Many gardeners have asked me lately what is happening to their roses. The leaves are turning yellow and then black spots appear. Then the leaves fall off. The second problem is one with the blooms that either fail to open properly or the flower is deformed. These are two common problems.
The first problem is a disease called black spot, a common and serious disease of roses. If the disease is not controlled, the plant will defoliate and become seriously stunted. The disease also can cause the plant to produce fewer and paler flowers.
The disease is caused by the fungus Diplocarpon rosae. It is more severe after long wet, warm periods in the spring. It starts to grow when the temperature is between 70 and 80 degrees. The spores of the disease need to be wet for 24 hours before they start to grow.
The symptoms occur on rose leaves as circular, black spots surrounded by a yellow area. Infected leaves often drop from the plant. Infection continues throughout the summer months. On the wood of first-year canes, the disease will develop raised, purple-red irregular blotches.
There are a number of ways this disease problem can be managed. The first is to plant one of the many varieties of roses that are resistant to black spot. Earth Friendly and KnockOut roses are resistant to this disease.
Another way to help control this disease is to practice good sanitation. Before spring growth begins, remove last year’s mulch and replace it. Prune the plants to remove old foliage and any of the stems that have lesions. During the growing season, remove any leaves that are infected with the disease.
To help reduce the amount of black spot, proper watering is important. Remember that the spores have to be immersed in water to start growing.
Deep watering that is done less often will help develop a healthier plant. Water at the base of the plant to keep the water off the leaves.
The last part of controlling this disease is the use of a spray program. But to have success, a rigorous spray schedule must be followed. This includes spraying the plants every seven to 10 days.
There are fungicides that can help control this disease, including captan, chlorothalonil, mancozeb, myclobutanil, propiconazole, triforine or copper fungicides. Most of these can be found in local garden centers. There are some products on the market that are called three-in-one products that contain a systemic fungicide. These will work if the fungicide is in the plant before the disease starts.
The second problem I am seeing on roses is an insect called thrips.
The two most common thrips are the flower thrips (Frankliniella tritici) and the Western flower thrips (Frankliniella occidentalis). Adult female thrips of both species are tiny, yellowish-brown insects with fringed or feathery wings. They are less than one-sixteenth inch long, and they are barely visible without a magnifying glass. Blowing lightly into the blooms and on the leaves causes thrips to move around, making them easier to see.
Immature and adult thrips feed by scraping surface cells to suck plant sap, feeding on both leaves and flower petals. The majority of their damage to roses occur from early to midsummer, but they can be a problem all summer and into the early fall if conditions are right.
Their feeding can result in distorted buds that open only partially or abort prematurely. Feeding on petals may result in petals streaked with silvery-white or brown, as well as petals with browning edges. White and light-colored rose blossoms appear to be particularly attractive to thrips.
Infested rose blossoms should be removed and destroyed. Grass and weeds in the area should be kept mowed or removed when possible.
The easiest way to control these pests is to use a systemic insecticide such as dinotefuran, disulfoton or imidacloprid. These will give six to eight weeks of control.
Timing of sprays is very important. They must be applied before thrips enter unopened buds. In addition, because rose blooms expand rapidly, it is difficult to keep them adequately covered with insecticide.
The following can be used to control thrips: bifenthrin, cyfluthrin, esfenvalerate, lambda cyhalothrin, permethrin, or spinosad. Insecticidal soaps will help control thrips, but thorough coverage is necessary.