crowded, preventing drying of the foliage. In addition, if you don’t get all-day sun on your roses, at least make sure they receive morning sun in order to dry off any dew.
Keep plants pruned properly to allow air circulation in the center of the plant. If you are going to water, water in the morning so that foliage dries and keep water off the foliage as much as possible.
Sanitation is very important and in the case of black spot, pick up and discard disease leaves and canes so that continual infection won’t occur.
The next is the pathogen or the fungus. In order to manipulate this, you must spray a preventive fungicide to keep the spores from entering the leaf surface.
All of these together help to minimize the impact of a disease.