In the Bluegrass, 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. In the Bluegrass’ warm humid summers, especially when there are heavy dews or the foliage stays wet overnight, are the prime times for black spot to develop.
As its name suggests, 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 also show streaking, red spots or distorted areas. Infected leaves drop off the plant very early. The plant then struggles to put out new foliage and causing the plant to weaken and reduce blooming.
Black spot overwinters on rose leaves, in 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, usually about June, the fungus germinates and infects the rose.
If you have had problems with black spot in the past or your area experiences lots of humid, hot weather, like the Bluegrass, you will want to plant roses with resistance to black spot. Some modern shrub roses have good resistance to black spot and rarely require treatment. If you do not like chemical spraying, choose resistant varieties of roses and hope for the best.
Resistant varieties are not immune to black spot. In really heavily infested areas with 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. Do not put these leaves in the compost heap because the disease will contaminate the compost. During growing season pick off any yellowed or spotted leaves and remove those that fall on the ground as soon as you see them. Do not 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 because roses are heavy feeders. You will also want to control rose insects such as rose chafers and Japanese beetles because 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 do not help already infected leaves but they do help new foliage stay healthy to make food for the plant.
According to Consumers’ Association Magazine, the following products were the best for controlling black spot: Bayer Garden Fungus Fighter Concentrate, Scotts Fungus Clear Ultra, and Bayer Garden Multi-rose 2 Ready-to-use. Researchers tried all the name-brand products versus using nothing at all and they found that any chemical product was better than no treatment. For people into organic gardening, 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, and alcohol, peroxide and other odd substances may actually harm roses.
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.