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Q. Every year I have a problem with rose chafers eating my rose leaves. These brown beetles devastate my roses. How can I control them?
A. "Rose chafers are related to Japanese beetles, and their life cycle and control measures are similar. Although their preferred foods are rose and peony blossoms, they can also be pests on apple, crabapple, grape, cherry, strawberry, raspberry, hydrangea, hollyhock, and many other ornamental plants and vegetables.
There are a number of different controls for rose chafers. You can hand pick them: As soon as you see the 1/2-inch-long, light tan, long legged adult beetles, usually the end of June or early July, go out to your rose patch daily to pick the insects off the plants and drown them in a can of water. The adults are only around for a few weeks, so if you are diligent, handpicking works.
For those without the time or temperament for handpicking, you can spray insecticidal soap. Avoid spraying during the hottest time of the day and don't spray directly on blossoms, as doing so may cause blossoms to deform and can harm pollinators.
Beneficial nematodes will also reduce the population of rose chafer larvae in the soil without harming earthworms or plants. Spray the nematodes on your lawn in spring or fall (when larvae are in the upper layer of the soil). Timing the spray is important. The nematodes are effective only against the larval stage of the chafer. Spraying when you see the beetles is useless."
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Q. There are strange holes in my rose's leaves -- almost as though someone came through with a hole punch. It doesn't look like caterpillar damage -- the edges of the holes are so perfect! What is making these holes?
A. Those small, perfectly circular holes cut into the foliage of your rose indicate that the leaf-cutter bee has visited. The bee uses the pieces of cut leaf to make its nest. Leaf-cutter bees are important pollinating insects so it don't be tempted to spray any insecticide -- although unsightly to some gardeners, the holes don't harm the plant. (Some gardeners delight in seeing evidence of this bee's work as a sign that spring has arrived!) Interestingly, leaf-cutter bees seem to prefer the dull leaves of old garden roses to the shiny leaves of modern hybrids.
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Q. I have several rose bushes in containers; on all of them the blooms turn brown before they fully open. Any ideas on what is causing this problem?
- J.G., Philadelphia, PA
A. Based on your description it is difficult to identify a definite cause of the problem, but here are a few possibilities. Drought stress can cause the symptoms you describe and insects can also cause buds to be deformed and fail to open.
Roses need a rich soil and regular watering so that they are kept evenly moist but not soggy, so it is possible that there is a soil problem. Container plants are a challenge when it comes to watering, sometimes needing it twice a day on very hot, dry and windy days or if the pots are too small for the plants. You might see if the texture of your potting mix has deteriorated over time or if it has been allowed to dry out it may require some care in rehydrating it thoroughly. (The roses may also have outgrown their pots.) A top-dressing of compost is helpful for ailing (and healthy) roses, and you might also consider fertilizing (either a long release granular or water soluble type specifically for roses) according to package instructions if you haven't already done so.
Finally, inspect the blooms very carefully for signs of aphids or other insects. Japanese beetles may be burrowing inside the blooms, as might thrips which are so small they are nearly invisible. A sharp spray of water from the hose will knock away aphids, handpicking or a neem-based spray will take care of the beetles, and insecticidal soap used according to the label instructions may work on the thrips. The soap is also effective in case of a serious infestation of aphids.
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Q. When is the proper time to spray my roses for aphids?
- D.H., Escalon, CA
A. Aphids are interesting critters. They are almost all female and can bear live young, which in turn are capable of bearing live young almost immediately. Therefore, the best defense is to control the aphids as soon as you notice their presence, and before they have a chance to multiply too much. Use a strong stream of water from your hose to dislodge them as soon as you're aware of them, and pay special attention to hosing them off the the buds, green growth tips, and the undersides of leaves. If you feel you need a stronger method of control, you can spray with an insecticidal soap.
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Q. How do I get rid of black spot on my roses?
- C.N., Riddle, OR
A. Black spot is a fungal disease that requires certain environmental conditions to infect plants. If you keep a close eye on your plants, you can catch the disease early and pinch off the affected leaves and dispose of them far from your roses. To help prevent further infection, spray with a solution of compost tea, made by mixing one part mature compost that contains some manure with 5 parts water. Let the mixture sit for 2 weeks, then filter and spray at 2-week intervals. An alternate spray recipe is 3 teaspoons baking soda, and 2 tablespoons of summer horticultural oil mixed in 1 gallon of water, sprayed every 10 days to 2 weeks. Compost tea contains organisms that fight the fungi, and baking soda changes the pH of the leaf surface, which discourages fungal growth.
Other routine maintenance can help prevent black spot: Avoid getting the leaves wet when you water your plants, and water early in the day so that moisture on leaves can evaporate during the heat of the day; keep the mulch around the bushes free of fallen rose leaves and replace it every spring; trim your roses to remove old blooms; in late winter, prune the bushes hard to remove all leaves left from the previous season and remove canes to open the center of the bushes. There are some rose varieties that are more tolerant of black spot, so you may want to look for them when buying new plants. Incidentally, yellow roses are the most susceptible to the disease.
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Q. My rose bush has transparent spots on the leaves. The leaf isn't eaten through -- in other words, these aren't holes, just places where the green is gone. What is wrong with my plant?
- J.T., Columbia, MD
A. "It's hard to identify the culprit without seeing the plant, but my first guess is an infestation of roseslugs. These aren't true slugs, but rather the larvae of the sawfly. The wasp-like adults lay eggs between the leaf layers, and the larvae emerge to feed, leaving translucent "windows" on the leaves. As they grow, the larvae consume entire leaves. Then they drop to the soil to pupate. In some places there may be multiple generations during a summer. Some people report good luck using a neem-based spray as a repellant. If the tiny larvae do emerge and start eating, hose them off with a blast of water daily or handpick and destroy. Natural enemies often keep them in check. If the infestation continues, a spray of neem or insecticidal soap should help. Be sure to spray the entire plant, including the undersides of leaves."
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