Q. I have black spots on the leaves of my roses and the leaves are turning yellow and dropping off. What are the spots? How can I get rid of them?
A. Black spot is a fungus that thrives in warm, damp conditions. Spores can be spread from plant to plant by splashing water, therefore you should not water roses by overhead methods. Spraying with fungicides containing chlorodialonil or triforine will give control, but must be done every 7-10 days during humid weather to be effective. Prune roses to improve air circulation, cut off infected canes and rake and clean area under shrubs. Consider planting varieties that are resistant to black spot.
Pest Note: Black Spot
Q. The flowers on my rose shrubs are not opening. They have brown spots and streaks on them. What is causing this?
A. Flower thrips are more than likely the problem. Check for their presence by shaking a rose bud over a white piece of paper. Thrips are tiny (less than 1/2 inch), long-bodied, greenish-black bugs. They live inside the buds of many garden plants and their feeding habits cause damage to the petal tissue which then turns brown and fails to open. They are difficult to control because they constantly move from plant to plant in the garden. Remove and destroy all infected buds and blooms. You can also try spraying with an insecticidal soap or pyrethrin-based spray-making sure to follow directions carefully.
The bad news is that sprays don't penetrate inside the buds. The other bad news is that systemic insecticides often contain an organophosphate such as disulftoton (Di-Syston) which is very toxic to humans. Use of oranophosphates in the garden is now being discouraged. The good news is that the thrip population is most active in spring and later bloom cycles should suffer less damage.
Pest Note: Thrips
Q. There are holes in the flowers on my roses (especially the light-colored roses) and on some of the leaves. What is the from?
A. It is probably damage from beetles. Several different beetles (rose chafer, etc.) infest roses and they can seriously damage the ornamental value of your shrubs. They usually overwinter in the soil underneath the plants or in plant debris on top of the soil. Good garden sanitation is important. You can spray with insecticidal soaps if damage is intolerable, making sure to follow label directions. Sprays are not generally recommended. It is difficult to obtain control because beetles are protected within the petals and most have to have contact with the insecticides to be killed. Systemic insecticides are not effective against adults because concentrations high enough to be toxic do not occur in the blossom. Hand-picking is recommended. The beetles are easy to spot inside the flowers and they don't bite-use tweasers to grab them if you're squeamish. They can be drowned in a jar of soapy water or just stepped on. They feed for 3-4 weeks in early spring and will disappear suddenly. Cultivate deeply around plants to expose overwintering larvae and pupae. The grubs may feed on lawns later in the summer. (See July-Lawn Fungi-Grubs)
Pest Note: Hoplia Beetle
Q. The new growth on my roses is covered with a grayish-white powder and the leaves are puckering. Is this a disease? If so, what can I do?
A. The problem is powdery mildew and it is a fungus that is one of the most serious and widespread diseases of roses. The spores of this fungus are carried by the wind to other plants making its spread difficult to control. All powdery mildew species can germinate in the absence of water, but they will proliferate in shady conditions and moderate temperatures.
Good cultural practices can help prevent infestation. Plant resistant varieties in sunny spots, prune to provide good air circulation and keep the garden cleaned up. At the first sign of problems, try to wash off as much of the powdery mildew as possible. Do not overfertilze-powdery mildew loves tender, new growth. Fungicide sprays containig sulfur or triforine can be used. Timing of application is important. Respray at 7-10 day intervals if mildew reappears.
Pest Note: Powdery Mildew