Pests And Diseases

There are a number of insects whose lifetime ambition is to suck or chew on rose bushes so that they can obtain all the vitamins, minerals and other necessities needed to produce a new generation to carry on their species. The gardener want to keep the roses healthy. You will need to make some important decisions: use insecticides or not. The average manual on roses contains a formidable list of chemical insecticides. Let's take a closer look at what happens to the ecology of a garden when insecticides are used. Most insecticides kill a wide spectrum of insect species, usually by affecting their nervous or digestive systems. They will kill not only insects inhabiting the leaves and branches of the bush but other insects and soil microbes as well when the chemicals wash into the soil. These other creatures may be playing important roles in keeping your garden healthy: as rain and sun wash away and degrade the insecticide residues, insect life returns to the sprayed areas. However, with the numbers of many insects reduced, the new balance is quite different.

Every insect has a predator. Without such predators, we would be scraping aphids off our cars in the morning. When we spray our roses we kill not only the pests we want to be rid of but their predators as well. When the surviving pests, or those arriving from other places, start to reinfest the rose bush, few predators remain to keep them under control. This often results in devastatingly high populations of pests, which, if not sprayed again, will do serious damage to your plants. Predators always reproduce more slowly than their food source, otherwise the predators would rapidly eat up their food source and die. In other words: there must always be a population of food present to maintain predator populations. The key to biological control of pests is to be able to maintain high enough populations of predators to keep pests from doing unacceptable damage to your plants.

When you spray insects with an insecticide, most of them will die, but not all. Every creature on Earth is unique, with its own set of characteristics, its own genetic code. So while most insects in a population will be killed by a particular poison, certain individuals may be tolerant to it. If they survive to reproduce, they pass on their tolerance to the new generation. After many generations, entire populations of insects may become tolerant to certain insecticides. This phenomenon is well documented and has caused much concern in conventional agricultural circles. For several years our nursery sprayed its plants with insecticidal soap. This substance is relatively nontoxic to mammals but is effective against many insect species. We noticed after several years that we seemed to need more and more soap to control our aphid populations. It took a while for the truth to sink in. We were creating a race of aphids that could tolerate soap.

If you wish to adopt a non-chemical approach to gardening, you must first realize that insects are an important part of the garden. Just because it crawls or flies, an insect is not necessarily an unwanted alien. Nearly all insects in your garden are benign or actually help the gardener by keeping other insect populations in check, by pollinating your flowers, by aerating the soil or by performing any number of countless tasks that keep the garden healthy. There will always be some unwanted insects that will feed on our roses. If we are to make a serious commitment to eliminating harmful chemicals from our gardens and our agricultural community, we must change our zero-tolerance approach toward insects. We must accept a certain amount of damage as nature's due.

The health of your plant is of paramount importance in reducing insect problems. A healthy plant reflects a healthy soil and a proper site. If you have looked after the basic requirements of your rose, you will have far fewer problems. An actively growing plant is less subject to insect injury. There is a growing body of evidence that plants under stress are more attractive to insects. This hypothesis is being substantiated by hard data that suggest that plants under stress emit substances and sounds that can be detected by insects. It makes perfect sense that a well-fed plant is more likely to remain healthy. Good parents feed their children well so that they do not suffer from disease. So it should be with our garden charges.

Gardeners constantly seek out varieties with beautiful flowers, unusual color, good vigor or pleasing form. The gardener who is committed to reducing the use of chemicals will pay strict attention to varieties that exhibit good resistance to insects and diseases. Roses differ dramatically in their tolerance to insects. When choosing varieties, select those roses that will make your job easier. It is encouraging that disease and insect resistance are now becoming important criteria for judging new varieties, a trend that is both welcome and long overdue.

Diversity in a garden is a tremendous asset. Most insects go through several phases during their lifetime. By offering an assortment of plants, you can provide sites for predators to complete their life cycles. As an example, in the fall our grapevines harbour clusters of ladybugs, which often number in the hundreds. Ladybugs prey on aphids. Although we do not know why the ladybugs are there, they use the shelter of the grapes to gather and perhaps mate. A healthy garden is a diverse garden.

Common insect pests of roses

If you choose to use biological methods against insects, a better understanding of insect pests is your best weapon. By learning about the life cycles of these insects you may discover ways you can interfere with cycles and prevent large populations from building up in your garden.

Aphids

Aphids are perhaps the most common pest of roses. They are small, soft-bodied, usually lime green creatures that puncture the soft new growing tissues with their mouths and suck the leaf juices. Severe infestations will cause the young leaves to curl and dry up. Aphids are nearly always found on the undersides of the leaves near the ends of the shoots. As they feed, they excrete a sticky residue that is attractive to ants. Certain types of ants feed on this "honeydew" and will even protect an aphid colony from other insects. If there is a great deal of "honeydew," it will often appear blackish on the stem's surface as molds and fungi begin to grow on it.
Aphids are one of the most prolific insects in existence and also have one of the most amazing life cycles. They overwinter as tiny blackish eggs on the stems, usually near a bud. In the spring small nymphs hatch from the eggs and quickly grow to full size. These first aphids are called stem mothers. They have the fascinating ability to hatch their live young without fertilization from a male. Several generations are produced in this manner. Then a generation is born with wings. These winged aphids, called migrants, fly to other plants, some to the same species of plant, others to a summer host plant, usually an annual of some kind. These continue to produce generations of unfertilized young throughout the summer. As the days grow shorter a generation is produced that contains both winged females and winged males. The females, called fall migrants, fly to the kind of plant on which they started in the spring, then give birth to wingless females, which must be fertilized by the males to produce eggs. The eggs are laid around the buds and crevices of the plant. In the spring these hatch out and the cycle is complete.
Each aphid is theoretically capable of producing millions of aphids by the end of its cycle. The reason we are not swimming in aphids is because so many other insects and birds consume aphids. Early in the spring small solitary spiders can be observed catching and eating aphids. Soon such predators as the small gall midge and the larvae of the syrphid fly begin feeding on them. As spring progresses, the most efficient enemy of the aphid appears. When aphids begin to multiply, adult ladybugs arrive at the aphid colonies and lay their eggs. After about two weeks, tiny, opaque and ravenous larvae hatch and begin feeding. They hold the aphids in their large mandibles and suck their insides out. In only a few days these larvae grow to nearly twenty times their original size and eliminate the colony of aphids. They move from colony to colony until they reach full size. They then form a hardened shell and pupate. In two weeks or so they emerge as the winged ladybugs that most everyone recognizes. They are rounded beetles, usually red or yellow with several dark spots on their wings. Although they eat some aphids at this stage, it is their young that are every aphid's nightmare.
It is imperative that you do nothing when aphids first appear on your roses. Spraying at this stage, even with soap or similar nontoxic substances, is a tragic mistake, for the ladybug's eggs or larvae, as well as other predators, will be killed. You must grit your teeth and bear them for a while. After two weeks or so you should begin to see the small ladybug larvae at work and will notice colonies of aphids reduced to empty white husks. Once the ladybugs establish a presence, the aphids will be kept to minimal levels. If your roses are growing in a light sandy soil, you may find that ants are protecting aphids from predators. This can be alleviated by spreading a thick layer of mulch in the garden. Ants prefer dry, well-drained conditions; under a mulch there is a great deal of moisture, and the ants will not be encouraged to build their colonies. If you are raising roses in a greenhouse, where ladybugs cannot enter, you have several options. It is possible to buy predators from companies that specialize in biological controls. Another simple but effective control is to hose plants down regularly with a well-directed and strong stream of water. Insecticidal soap can also be effective against aphids, and may be necessary in the greenhouse, where normal insect relationships are disrupted.

Gall wasps

Gall wasps are tiny insects, usually black or orange. They are so small that a hand lens is necessary to see them well, and they are usually noticed only when the gall forms around the larval stage. The galls interfere with the flow of water and nutrients to the sections of stems above them, and occasionally large numbers of galls are noticeable. Where no control is practiced, infestations can build up to levels that can seriously reduce the vigor of your roses.
In the spring, adult wasps lay their eggs in the stem of the rose. These eggs hatch in approximately four days. Once the larvae begin feeding, the plant reacts by producing masses of tissue around the larvae. The larvae overwinter in these protective galls. In spring they pupate. When their host plants are at the proper stage, the adult wasps eat holes through the sides of the galls, emerge, then lay their eggs to begin the new generation.
Gall wasps are seldom a severe problem. To control them, remove the galls as soon as you notice them. If you cut the gall in half with a knife, you will be able to see the larvae inside. Bury or burn the removed galls to ensure there is no threat from another generation. If you keep an eye out, you will notice galls on oak trees, goldenrod and other plants.

Leafhoppers

Leafhoppers are small, pale green to greenish white and very active creatures that eat foliage with sucking mouthparts. They are found on the undersides of leaves. Through a hand lens, they appear to have a large head with a somewhat triangular body, the wings coming to a high ridge along their backs. Although they are not usually a serious problem on roses, they can reduce the vigor of badly infested plants.
Leafhoppers overwinter both in the egg stage and as adults. Adults become active early in spring and often mate even before leaves appear. They push their eggs into the midrib of the leaves. The first generation appears about the time leaves become full-size, and they begin feeding by sucking sap from the undersides of the leaves. If disturbed, they will live up to their name and hop from leaf to leaf. If an infested plant is shaken, a small cloud of leafhoppers will be airborne, but they settle quickly, as they are not flyers. Several generations are produced each year, and as each leafhopper grows, it sheds its skin in the molting process. These small white skins cling to the undersides of the leaves, confirming the presence of leafhoppers.
Leafhopper populations vary from year to year. The best general advice for leafhoppers is to keep your plants actively growing. They have little effect on a well-fed, well-watered rose.

Mites

Mites are closely related to spiders. Several types can infest roses. The most common in northern gardens is the European red mite. The two-spotted mite and the spider mite can be pests in the greenhouse. Mites are quite small and you'll need a hand lens to see them. Their presence is indicated by a general yellowing of the leaves and fine web structures on the undersides of the leaves, where they feed. They are sucking insects that destroy the inside tissues of leaves.
Mites spend the winter as small red eggs on branches, around bud scars or in crevices. About the time apple blossoms are opening, the eggs hatch and the first generation begins to feed. If conditions are right, the mites will begin to multiply rapidly. Their entire life cycle can take as little as four days to complete and several generations a year may occur, although the northern gardener is blessed with a shorter, cooler growing season, which helps to limit the number of generations.
Mites can be thought of as a signal that your roses are not receiving enough water. A well-watered, vigorous rose is rarely affected by mites. Mulching roses will go a long way toward ensuring that your soil stays adequately moist. If the weather is extraordinarily dry, be sure to give your roses a good soaking. If you notice a buildup of mites, wash the foliage. Once off the leaf, mites cannot regain their place on the plant. Mites have numerous predators. Certain species of thrips have a voracious appetite for mites. They eat both the eggs and the larvae. There are also predator mites, which feed on the pest mites. Ladybugs and related insects also enjoy a good meal of mites.

Sawflies

Several species of sawfly eat roses. Sawflies are in the larval or caterpillar stage when they do their damage. They are smallish, usually about 1/2 in (1.3 cm) long, somewhat enlarged at the front and have a characteristic habit when active of holding on to the leaf with their front feet and curling the remainder of their bodies into the air. When resting they remain curled on the underside of the leaf. Some sawflies roll the leaves as they eat them. Others eat all the leaf except the main veins. Populations vary from year to year, but, when dense, sawflies can cause a great deal of damage to rose foliage and are one of the most aggravating pests.
The adult sawfly lays its eggs on the leaves. These hatch out into tiny larvae, which begin feeding immediately. When they reach maturity the larvae fall to the ground, where they spin cocoons around themselves, remaining in the leaf litter until spring, when they emerge with wings and lay eggs to begin the cycle again.
Populations vary so much from year to year that some years sawflies may be no problem at all, while in other years they may strip whole bushes clean. At the first sign of infestation, a thorough hand picking should be done. The more sawflies you pick, the fewer will get a chance to reproduce. A simple but effective technique of lowering sawfly populations is to beat infected rose bushes with a padded stick or to kick them. The caterpillars will drop to the ground. There, away from their habitat, they become easy prey for ground-patrolling birds and insects. Severe infestations can be treated with materials such as rotenone, pyrethrins or insecticidal soap. However, use these as a last resort, as they will disrupt other insect populations. If you have a severe infestation, it would be a good idea to work the soil around the roses deeply or add a thick layer of mulch; this will kill many of the overwintering cocoons by burying them in damp soil. Birds should be encouraged, as they are one of the primary predators of the sawfly.

Spittle bugs

Spittle bugs are also known as froghoppers. They are usually noticeable only when they surround themselves with a protective layer of white foam, which looks just like spit. They have sucking mouthparts that draw sap from the stems of many plants, including roses.
They begin life as eggs, which hatch into small green nymphs. During the year they molt, gaining size as they do so. Eventually they mate and lay eggs in the fall for next year's generation.
Although noticeable, spittle bugs do little harm to roses. Occasionally a larger than normal number may appear on bushes, but these can be easily sprayed off with a garden hose or picked off by hand.

Other pests

There are many other insects that attack roses, such as Japanese beetles, chafer beetles and earwigs. Patrol your roses and learn to recognize the presence of such pests. Usually a hand picking will take care of an infestation if caught early. The earwig, a nocturnal species, can be caught in traps made of strips of corrugated cardboard. They crawl into the corrugation at daybreak and can then be disposed of.

Diseases of roses

Roses have a reputation as troublesome plants that need to be constantly coated with various fungicides to prevent the diseases to which they are prone. In part, this reputation is deserved. Many roses, particularly in the more humid climates, fall victim to rusts, mildews and that most ubiquitous of rose diseases, blackspot. The modern hybrid teas and floribundas are quite susceptible to fungal diseases. In general fungi, which are among the more serious diseases, are spread in damp conditions. While it is impossible for you to change the weather, you can help discourage fungus. The placement of roses is so important. If your roses are placed in a garden where there is very little air movement, they will take much longer to dry after a rain and will tend to hold the humidity around them. Roses placed where there is good air movement will dry out more quickly and will, therefore, have drier foliage. This will often make the difference between a heavy infestation of fungal disease and a light occurrence or even total absence of disease. Sunlight helps to inhibit fungal growth, and the availability of sun is of great importance in preventing it. Pruning roses to open up the bush will increase both air movement and sunshine within the bush.

Blackspot

Without a doubt, blackspot is the most common scourge of roses. Most varieties are at least partially susceptible, and some varieties can become defoliated if it is not controlled. As its name suggests, the symptoms are black or brown spots, which begin to appear in early summer. The previous year's infected leaves release millions of spores into the air, which settle on the leaves and begin to grow. Blackspot generally shows up first on the older, lower leaves, and can eventually infect the entire shrub.
It is helpful to remove and burn or compost any infected foliage on or under your rose bushes. This will reduce the number of spores the next season. A new layer of mulch each fall will bury overwintering spores and prevent them from dispersing. Keep susceptible varieties pruned to an open shape and try to get as much air movement as possible around the plants. If you wish to grow some of the more susceptible varieties, you will need to begin a preventive spray program in mid-spring. Wettable sulfur powder provides reasonably good protection and does not have the toxicity of fungicides. Spray after each rain or wet period. Use a few drops of a liquid soap in your sprayer to help spread the sulfur evenly over the foliage. Without a spreader the sulfur will tend to bead and roll off the leaves. An old remedy uses baking soda (sodium bicarbonate) as a preventive spray. Mix 1 oz. (30 g) of baking soda in 10 gallons (40 L) of water and apply after damp periods.
Rosa rugosa is a species commonly used in the breeding of hardy roses and is one of the most resistant to blackspot.

Canker

Cankers usually appear as the result of poor sanitation and pruning. The cankers show up as brown or orange spots on dead wood and then spread to the adjacent live wood. By pruning out dead wood, canker can be almost wholly prevented. Use sharp pruning shears that will not tear wood, and cut cleanly and closely to the nearest live branch. Burn canker when it is found.

Crown gall

Crown gall is found on the roots of roses and related plants. It is caused by a bacterium (a microscopic one-celled organism) that is found in nearly all soils and that gains entrance through a mechanical injury or insect damage. The galls are irregular, bulbous growths, which over time can become quite large. There is some debate over how much crown gall harms the plants it infects, but the concensus is that it does affect the vigor and longevity of the plant. Do not use gall-infected plants if you can avoid it. If for some reason you have to use a plant that shows galls, cut off the infected root. A biological control for crown gall is now available. Before planting, the roots are dipped into a solution of water and a bacterium (Agrobacterium radiobacter), which inhibits the growth of the crown gall bacterium. This specific bacterium is quite safe to use.

Powdery mildew

Powdery mildew shows up as a grayish-white coating on the surface of infected leaves. It is particularly troublesome in late summer. Susceptible varieties can be severely harmed unless measures are taken.
Mildew, as is the case with so many diseases, tends to show up on stressed plants more than on healthy ones. Roses that are either poorly nourished or over stimulated by heavy feeding of nitrogen fertilizers are particularly prone to mildew. Plants that do not receive enough water are likewise affected. Keep your plant mulched and well fed with compost. This will promote a balanced growth that will help to prevent mildew. Place susceptible varieties in areas where they will get good air circulation. If you are growing a particularly susceptible variety, you may find it necessary to adopt a spray program using wettable sulfur. Bicarbonate of soda can be used as well.

Rose midge

The rose midge is one of the most frustrating of rose pests. The adult midge is a very small, yellowish fly that lays its eggs in the stem just below a forming bud. When the egg hatches, the tiny larva begins feeding. As it feeds, it disrupts the flow of nutrients and water to that side of the bud on which it is feeding. The result is that the bud bends to one side. The lack of food and water is usually fatal to the bud, and it blackens and dies before opening. Careful and consistent examination of your roses is critical to keeping this insect at bay. Any stems that show the characteristic bent bud should be removed immediately and destroyed. This will disrupt the cycle and prevent further generations from developing. Diligence is the gardeners best weapon against this pest.

Rust

Rust shows up as orange patches on the undersides of leaves and can, if unchecked, spread to the stems. It shows up in warm, moist weather. It does not seem to be as widespread in the more northern areas. This may reflect a difference in the types of roses grown, or perhaps rust is unable to overwinter in colder temperatures.
The life cycle of rust is much the same as that of blackspot, overwintering on leaves. If you notice rust, immediately remove and burn any infected foliage. Infected leaves should likewise be removed from the ground to prevent re-infection. A new layer of mulch will help prevent dropped leaves from dispersing spores into the air.

Viruses

Viruses are mysterious organisms that show up throughout the animal and plant world. Virus-infected roses will show symptoms such as yellowed and streaked leaves. The flowers may tend to be small and few, and foliage may drop prematurely. You cannot cure virus. It is most often spread when infected stock is used for nursery propagation. If you are sure you have a virus-infected rose, and not simply a rose that is suffering from pests, lack of water or nutritional deficiencies, rip it out. Sucking insects such as aphids can spread a virus to healthy plants.