Roses are a show stopper for any garden. They bloom spring through the frost (sometimes longer depending on what zone you garden in), and are one of the easiest shrubs to care for. There are hybrid tea roses, floribunda roses, grandiflora roses, climbing roses, and the list goes on. As much as I wish I could say they are as easy as advertised to care for, and they are never susceptible to disease….I can’t. One major issue roses tend to present is a powdery mildew that attacks the shrub called Podosphaera pannosa. This pathogen effects roses literally all over the world, and has devastating results on the quality of greenhouse roses. Not to mention economic aftermath. Podosphaera pannosa symptoms are pretty easy to notice. The powdery mildew looks just like it sounds. A powdery gray or white mildew that sits on the foliage causing them to distort at the edges. The mildew also spreads to the new younger shoots where the tissue is more supple. Even blooms can be seen covered in the pathogen as shown in the photo. Blooms won’t open properly and eventually decay. The mildew inhibits proper photosynthesis from taking place when the leaves distort. It depletes the nutrients and moisture causing the entire shrub to slowly die, an ugly death.
This unsightly display can be remedied using various fungicides. Great care needs to be taken to ensure one doesn’t spread this mildew pathogen any more than its capable of doing on its own. The first line of defense is to prune the stems showing signs of the disease. Be sure to clean pruners thoroughly with alcohol after removing all diseased stems. This step is very important, otherwise the pruners will spread the mildew to other plants in the future. Keep in mind that over-pruning will also negatively impact the shrub, so don’t prune more than 20% of the shrub. After pruning is complete dispose the infected stems immediately, and apply a fungicide.
Other factors can also help in the fight against Podosphaera pannosa. Some include planting shrubs far enough apart, and away from walls or fences, so that air can properly circulate. Pruning them when overcrowding occurs can also help in air circulation. Fertilizing and proper watering will help shrubs fight off disease better than shrubs neglected of these key steps. Watering during the morning hours will help the foliage dry and avoid being infected by the mildew, especially in humid conditions.
The key is to really be proactive in preventing the mildew from forming in the first place. Fungicides used at the start of the growing season that can not only kill, but go a step further, and prevent the mildew from forming is the best line of defense.
One last tip, more like a public service announcement really. Whether using a synthetic or organic fungicide; be sure to wear proper protective clothing when applying fungicides. Long sleeve clothing, gloves, and a mask should be worn. Also avoid spraying on windy days. Try to be conscientious as to what time of the day you spray, as some fungicides are harmful to honey bees.
Roses are a beautiful shrub to include in any garden. They offer great interest to any garden, and wonderful fragrance. Keeping them healthy will allow them to perform at their best, and for you to enjoy them for years to come.
Sources
Powdery Mildew on Ornamentals. University of California Agriculture and Natural Resources, Statewide Integrated Pest Management Program. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7493.html