One of the most common diseases of roses is Black Spot. Black spot is a disease caused by the fungus, Diplocarpon rosa. It is extremely common in older varieties of tea roses but when conditions are right even roses marked as resistant to Black Spot can develop a mild case of the disease. Warm humid conditions, especially when there are heavy dews or the foliage stays wet overnight are the prime times for Black Spot to develop.
Black Spot causes black spots on the upper surfaces of rose leaves, surrounded by a yellow area. If the fungus is heavy the leaves may look almost totally yellow, sprinkled with black spots. Rose flower petals may show streaking, red spots or distorted areas. Infected leaves soon drop off the plant. The plant struggles to put out new foliage and this weakens the plant and reduces blooming. Infected plants may not survive the winter as well as those with mild or no infection. And half bare, yellow leaved plants just aren’t very attractive.
Black spot overwinters on rose leaves on the soil or on the rose canes. Rain and wind move the spores to new foliage on the roses in the spring. When conditions are right, (warmth and humidity), usually about June, the fungus germinates and infects the rose.
How to control Black Spot
If you have had problems with Black Spot in the past or your area experiences lots of humid, hot weather you’ll want to plant roses with resistance to Black Spot. Some modern shrub roses have pretty good resistance to Black Spot and rarely require treatment. Even some tea roses have recently been bred that have some resistance. Older roses with resistance are the gallicas, rugosa’s and albas. If you don’t like chemical spraying choose resistant varieties and hope for the best.
Resistant varieties are not immune to Black Spot. In really heavily infested areas and ideal conditions even resistant roses may get Black Spot. Some resistant varieties perform better in some geographical areas than others. If one variety of rose always seems to get infected in your garden try another rose or at least another location in the garden.
Other ways to control black spot are to remove all rose leaves from under the plant in the fall or early spring before the plant leafs out. During growing season pick off any yellowed or spotted leaves and remove those that fall on the ground as soon as you see them. Don’t crowd the roses, they need good air circulation and roses against buildings or with hedges behind them may have more problems with black spot.
Water your roses at the base, trying not to wet the foliage and do so early in the day so the foliage dries before evening. Keep roses healthy by planting them in full sun and regularly fertilizing them as roses are heavy feeders. You’ll also want to control rose insects such as rose chafers and Japanese beetles as these weaken the plant and make them more susceptible to damage from black spot.
There are many chemical controls for Black Spot and other fungal diseases of roses. If you are going to use fungicides begin spraying in June or as soon as you see even one infected leaf. If you want nice looking tea roses you will probably want to start a spray schedule soon after the plants leaf out. Follow label directions and keep up the schedule for best results. The sprays don’t help already infected leaves but they do help new foliage stay healthy to make food for the plant.
The Consumers’ Association Magazine put eight fungicides to the test between April and October last year to combat black spot on roses. The products they found did the best job were Bayer Garden Systhane Fungus Fighter Concentrate, Scotts Fungus Clear Ultra, and Bayer Garden Multirose 2 Ready-to-use.
The test was conducted on a variety of modern rose called ‘Silver Jubilee’ which has some resistance to black spot. Black spot was noticed in June on the roses and treatment according to label directions was begun. Some roses were left untreated as a control. The researchers found that any chemical product was better than no treatment. There are organic treatments on the market but many of these do little or nothing to control fungal disease.
Beware of so called “home remedies” for Black Spot on roses. Soap solutions do not work on fungal disease. Alcohol, peroxide and other odd substances may actually harm roses, and you won’t see the experiences of the people who have had that happen in the advertisements for books and magazines listing these miraculous cures.
With the many disease resistant varieties of roses on the market and a little tolerance for less than perfect foliage almost everyone can grow roses, even without chemical sprays. Don’t let the fear of Black Spot or other rose diseases keep you from enjoying them in your garden.
Here are some additional articles you may want to read.
How to choose roses
How to identify 10 common garden weeds
How to grow and use Lemon Verbena
If you would like to receive a weekly email garden newsletter by the author of this article please email her at email@example.com