I must admit, for years I was not a big fan of roses. It seemed to me they were a whole lot of work for not much. And then, as if by magic a new breed of shrub roses hit the market. And still I was skeptical. So I waited. I took the plunge about five years after they were introduced, and I was impressed. They were fabulous! Who wouldn’t like a shrub that blooms beautifully from spring until frost, requires minimal care, is disease and insect resistant and comes in different perfect landscape sizes? I was hooked! Sadly my euphoria didn’t last long. We were hit with Rose Rosette Disease and the love story ended. Even the disease resistant roses don’t always resist Rose Rosette Disease.
No gardener wants to think about diseases that can’t be cured, especially with plants that are our favorites. Some problems like Japanese beetles are simply a nuisance and can be taken care of relatively easily. Others like Rose Rosette Disease cannot be cured. The offending plant must be removed and in fact it is recommended roses not be planted there for many years to come. We are going to look at three common threats to the rose family: Rose Rosette Disease, also known as Rosette Virus; Japanese beetles; and Black Spot. Let’s start with some fun facts: Insect damage does not normally kill a plant, but the damage done will compromise the immune system. Once the plant is weakened, it is susceptible to any threat floating by. Fungal diseases and bacterial diseases can be treated, but there are no treatments for viral diseases.
Rose Rosette Disease.
What is it and where did it come from?
Rose Rosette Disease, caused by a windborne mite that inhabits the multiflora rose, has become more prevalent with the spread of multiflora rose, which was first brought to the U.S. from Japan as an under root stock for ornamental roses.
This story is a classic case of a good deed gone bad. Rose rosette disease, also known as rosette virus or witches’-broom of rose, is a virus, or virus-like disease spread by grafting and by a very small wingless mite that can travel passively in the wind, called the eriophyid mite. This little mite was a “cling on” to the multiflora rose which was brought into this country from Japan in 1886, as an under root stock for ornamental roses. So far so good, seems harmless enough. In fact, the multiflora roses were also used in the 1930s by the U.S. Soil Conservation Service for erosion projects and as a way to confine livestock. Here’s where the trouble starts: The multiflora rose (Rosa multiflora) is an extremely prolific plant. It invades and damages pastures and unplowed lands, crowding out vegetation and creating dense, impenetrable thickets. Cattle are often reluctant to enter fields dominated by multiflora rose; its hedges cause lower crop yields on adjacent fields by competing for nutrients. Hedges of multiflora rose have also been used as a crash barrier and to reduce headlight glare in highway medians. I first heard about rose rosette disease in master gardener school in 2004, but because the disease had not yet reached our area I didn’t think much of it. A few years ago though, a friend’s roses started showing the earliest symptoms of rose rosette disease, which include red pigmentation on the underside of leaf veins followed by sharply increased growth of vegetative shoots, which are typically more succulent than normal and appear in various shades of red. As the disease progresses the leaves will become deformed, crinkled, and brittle with yellow mosaics and red pigmentation. Leaves then become very small, petioles are shortened, and most lateral buds grow, producing short, intensely red shoots. The disease causes the plant to be exceptionally susceptible to freeze damage. Symptoms on cultivated roses are typically less severe than on the multiflora rose. Cultivated roses show symptoms of thickened, succulent stems and a proliferation of thorns.
So, what to do? First of all if you have multiflora roses growing near by, say in a wooded area, pass on putting roses in your garden. If you already have roses they can be pre-treated with a new predator mite recently introduced to the market place by the Bartlett Tree Experts. It’s not 100 percent effective, but it’s a lot better than doing nothing. You can also try pesticides such as carbaryl (Sevin), bifenthrin, or the organic strategies, which are horticultural oils and insecticidal soap. These may provide some protection when applied at weekly intervals during the months of June and July.
If you find your roses have rosette disease, the entire plant including the rootstock should be removed and destroyed. Roses cannot be planted there for many years to come. You don’t have to go without beautiful blooms, though. A great full sun alternative to plant in the place of your roses, are the new compact butterfly bushes. They come in many colors, and at 3-by-3 feet or smaller, they are right sized for many landscape uses. These beautiful shrubs bloom from mid-June until frost and do not require dead heading. Inthe late fall mulch their root zones with a good solid layer of fiveinches of leaves to protect against extreme cold and our Pennsylvania freeze/thaw cycle. In the spring they leaf out later than many other shrubs, so wait until you see new growth and then trim out the dead crossing branches.
Japanese Beetles on Roses. a pest management problem
When I was a kid growing up on Altadena Drive I remember my nana picking beetles off her roses and drowning them in soapy water. Ew. Here’s the deal about Japanese beetles though: They may be gross and leave the leaves on your roses looking like lace, but that’s about it. In and of themselves, they will not kill your rose. On the other hand, defoliation isn’t entirely without consequence. If all the leaves are destroyed, the plant cannot perform photosynthesis and it will start to decline.
So where do these little pests come from and what can we do? Japanese beetles are the adult form of the grub we hate, the one sthat eat the roots of the grass in our lawns. They lay their eggs the year before and emerge the following in June, so if you are diligent about lawn care and treat for grubs your problem will be lessened. Except for one big problem: You have to convince all your neighbors to treat their lawns too.
And what about those popular pheromone traps, those bags that you see on posts around everyone’s gardens? Big mistake! Research shows that the amount of beetles coming to your trap far exceeds the number you would have had if you hadn’t put out the traps. And your neighbors will hate you for that. Whatever you do, don’t squish the beetles and throw them on the ground—crushed female beetles emit a powerful sexual attractant that brings every male in the neighborhood right to your rose bed.
Of course I recommend an integrated pest management ( IPM) approach. Meaning you use the least toxic methods first—pick the beetles off and put them in soapy water—and work your way down through the list of options. Only use chemicals as a last resort.
Black Spot: a fungal disease
Black Spot is a fungal disease, so it can be treated. The IPM approach advocates choosing plants that have been developed to be disease resistant. In the case of roses, the newer varieties like Flower Carpet and Knock Out are resistant to Black Spot. If your heart is longing for climbers and tea roses, get ready to do some battle because controlling black spot is crucial to any rose grower. Black Spot is characterized by black spots on the upper side of leaves. Tending to occur in warm, wet weather, usually during wet summers, the leaves of infected roses turn yellow and fall off. This weakens the plant by making it more susceptible to other diseases or opening it up to injury next winter. The organism responsible for Black Spot spreads quickly and can move from plant to plant if proper care is not taken. Strongly consider horticultural conditions. Site your roses in full sun—morning sun is best—and leave enough room in between each one for good air flow. Keep the floor of your rose bed clean and disease free by picking up the fallen leaves and throwing them away, do not compost them. Finally, make sure your soil is well dug, friable and well amended with leaf mould and compost. I have found copper
soap to be an effective organic remedy for black spot.
One final tip, according to horticulture professor Jeff Gillman, who has conducted extensive research on blackspot remedies, a spray composed of one part milk and two parts water is the best answer to the disease. When applied weekly, the solution controls Black Spot as well as any synthetic fungicide.
Claire Schuchman is a 2004 Phipps Master Gardener, reach her at Claire.CS@ExceptionalGardens.net