Read an Excerpt
Outsmarting Rose Pests and Diseases
In This Chapter
- Common-sense pest prevention
- Preventing problems before they start
- Common rose pests . . . and how to control them
- Pitting beneficial bugs against troublemakers
- Managing pest levels
- Choosing safe and sane pest control products
- Heeding pesticide cautions
Certain insects and plant diseases probably like your roses as much as you do -- but not more so than they like many other kinds of plants. Yeah, right. Then how come every time you walk into a nursery, garden center, or even a grocery store, all you see are rose care products -- sprays, dusts, combination fungicides and insecticides, and preventative three-way cure-alls piled up to the roof?
Big surprise. Roses are, after all, among the most popular garden plants, and gardening is big business. Millions of dollars are made each year from products that catch the eye of rose growers, whether growers actually need those products or not.
The truth is that more than one approach to controlling rose pests exists. Some gardeners are determined to have their plants produce perfect flowers; these growers are looking for the perfect flower to display at a rose show and spray their plants every seven to ten days to prevent every insect and disease from touching their roses. On the other hand, many gardeners never spray with strong chemicals and still have beautiful rose gardens.
We recommend a flexible, common-sense approach to controlling rose pests. Take a little and give a little. We can live with a few bugs -- they make for a more diverse garden. But if they try to wipe out our plants, we do something to stop them. But even then, we use only products that have the least impact on the environment. This common-sense approach to pest control is what this chapter is all about.
You can do several things to prevent insects and diseases from becoming problems on your roses:
Grow healthy plants. A good, strong rose plant is less likely to be seriously bothered by pests or diseases than one that is weakened by under- or overwatering or that is planted where it doesn't get enough sunlight. Proper pruning to keep the plant open and free to good air circulation helps prevent disease. Even too much of a normally good thing -- like nitrogen fertilizer -- can result in excessive, lush growth that attracts insects like aphids. The tender leaves are just too luscious to pass up. So read Chapters 16 through 20 on caring for roses; they're the first step in preventing insects and disease.
Plant problem-free varieties. Many rose varieties, especially the newer ones, have natural resistance bred into them. On the other hand, some of the loveliest roses are also the most disease prone. It's like anything else -- for the best you have to pay a price, in this case more pests. Roses of all types vary in their susceptibility to pests, and where you live plays a part, too; some problems that are common in one region are rare in others. Regional disease problems are discussed in Chapter 4.
Encourage and use beneficial insects. Beneficial insects are the good bugs in a garden -- the insects that feed on the bugs that bother your roses. You probably have a bunch of different kinds in your garden already, but you can also purchase them and release them into your garden. The more beneficials, the fewer the pests. File that in your memory for now; we give you specifics later in this chapter.
Keep your garden clean. Many insects and diseases spend the winter or go through various stages of their life cycle in garden debris, like fallen leaves or prunings left on the ground. If you remove these hiding places, you also likely reduce the number of future pests. So occasionally rake around the base of your plants to clean up fallen leaves, and always discard or burn your prunings. And apply a mulch to prevent water from splashing disease spores onto the foliage.
Know the enemy. The more you know about specific pests and diseases common to your area -- when they occur and how they spread -- the more easily you can avoid them. For example, the fungal disease black spot runs rampant on wet foliage. By simply adjusting your watering so that you don't wet the leaves of your plants, or by watering early in the day so that plants dry out quickly, you can reduce black spot's occurrence.
Apply a dormant spray. This application is the most important preventive spray you can make, and the only one we recommend that you apply every single year. Usually a combination of fairly benign horticultural oil and a fungicide like lime sulfur or fixed copper, a dormant spray smothers insect eggs and kills disease organisms before they become a problem. Apply the spray right after you prune in late fall or winter, and next summer will be much easier.
If a rose has good pest resistance, we say so in the variety descriptions beginning with Chapter 8. Rose mail-order catalogs (see Appendix C) are also a good source of information about pest and disease resistance.
Rose Pests and How to Control Them
Before you wrestle with any insect or disease problem, make sure that you properly identify the problem. For a start, consult our list of common insects and diseases that follow, and also take a look at the information we give about the best products and materials to use. If you need further help, contact a local nursery that has a variety of reference books to consult and is familiar with local problems. Nearby botanical gardens and your local cooperative extension office also may be able to help.
Or better yet, contact the American Rose Society (see Appendix A) and ask about their Consulting Rosarian Program. Consulting Rosarians are trained rose experts, and the ARS can put you in touch with the one nearest you. You can usually find several in every good-sized city, and nothing is better than asking a local expert for help. And the advice is free. (Appendix A also lists contact information for similar organizations around the world.)
If you like to look things up for yourself, ask at your local nursery or library for the Ortho Problem Solver. It's a 1,000-page encyclopedia of garden pest problems, each one with a color picture.
Insects that prey on roses
Here are the most common insect pests that you are likely to find infesting your roses and the best ways to control them:
Aphids are tiny, pear-shaped pests that come in many colors, including black, green, and red. (See Figure 21-1.) They congregate on new growth and flower buds, sucking plant sap with their needlelike noses. They leave behind a sticky sap that may turn black if infected with sooty mold.
Cucumber beetles are easy to recognize -- they're about 1/4 inch long and yellowish green, with black stripes or dots on their backs. Two different types exist. They feed mostly on cucumbers and vegetable plants, but they also love rose blossoms and take big bites out of them just as they open.
Japanese beetles can really be a serious problem east of the Mississippi River. The 1/2-inch-long beetles have coppery bodies and metallic green heads. (See Figure 21-2.) They feed on both flowers and foliage, often skeletonizing the leaves.
June beetles are about an inch long and reddish brown to black. They usually feed at night and prefer the foliage of various trees, but they also feed on roses. Control is the same as for Japanese beetles, but milky spore is not effective against June beetle grubs.
Caterpillars are the larvae of moths or butterflies. They occasionally feed on the foliage or flowers of roses. You can control them with Bt or by releasing trichogramma wasps. Acephate and diazinon are traditional chemical controls that may be effective.
Rose midges are small, almost invisible pests that rasp new growth, especially flower buds, causing it to shrivel and turn black. If your rose plants look healthy but do not produce flowers, suspect rose midge. Insecticidal soaps sometimes work. For sure-fire control, attack the soil-borne larvae with diazinon.
Rose chafers are tan-colored beetles with long legs, as shown in Figure 21-3. Again, control is the same as for Japanese beetles and June beetles, but milky spore is not effective against the grubs.
Rose stem borers (shown in Figure 21-4) are tiny, wormlike larvae that bore into recently cut and new canes and feed inside, causing the cane to die. Borers are hard to control. First, cut off the wilted stem well back into healthy tissue. You may be able to see a small hole where the borer entered the stem. Cut back below that and look to see whether tissue inside the cane is damaged. If so, cut lower still until the inside of the cane is normal. If the borer has reached the base of the cane and bored into the bud union, you may lose the plant. Few sprays of any kind are effective, although you may get some of the larvae by using parasitic nematodes near the base of the plant. After feeding, the larvae drop to the ground to mature.
Spider mites are tiny, spiderlike arachnids that you can barely see without a magnifying glass. If the population gets big enough, you can see their fine webbing beneath the leaves. And as they suck plant juices, the leaves become yellowish with a silvery stippling or sheen. If things really get bad, the plant may start dropping leaves. Mites are most common in hot, dry summer climates and on dusty plants.
Thrips are another almost-invisible troublemaker. They feed on flower petals, causing them to be discolored and the buds to be deformed as they open. Often the first sign you see is an unopened bud that is bent over at a sharp angle. Thrips like all roses but are particularly fond of light-colored varieties.
Aphids are easy to control. You can knock them off a plant with a strong jet of water from a hose or use insecticidal soap. The soap helps to wash off the sooty mold, too. But usually, if you just wait a week or two, the aphid population boom is followed by a buildup in beneficials, especially lady beetles, and they take matters into their own hands before serious damage occurs. Malathion, diazinon, and acephate are traditional chemical controls for aphids.
To control these pests, spray with pyrethrin, neem, or insecticidal soap. Parasitic nematodes prey on the soil-borne larvae. Carbaryl is a traditional chemical control.
Control can be tough. Treating your lawn and garden soil with parasitic nematodes or milky spore may reduce the white, C-shaped larvae, but more adults will probably fly in from your neighbors' yards. Turning the soil to expose the grubs to birds may also help. Floral-scented traps that attract adult beetles are available, but the traps may bring in more beetles than you had before. If you try traps, keep them at least 100 feet away from your roses.
Neem, insecticidal soap, and pyrethrins are effective alternative sprays for controlling adult beetles. Traditional chemicals that may help include carbaryl and acephate. You can also just pick them off your roses and stomp on them.
If borers are really giving you fits, put a drop of Elmer's Glue-All on the top of the cane after you cut a flower or prune.
A daily bath with a strong spray from a hose should keep infestations down. You can control spider mites with insecticidal soaps, which also help to clean off a plant's leaves. Summer oil is also effective, as is releasing predatory mites. If the pests get completely out of control, you may have to use a miticide, such as Avid (see Table 21-1).
Many beneficial insects feed on thrips, especially lacewings. Insecticidal soaps are also effective, as are several stronger insecticides, including acephate.
Troublesome rose diseases
The following sections talk about five of the most common rose diseases and suggest some techniques for controlling them.
Like its name says, this fungus causes small black spots on rose leaves and stems, as shown in Figure 21-5. The edges of the spots are fringed, and the tissue around the spots often turns yellow. In bad infections, the plant may drop all its leaves. This disease is most common in warm, humid climates with frequent summer rain.
The best advice to prevent black spot (besides planting disease-resistant varieties) is to clean up your winter prunings -- the most common source of reinfection -- and use a dormant spray that includes lime sulfur. Also, avoid overhead watering or water early in the morning so that the leaves can dry out quickly. The baking soda-summer oil spray mentioned later in this chapter provides some control, as does neem oil. Effective traditional fungicides include triforine.
Often confused with powdery mildew and black spot, but much more serious, downy mildew has the capability to defoliate a plant in 24 hours. Grayish white fuzz forms on the bottoms of the leaves. Round, purple blotches with yellow edges form on the tops. Leaves often turn brittle and drop. Fortunately, downy mildew is less common than other rose diseases. It usually shows up after long periods of cool, wet weather and then clears itself up when the weather warms.
The disease needs moist conditions to spread, so avoid overhead watering and water early in the morning -- that way, everything has time to dry out. Good cultural methods are the best control -- prune to increase air circulation and clean up plant debris. In late fall or winter, use a dormant spray.
Grayish white, powdery fungus infects new leaves and flower buds, causing them to become distorted and crinkled-looking. (See Figure 21-6.) Unlike most other fungal diseases, powdery mildew spreads on dry foliage.
Many rose growers prevent its spread by overhead watering or with a sprinkling down each morning, thus washing the spores off the leaves before they can establish themselves. Other preventive measures include planting resistant varieties, planting in full sun, and pruning to encourage air circulation. Effective preventive sprays include antitranspirants, the baking soda-summer oil mentioned later in this chapter, and neem oil. Triforine is one of several traditional chemical fungicides used to control powdery mildew.
On a plant that has rust, small, orange pustules form on the undersides of the leaves. Yellow spots appear on top. (See Figure 21-7.) If the rust is severe enough, the plant can lose all its leaves. This disease is most troublesome when days are warm but nights are cool; prolonged hot, dry weather usually stops its development.
Prevention is similar to black spot -- winter cleanup and dormant spray. Make sure that you also strip off infected leaves. Neem oil provides some control, as do traditional fungicides like triforine.
Rose mosaic viruses
Rose mosaic viruses cause yellow mottling on the leaves and deformed new growth. There's not much you can do about it. Most of the time, it shows up on a few leaves and doesn't do much harm to anything. It can't spread from plant to plant, but you should definitely avoid taking cuttings from or hybridizing with infected plants. The virus is passed on to offspring.
All reputable rose nurseries will replace a plant with a virus. Large commercial nurseries that grow roses now supply plants that are 95 percent virus free, but some older plants may be infected. Severely infected garden plants should be destroyed.
Encouraging Garden Good Guys
Okay, open your mental file on beneficial insects -- the garden good guys -- and fill it with some specifics. Following are some things you can do to encourage beneficial insects to populate your garden and reduce the number of rose pests:
- Avoid indiscriminate use of broad-spectrum pesticides, which kill everything, the good guys and the bad. If you do spray, use a spray that specifically targets the pest you want to eliminate.
- Have a diverse garden with many different kinds and sizes of plants. Doing so gives the beneficials places to hide and reproduce. Variety can also provide an alternate food source, because many beneficials like to eat pollen or flower nectar, too. Some plants that attract beneficials include Queen Anne's lace, parsley (especially if you let the flowers develop), dill, clover, fennel, and yarrow. You'll find many of these plants described in Gardening For Dummies.
If beneficial insects are not as numerous in your garden as you would like, you can buy them from mail-order garden suppliers (we list several in Appendix C). If you know that a particularly difficult pest is likely to appear, order in advance. That way, you can release the beneficials in time to prevent problems.
Following are some of the good insects that you can buy to help control rose pests:
- Lady beetles: These are your basic ladybugs. Both the adult and the lizard-like larvae are especially good at feeding on small insects like aphids and thrips. Releasing adults is sometimes not very effective because mother nature has preprogrammed them to migrate on down the road, so they leave your garden quickly. Try preconditioned lady beetles, which have been deprogrammed (you don't want to know how); they are more likely to stick around. And release them just before sundown. That way, they'll at least spend the night. Release a few thousand of them in your garden in spring as soon as you notice aphids.
- Green lacewings: Their voracious larvae feed on aphids, thrips, mites, and various insect eggs. These insects are one of the most effective beneficials for garden use. Release them in your garden in late spring after danger of frost has passed.
- Parasitic nematodes: These microscopic worms parasitize many types of soil-dwelling and burrowing insects, including the grubs of Japanese beetles, June beetles, and rose chafers. Because grubs usually inhabit lawns, you have to apply these worms there, too, as well as around the base of your plants. Apply parasitic nematodes to the soil around the base of your plants once a year in the spring.
- Predatory mites: This type of mite feeds on spider mites and thrips. Add them to your garden in spring as soon as frost danger has passed.
- Trichogramma wasps: Tiny wasps (harmless to people) that attack moth eggs and butterfly larvae (that is, caterpillars). Release these garden good guys when temperatures are above 72° F (22° C).
Aphid Hors d'Oeuvre
Good bugs are no dummies; they hang out in gardens that offer the most diverse and reliable menu. That's why eliminating every last insect pest from your garden makes no sense.
As we said earlier, our approach to pest control is to have maximum diversity in the garden. That's why having some "bad" bugs around all the time is important. Aphids are like hors d'oeuvre for so many helpful insects, so you always hope to have a few in your garden. Otherwise, what will the good bugs eat? But accepting the bugs also means that you have to accept a little pest damage once in a while. So you're really just trying to manage the pests, not nuke them off the face of the earth. You want to keep them at acceptable levels, without letting them get out of control.
Spend time in your garden. Poke around, turn leaves upside down. Investigate. Knowing what's out there is important.
To manage insects and diseases successfully, you have to be a good observer, checking your roses frequently, if not daily, for developing problems. If an insect or disease does get out of hand, you want to treat it effectively without disrupting all the life in the garden. To do that, we start with what we consider our first line of defense against pest outbreaks: pesticides that can be very effective against a certain pest, are pretty safe to use, and have a mild impact on the rest of the garden's life forms.
In general, these products are short-lived after you use them in the garden -- that's what makes them so good. However, in order to get effective control, you often have to use them more frequently than stronger chemicals.
Here are our favorites:
- Biological controls: This method involves pitting one living thing against another. Releasing beneficial insects is one example of biological control, but you can also use bacteria that, while harmless to humans, make rose pests very sick and eventually very dead. The most common and useful to rose growers are forms of Bacillus thuringiensis, or Bt, which kills the larvae of moths and butterflies -- that is, caterpillars. However, some strains of Bt control other types of pests. For example, one type (sold as milky spore) kills the larvae of Japanese beetles.
- Botanical insecticides: These insecticides are derived from plants. The following two are especially useful against rose pests.
- Neem comes from the tropical neem tree Azadirachta indica. It kills young feeding insects and deters adult insects but is harmless to people and most beneficials. Neem works slowly and is most effective against aphids, thrips, and whitefly, but it also repels Japanese beetles.
- Pyrethrins are derived from the painted daisy, Chrysanthemum cinerariifolium. It is a broad-spectrum insecticide, which means that it kills a wide range of insects, both good (spray late in the evening to avoid killing bees) and bad. That's the downside. The upside is that this insecticide kills pests like thrips and beetles quickly, breaks down rapidly in sunlight, and has low toxicity to mammals, which means that it's essentially harmless to people, pets, and the environment.
- Horticultural oils: When sprayed on a plant, these highly refined oils smother pest insects and their eggs. The words highly refined mean that sulfur and other components of the oil that damage plants are removed. They are relatively nontoxic and short-lived. Two types exist:
- Dormant oils are sprayed on roses when they are leafless in winter. They are often combined with a fungicide like lime sulfur or fixed copper to help kill overwintering disease spores.
- Summer oils usually are more highly refined (or further diluted) than dormant oils -- they're thinner. They can be used on roses during the growing season, as long as the plants have been well watered and temperatures are not above 85° F (29° C).
- Insecticidal soaps: Derived from the salts of fatty acids, insecticidal soaps kill mostly soft-bodied pests like aphids, spider mites, and whiteflies. They can also be effective against Japanese beetles. They work fast, break down quickly, and are nontoxic to humans. Insecticidal soaps are most effective when mixed with soft water. Soaps sometimes burn tender foliage.
- Baking soda (sodium bicarbonate): Regular old Arm & Hammer has been a popular powdery mildew remedy (partially effective against black spot) in the rose underground for the past few years. If you want to try it, do it like this: Mix 1 rounded tablespoon of baking soda with 1 tablespoon of summer oil in a gallon of water. Apply weekly to well-watered plants, and don't spray if temperatures are above 85° F (29° C). The combination of the spray with the heat damages leaves. Ongoing research has revealed that potassium bicarbonate might work a little better and be less prone to damage leaves. We expect to see products containing potassium bicarbonate and summer oil soon.
- Antitranspirants: When sprayed on plant foliage, these form a thin, waxy layer that can prevent fungal disease like powdery mildew from invading the leaves. Antitranspirants don't kill disease, but they may prevent a disease from getting worse.
We prefer neem oil over neem extract (check the product label) because oil is also effective against all three common rose diseases: black spot, powdery mildew, and rust. Neem oil gets thick when cool, so you need to warm it up a bit before trying to mix it with water.
Use either kind of neem before you have a major pest problem. Neem is most effective when applied in early morning or late evening when humidity is highest. Reapply after rain.
Currently, you can buy neem oil only from the Green Light Co., Box 17985, San Antonio, TX 78217; 210-494-3481. An 8-ounce container costs about $13, and you need to use 2 tablespoons per gallon of water.
The terminology can be confusing, however. Pyrethrum is the ground-up flower of the daisy. Pyrethrins are the insecticide components of the flower. Pyrethroids, such as permethrin and resmethrin, are synthetic compounds that resemble pyrethrins but are more toxic and persistent. Consequently, we prefer to avoid pyrethroids for home garden use.
Avoid using oil sprays when temperatures are likely to reach above 85° F (29° C). When it's that hot, the oil can damage plant leaves.
Baking soda can burn leaves. Apply it in the early morning and not at all during very hot weather.
People call pesticides such as carbaryl, diazinon, and malathion the "traditional" pesticides. Such a label would make George Orwell proud. These pesticides are not traditional at all, unless you figure that only what happened after World War II is "traditional." Most of the petroleum-derived pesticides that line the shelves in garden centers weren't available to home gardeners until the 1950s.
Many chemical pesticides are labeled for controlling rose pests. They are generally effective but usually kill beneficial insects as well as pests. And in some cases, pests have developed resistance to a particular spray, so the spray no longer provides adequate control. When we mentioned a traditional pesticide in the earlier section on controlling pests, we used the generic name that is listed on the product label under "active ingredients." Most of these chemicals also have trade names, but they often vary by manufacturer. Table 21-1 lists some spray pesticides by trade name (and by chemical name in parentheses) and tells which pests they target.
Table 21-1 Spray Products for Perfect, Pest-Free Roses
|Product||Pests It Controls|
|Avid (abamectin)||Spider mites only|
|Fungi-Gard (chlorothalonil)||Black spot|
|Funginex (triforine)||Many fungus diseases|
|Malathion||Aphids and many other insects|
|Mavrik (fluvalinate)||Mites, tobacco budworms, black aphids, and cucumber beetles|
|Orthene (acephate)||Thrips, whiteflies, aphids, and many other insects|
|Orthenex (several chemicals)||Insects and diseases|
|Protect T/O (maneb)||Severe black spot|
|Rubigan (fenarimol)||Severe powdery mildew|
|Sevin (carbaryl)||Japanese beetles, rose chafers, and cucumber beetles|
|Subdue 2E (metalaxyl)||Downy mildew|
Most of the pesticides in Table 21-1 are widely available. A few, such as Avid, Rubigan, and Subdue, are hard to find. Check with your local rose society or a mail-order supplier such as Primary Products, 100E Tower Office Park, Woburn, MA 01801; 617-932-8509.
In the pest descriptions earlier in this chapter, we included both traditional chemical controls and less toxic alternatives, such as botanical insecticides and beneficial insects. We prefer to use alternatives first and turn to toxic materials only if the survival of a plant may be threatened. We do not believe in using combination products (fertilizer, insecticide, and fungicide). They are a shotgun approach to pest control that often results in excess use of unneeded chemicals, which can harm the environment.
A Rosarian's Approach to Pest Prevention
Serious rose growers -- less charitable folks might say obsessive -- follow a preventive spray program to keep their roses in perfect shape and virtually pest free. They often use chemicals that are not widely available in nurseries and garden centers but that can be purchased through the mail (see Appendix C).
We've already stated our approach to controlling pests, but if you want "perfect" roses, you may decide that you want to go in a different direction. That's your choice. Here's one very knowledgeable rosarian's recommendations for preventing pests:
Spray every ten days with a good insecticide-miticide-fungicide combination. Whether you prefer to choose your own materials -- such as acephate (same as Orthene) or malathion for bugs, triforine (same as Funginex) for fungal diseases, and Avid for spider mites -- or use a combination product like Orthenex, which has three different chemicals in one bottle, a ten-day spray program plus good cultural practices will keep your roses looking great from spring to fall.