Dealing with Rust, Black Spot and Powdery Mildew in Your Rose Garden
Fungal diseases of roses are among the most frustrating of gardening problems. They are not as difficult to manage as one might think.
Rose diseases are often caused by fungi. These fungal agents vary from area to area depending upon temperature and moisture. Three of the most common diseases may not kill the rose; however, they can disfigure and act as serious stressors on a plant trying to survive a blazing summer, an insect infestation or a hard winter.
Rust appears as reddish, blister or wart like spots on the underside of the leaf with blotchy yellow areas on the upper side. As long as rust is limited to the lower leaves it remains fairly innocuous. However, when it moves up to the younger rose foliage these tender leaves can be seriously damaged. Stripes of rust may also form on young canes. If not removed these stripes will become large pools of rust warts, turning black in autumn and total defoliation of the rose may result.
Rust thrives in climates where rain, fog and mist are prevalent. Optimal temperature conditions are cool (around 64 to 70 degrees Fahrenheit). East of the Rocky Mountains rust is not seen as often.
Frequent cleaning of rose beds is one way to help prevent rust infections. Fungal spores remain on leaves that have fallen from the plant. Communication of rust from one rose to another can occur via the wind.
Black Spot as its name would imply presents on the upper surface of the rose’s leaf in irregularly shaped black spots. The culprit here is a fungus known to proliferate in water. People growing roses in moist climates are at risk as well as people who water their roses overhead.
While it is recommended that roses be watered at their base, people with large or mature landscapes and many roses might not be able to do this. Rule of thumb is that overhead watering should be done in the morning – not the evening. Allowing the leaves to dry in the sun will cut down the incidents of black spot and other fungal diseases. Infected leaves should be cut from the plant immediately.
Black Spot survives on dropped leaves and rose canes. It over winters in most climates, therefore, affected canes should be cut and leaves removed from the rose bed.
Powdery Mildew presents as a thin white film over the rose leaf. It can be mistaken for soil or water spots. However, unlike soil or spots it won’t rub off. Powdery mildew cannot be hosed off the rose. It is a fungal infection not a localized infestation. This fungus eats into the rose’s leaf tissue between the veins and will eventually destroy as well as disfigure the plant. It is believed that there are different kinds of powdery mildews and that what affects your roses will not affect your grapes, for instance. Regardless of type, you don’t want this stuff in your garden. It is a prolific spreader and can be very hard to control on certain plants including some trees.
Conventional wisdom holds that watering roses overhead is the prime cause of powdery mildew, along with excessive rain and humidity. New thinking is that good pruning, plenty of airflow within the garden, conscious selection of resistant rose varieties, vigilant monitoring and prompt action when problems are seen will decrease the incidents and severity of infection, watering practices notwithstanding. Watering in the morning is still recommended if overhead watering is necessary.
Removing leaves and canes from the roses that are infected is advised. Also, make sure that infected fallen leaves are removed from the rose bed. Powdery Mildew will over winter, and the spores will become active in spring.
In addition to newer and ever more resistant rose varieties, gardeners – whether organic or inorganic – have a panoply of ways to deal with the fungal diseases of roses. If you are an organic gardener you might want to speak to the University Agricultural Extension or gardening center professionals in your area for ideas on how best to deal with fungi. If you are an inorganic gardener you can choose a spray anti-fungal or a systemic. The spray can be found in a can or used with a garden sprayer. The systemic is administered at the base of the rose four to six times a season. The gardening professionals in your local nursery will be of great help in deciding which is best for you.
There is a hygiene to gardening, most definitely when dealing with diseased plants. Keep hands, tools, gloves and clothes clean. After working with an infected rose wash your hands and tools with antibacterial soap and rinse with warm water and rubbing alcohol. Empty clippings from a diseased rose out of your gardening basket or can before approaching any other rose in the garden.
It can’t be stressed enough that rose beds should be laid out in such a way as to give roses plenty of space and airflow. Not only does this reduce the incidence of fungal diseases but it lessens contamination from one rose to another.