Roses are susceptible to all sorts of diseases and bug problems. To grow the most beautiful, healthiest roses, we'll eventually need to spray. But spraying only works if we use the right spray at the right time.Fungus diseases
Roses in Atlanta will get powdery mildew and blackspot. Neither disease will kill a rose outright, but it will sap the bush's strength, resulting in fewer flowers. Powdery mildew is most common in the spring and fall, and looks like the plant has been dusted with flour; the fungus distorts new foliage and blooms. Black spot appears as black spots on leaves; in time the leaf turns yellow around the spot, then the whole leaf turns yellow, and eventually it drops off the plant. For either disease, there is no cure - prevention is the key!
To prevent diseases, or at least to minimize them, we need to spray a fungicide throughout the growing season, every 7 days or so, as long as there are leaves on the bush. It is helpful to alternate fungicides so that diseases do not build up resistance to them. Suitable fungicides include Daconil, Fungi-Gard, Funginex, Mancozeb, Immunox, Banner Maxx, and Compass. All should be used at 7-day intervals except for the last two, Banner Maxx and Compass, which can be sprayed at 14-day intervals. Spray at the concentration listed on the label, and be sure to spray both the top and bottom sides of the leaves. Be careful with sprays - to protect ourselves as well as our roses we need to use goggles, a respirator, gloves, and full clothing or protective suits.
Some "natural" alternatives to these chemicals exist, but they seem to be less effective in our climate. Among them are baking soda and Rose Defense sprays. To be most effective, these must also be sprayed throughout the season, beginning before disease signs emerge.Insects
Our world is full of bugs, and many of them eat our roses. It's not possible or desirable to kill all insects - some of them are helpful in the garden, and in any case the chemicals that would kill the bugs are unhealthy for us, too - so we need to be selective in when and how we try to kill them. If we use too many insecticides, we'll kill off the beneficial insects and wind up with even worse bug problems. So don't spray insecticides until we can see the little buggers and the damage they're doing! Insecticides we may use include Orthene, Cygon, Sevin, and Malathion, being careful to follow label directions and protect ourselves.
Try washing aphids (small green/brown sucking bugs) off the bush with a hard spray of water before resorting to an insecticide. Control thrips, which leave brown shredded edges on light-colored flowers, by spraying just the bud, not the whole bush, with an insecticide. We should try to identify other bugs before spraying, so we can choose the least toxic, most effective insecticide to spray.
The most obnoxious insect we deal with in Atlanta is the Japanese beetle, which feeds on roses in early summer. No one method will get rid of them all, but we can use an assortment of techniques to keep them at bay - spray with an insecticide at frequent intervals, lure them away with beetle traps, pick them off by hand, or try to interrupt their life cycle by attacking the grub stage with milky spore disease.
Less toxic or "natural" alternative insecticides exist, including Safer insecticidal soap, Rose Defense (neem oil), and horticultural oils, but nothing is completely safe or completely effective.Spider Mites
We get spider mites when the weather is hot and dry and when we've been using a lot of insecticides. They can kill a rose bush rapidly, so we have to learn to control them. A forceful spray of water to the undersides of the leaves, repeated every other day, will control mild outbreaks. We may need more potent chemicals to kill them, like Avid, Vendex, and Kelthane, once an infestation is apparent. We may then need to spray throughout the summer to keep mites under control, being careful to follow the label and to protect ourselves.
If you have any questions, contact Bobbie Reed or any of our Consulting Rosarians.