Roses have not always been plagued by disease. If that were the case, they would not have survived for thousands of years throughout the world as native plants.
Man, in his quest for the new and different, has breed much of the disease resistance out of the modern roses we are familiar with today.
So what can we do about rose disease? First, select roses that are known to be disease resistant in this area – Earth Kind™ roses are good examples of those. This is not to say that an Earth Kind™ rose will not get a rose disease. It is to say that the rose will be less likely to pick up the disease and if it does, will continue to perform well in spite of it. Often, such roses will simply outgrow the problem, such as is the case with powdery mildew.
Second, pay close attention to airflow around the bushes. This comes into play not only when choosing a location, but also when planting the bushes. A good location will be of minimal benefit if the bushes are planted too close together. Listed height and width dimensions are simply an average and do not take into account our longer growing season. A good rule-of-thumb is to plan on the bush growing 1/4th to1/3rd larger than the size listed and plant accordingly. Prune in the spring with attention to airflow. Are there several canes growing close together or crossing each other? Remove enough canes to allow air to flow into the center of the bush.
Third, keep a clean garden by removing old leaves and debris from around the plants. Disease spores can overwinter and re-infect the plant when water droplets splash on the debris, then on the new leaves.
Fourth, learn what the diseases are that affect roses in this area. Two fungal diseases – powdery mildew and blackspot are the ones that plague local gardeners the most.
Powdery mildew affects the young, tender, new plant growth in the spring. Cool temperatures combined with wet, damp spring weather brings about the worst powdery mildew. However, since we typically do not have long-lasting spring weather, plants will outgrow most powdery mildew afflictions when the temperatures become warm and the days dryer. Fungicides can be used, if the problem is severe enough – just follow the label instructions.
Blackspot, however, can be a plant killer. This is due to toxins released by the fungus that cause leaf death and drop. After this occurs, a rose bush can no longer manufacture food; protect its canes from intense sunlight; keep its roots cool; or transpire effectively. No fungicide will get rid of the spots already on a leaf – these are actually spore cells waiting to be released. Prevention is the only real way to ensure minimal blackspot issues within the rose garden, either by applying the aforementioned techniques and/or fungicide application throughout the growing season.
Two other diseases of note are rust – a reddish powdery fungus seen on the underside of the leaves; and galls – growths (that have the appearance of cauliflower) on the stems. Rust can be dealt with using a labeled fungicide. Gall, however, typically involves removal of the canes infected (if feasible), followed by sanitation of all equipment, or, if severely infected, destruction of the bush.
Purchasing healthy, disease resistant plants, planting in sunny, open areas (with an eastern exposure) – providing adequate distance between each bush, and removing dead leaves from under them on a regular basis will often be all that is necessary to keep a rose garden looking beautiful. If a fungicide is needed, be sure to carefully read and follow all the label’s instructions, verifying that it is safe for use on roses and that it will treat the disease problem you are faced with.