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Roses Insects & Diseases

Rose Insects, Diseases & Related Pests


With their showy and often fragrant blooms, roses are easily one of the most popular flowering plants grown. Unfortunately, the numerous insects and related pests that attack them can make growing them "interesting ", if not outright challenging. As with any plant, the first priority should be to provide the rose with the cultural conditions that it requires. A vigorously growing rose is much more likely to survive pest damage than a stressed plant.


When trying to control insects and related pests on roses, it is essential that the plants be thoroughly inspected on a regular basis. These inspections increase the likelihood that a pest infestation will be detected early, when pest numbers are low and control is easiest. In order to choose the best control method, it is necessary to correctly identify a pest first. Often, more than one control option is available for a pest. Whenever possible, physical control measures should be tried first. If a chemical control is necessary, the least toxic chemical should be used, being sure to apply it when a susceptible stage of the pest is present. When applying a pesticide, thorough coverage is important. Always be sure to read the pesticide label before purchasing. Apply all pesticides according to label instructions, following all precautions.


Aphids


Various species of aphids feed on roses, but the predominant species is the rose aphid (Macrosiphum rosae). Rose aphids are small (about 1/8 inch long). They are soft-bodied, pear-shaped, pink or green insects that are found in clusters on new growth of buds, leaves and stems.


Aphids feed on plant sap with their piercing-sucking mouthparts. A low population of aphids does little damage to a rose bush; however, aphids reproduce very rapidly and can quickly reach numbers that cause damage. Their feeding results in distorted growth. Heavy infestations can reduce the number and quality of blooms. As they feed, aphids excrete honeydew, a sugary substance that attracts ants and wasps. The honeydew supports the growth of unsightly, dark-colored sooty mold fungi on the leaves.


Control: Aphids have several natural enemies, including parasitic wasps, ladybird beetles (ladybugs) and larvae, and green lacewing adults and larvae. Their natural enemies tend to keep aphid populations under control except in cool weather. Ants are sometimes associated with aphid infestations and will protect them from their natural enemies. If ants are present, they should be controlled.


Aphids can be hosed off with a strong stream of water directed above and below the leaves. Spraying with water should be repeated frequently as needed, focusing in particular on new growth. Roses can also be sprayed with insecticidal soap to control aphids. Insecticidal soap must be sprayed onto the aphids to be effective. Repeat spray three times at 5-7 day intervals. Higher toxicity insecticides are available. However, it is important to note that aphids are very difficult to control because they multiply so rapidly. Leaving even one aphid alive can result in a large population very quickly. In addition, these insecticides kill the natural enemies of rose aphids.


If insecticides are deemed necessary, the following are available in homeowner size packaging. Sprays containing bifenthrin, cyfluthrin, esfenvalerate, horticultural oil, insecticidal soap, lambda cyhalothrin, malathion, neem oil, permethrin, or pyrethrin will control aphids. Soil drenches or granular applications of imidacloprid, dinotefuran, or disulfoton will control aphids and last longer within the plant to prevent future infestations.


Beetles


A number of different beetle species feed on roses. Many of these beetles feed mainly on flower buds or open blossoms, but can feed on leaves. Since many beetles feed mainly at night, the gardener rarely sees them, only the damage that they cause.


Japanese beetles (Popillia japonica) feed during the day and are perhaps the most readily recognized of the beetle pests that feed on roses. An adult Japanese beetle is about ½ inch long and has a metallic green body and legs with coppery-brown wing covers. It can be distinguished from similar beetles by the tufts of white hair that are clearly visible at the end of its abdomen.


The adults begin emerging from the soil in mid-May and are present through August. They can live from 30 to 45 days. They lay their eggs in the soil. Grubs hatch from the eggs and feed on grass roots. As the weather cools, the grubs move more deeply into the soil to overwinter (survive the winter). In the spring, the grubs migrate back up to the root zone and continue to feed. They pupate (change to adult form) in late April and May.


Japanese beetles have chewing mouthparts and feed on flowers, buds and leaves of roses (as well as numerous other plant species). Partial or entire flowers and buds may be eaten. Typically, flowers and buds that have been fed on have ragged edges and/or holes in the petals. Affected buds may fail to open. Rose leaves are typically skeletonized (only leaf veins remain) by the feeding. Leaves with tender veins may be eaten completely.


Control: Various non-chemical control options are available for Japanese beetles. They can be handpicked and destroyed by dropping into soapy water. When only a few plants are involved, fine netting, such as tulle fabric, can be placed over the bush or individual blossoms to exclude the beetles. Japanese beetle traps are available commercially, but should be used with caution. They can be effective at reducing adult populations, but they should be kept at least 50 feet from the plant(s) that you are trying to protect. The traps have the potential to create more of a problem by attracting numerous beetles to the area. Also, traps must be emptied frequently as beetles are repelled by the smell of ammonia which is released by dead, rotting beetles.


Numbers of adults may also be reduced by using the product, Milky Spore, against the grubs in the lawn. This product contains a disease-causing bacterium (Bacillus popilliae) that specifically infects the grubs of Japanese beetles. It is applied to turf and once established, can be effective for 20 to 30 years. However, as the adults are strong fliers, they can fly in from nearby lawns and pastures.


It is important to keep in mind that rose blossoms openly quickly and are very attractive to Japanese beetles. These circumstances make it difficult to keep the blooms adequately covered with insecticide to protect them. Insecticides that are labeled for homeowner use include sprays containing bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda cyhalothrin, malathion, neem oil, permethrin, or pyrethrin to control beetles. Soil drenches or granular applications of imidacloprid, dinotefuran, or disulfoton will control Japanese and other beetles and last longer within the plant to prevent future infestations.

 

Mites


Mites are not insects but are more closely related to spiders with eight legs as adults instead of six. They are extremely small (about 1/50 inch long) and are somewhat difficult to see without a magnifying lens. One way to detect them is to hold a piece of white paper under a branch and then tap the branch sharply. Wipe your hand over the paper. If mites are present, red streaks will be seen.


Two-spotted spider mites (Tetranychus urticae) and southern red mites (Oligonychus ilicis) are most common pests on roses. Two-spotted spider mites are more of a problem during hot, dry weather and susceptibility increases when a rose is drought stressed. Southern red mites are more of a problem during cool weather in spring and fall, and their populations drop during summer.

 

Mites have piercing-sucking mouthparts. They suck plant sap, typically feeding on the lower surface of a leaf. Early damage is seen as yellow or white speckling on the leaf's upper surface. Fine webbing may be seen on the undersides of leaves. With severe infestations, leaves may develop a grayish green or bronze color, and webbing may cover both sides of leaves as well as branches. Severely infested leaves may drop prematurely. Webbing can collect dust, making the plant look dirty.
Control: Both beneficial insects, such as lacewings and lady beetles, and predatory mites prey on spider mites. Predatory mites are about the same size as spider mites but can be distinguished from spider mites by their long legs and the speed with which they move. Several species of predator mites are available commercially for use as biological control agents.


A strong spray of water is a non-chemical control option that removes eggs, larvae (six-legged immature stage), nymphs (eight-legged immature mites) and adult mites. Be sure to spray lower surfaces of leaves and repeat as needed. This method is most effective with light infestations as seen with early detection. An important advantage of this control method is that populations of natural enemies are not harmed.


Insecticidal soaps and horticultural oils are effective control options for spider mites, and are essentially nontoxic to humans, wildlife, and pets, and only minimally toxic to beneficial predators. When using these products, good coverage is critical to ensure contact with the pest, and reapplication may be needed as determined by follow-up monitoring for the pest. Foliar injury from soaps and oils may occur on plants under drought stress. Water the plants well prior to spraying. Do not spray with soaps or oils if the temperature exceeds 85 degrees.


When growing roses, the use of broad-spectrum insecticides should be avoided as much as possible as these products can kill off natural enemies that help keep spider mite populations in check. Also avoid pesticides that claim to "suppress" mites as they tend to be weak miticides. When stronger chemical control is needed, the following insecticides/miticides are available in homeowner size packaging: tau-fluvalinate or bifenthrin sprays or granular applications of disulfoton

 

Thrips


Various thrips species feed on roses. Two of the most common are flower thrips (Frankliniella tritici) and western flower thrips (F. occidentalis).
Adult female thrips of both species are tiny, yellowish-brown insects with fringed or feathery wings. At less than 1/16 inch long, they are barely visible without a magnifying glass. However, blowing lightly into the blooms and leaves causes thrips to move around, making them easier to see.


Both immature and adult thrips feed by scraping surface cells to suck plant sap. They feed on both leaves and flower petals with the majority of their damage to roses occurring from early to midsummer. Their feeding may result in distorted buds that open only partially or abort prematurely. Feeding on petals may result in petals streaked with silvery-white or brown as well as petals with browning edges. White and light-colored rose blossoms appear to be particularly attractive to thrips. Young leaves may be distorted and flecked with yellow as a result of thrips feeding.


Control: Control of thrips is difficult. Infested rose blossoms should be removed and destroyed. Grass and weeds in the area should be kept mowed or removed when possible. Insecticides are available but timing of sprays is very important. They must be applied before thrips enter unopened buds. In addition, because rose blooms expand rapidly, it is difficult to keep them adequately covered with insecticide. If it becomes absolutely essential to spray an insecticide, the following are available in homeowner size packaging: acephate, bifenthrin, cyfluthrin, esfenvalerate, lambda cyhalothrin, permethrin, or spinosad. Insecticidal soaps will help control thrips, but thorough coverage is necessary. The soap spray must contact the pest to be effective, and may require three sprays at 5 to 7 day intervals. Soil drenches or granular applications of dinotefuran, disulfoton or imidacloprid will give thrips suppression.

 


Chilli Thrips


Roses just got harder to grow because of a new out-of-control pest. Even the tough, durable and previously problem-free Knock Out rose is no challenge for this critter -- neither are many common ornamentals. Meet the chilli thrips that seem to be everywhere.


Plant foliage usually becomes distorted and often bronze to brown in color ,as noted in the most central portions of the rose plant pictured. The discolorations may also appear as streaks and blotches. Unlike many other thrips, which are often brown to black in color, this species is pale yellow to greenish-white and very small. In most cases, you will miss seeing the insects without a hand lens or a microscope.


Chilli thrips appear to be here to stay, having arrived in force about a year ago. It's probably best to check all your plants frequently during the warmer months for signs of this pest.

 

Now the good news: Chilli thrips are controllable. At least one naturally derived insecticide marketed as Conserve is giving good control. It's available at local independent garden centers as Fertilome Borer, Bagworm and Leafminer Spray or Southern Ag Conserve.

 

Some synthetic insecticides are doing a good job, too, including the synthetic pyrethroids marketed by most companies and Merit, which is found in several systemic insecticides.


Check the label of the products to make sure the plants you want to treat are listed before making a selection. Then when applying the pesticide, follow the label instructions to be safe and prevent major damage to your plants while controlling the Chilli thrips.

 

Rose Scale

Adult scale insects have an unusual appearance. They are generally small and immobile, with no visible legs. They secrete a waxy covering, making some appear white and cottony while others appear like white, yellow, brown or black crusty bumps. The waxy covering or "scale" protects adult scale insects from many insecticides. Their immature forms, called crawlers, are susceptible, however.

 

Several species of scale are pests of roses, but rose scale (Aulacaspis rosae) is one of the most serious. Female rose scales are round, gray to white and about 1/16 inch long. Males are elongate, white and much smaller than females. These insects overwinter as eggs under the waxy covering of the mother.


Rose scales are usually found on rose canes where they feed on sap with their piercing-sucking mouthparts. With a heavy infestation, rose scale can cause cane decline or twig dieback.


Control: Various natural enemies, including ladybird beetles (ladybugs) and parasitic wasps, usually keep scale insects under control. With light infestations, scale can be scraped off by hand and destroyed. Pruning out and destroying heavily infested canes is helpful. Horticultural oils (also called supreme, superior or summer oils) work well to control armored scales, such as the rose scale, by penetrating their waxy covers and smothering them. Horticultural oils applied at higher rates of 3% to 4% during the dormant season (i.e., to a rose bush that has lost its leaves) will penetrate the thick waxy covers of the overwintering adults. Applications at lower rates of 1% to 2% can be used during the spring to target the crawlers (immatures) and the newly settled scales with thin waxy covers. It is best to spray when temperatures are between 40 and 85 degrees.


Monitor the crawler emergence in the spring with sticky cards, double faced tape wrapped around a branch, or by putting an infested shoot into a baggie and watching for crawler movement. The presence of crawlers can sometimes be determined by sharply tapping an infested twig on a piece of white paper. Crawlers are very small and will appear as moving specks of dust.


Avoid using insecticides as much as possible as they will often kill the naturally occurring enemies of scale. When insecticides are necessary, they should be applied only when the crawler stage is present. The following insecticidal sprays are effective against crawlers only: acephate, bifenthrin, cyfluthrin, esfenvalerate, lambda cyhalothrin, malathion, or permethrin. Soil drenches of imidacloprid do not control these armored scales, but soil granular applications of dinotefuran may give some control.

 


Rose Leafhopper


Adult rose leafhoppers (Edwardsiana rosae) vary in color from white to gray to yellow to green. They are wedge-shaped and between ¼ - ½ inch long. When a plant is disturbed, they hop or fly away quickly.


The adult female deposits eggs within the bark of rose canes in the fall. Dark, purple, pimple-like spots on the bark indicate the presence of eggs. In the spring the young nymphs (immature forms that resemble adults but are wingless) emerge from the cane. The wounds that remain in the bark as they emerge, as well as wounds made during egg-laying, can provide openings for stem canker-causing fungal pathogens to enter. Stem canker can result in plant death.


Nymphs and adult leafhoppers feed on the undersides of leaves, using their piercing-sucking mouthparts to suck plant sap. Their feeding causes white stippling (small dots) on the upper surface of the leaf. The stippling spots may merge, causing leaves to appear almost white. Damaged leaves may drop prematurely. Between feeding by the nymphs and adults, and egg laying by adult females, a severely infested rose bush may be killed.


Control: Natural enemies of rose leafhoppers include damsel bugs and assassin bugs. As such, broad spectrum insecticides that may kill these beneficial predators should be avoided. When an insecticide is necessary, be sure to spray lower leaf surfaces thoroughly. The following insecticidal sprays are effective against rose leafhoppers: acephate, bifenthrin, cyfluthrin, esfenvalerate, lambda cyhalothrin, malathion, or permethrin. Soil drenches or granular applications of dinotefuran, disulfoton or imidacloprid will suppress leafhopper populations.


Rose Slugs


Rose slugs are the larvae (immature forms) of sawflies, non-stinging members of the wasp family. Three species of sawflies, the roseslug (Endelomyia aethiops), bristly roseslug (Cladius difformis), and curled rose sawfly (Allantus cinctus), are pests of roses. The larvae of some sawfly species are hairy and often mistaken for caterpillars. Others appear wet and shiny, superficially resembling slugs. The larvae generally reach about ½ to ¾ inch in length.

 

Generally rose slugs feed at night. Depending on the species, young rose slugs feed on the upper or lower surfaces of leaves between veins, leaving a 'window' of translucent tissue that turns brown. As some species of rose slugs get larger, they chew large holes or the entire leaf with only the midrib remaining. Regular inspection of roses is important because feeding typically progresses quickly, and extensive leaf skeletonizing can occur if infestations are not noticed. In addition, with their coloring, they can be very difficult to spot on leaves.


Control: Rose slugs can be controlled by handpicking. They can also be removed by spraying with water. Once dislodged, they cannot climb back onto the plant. Insecticidal soap and horticultural oil are also effective against rose slugs. Other insecticidal sprays that are labeled for homeowner use include acephate, bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda cyhalothrin, permethrin or spinosad. Sprays should thoroughly cover both upper and lower leaf surfaces. Soil drenches or granular applications of dinotefuran or imidacloprid will control sawfly larvae. Bacillus thuringiensis will only control true caterpillars and not the larvae of sawflies.


Leafcutting Bees


Leafcutting bees (Megachile species) are similar in size to honeybees, but are a blackish or metallic purple or green color. The females cut out semi-circular sections of leaves, which they use to line their nests. The cut surface is very smooth as compared to the ragged edge that results with most leaf feeding insects.
Control: No control is recommended because the damage leafcutting bees cause is minimal, and the bees are important as pollinators.


Caterpillars


Infrequently caterpillars (immature stage of moths and butterflies) will be found feeding on rose foliage. Damage will appear as holes or irregular-shaped areas of the leaf blade that have been eaten. Several caterpillars may feed upon rose foliage, including the corn earworm, eastern tent caterpillar, stinging rose caterpillar and puss caterpillar.


Control: Insecticidal sprays of Bacillus thuringiensis, acephate, bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda cyhalothrin, malathion, neem oil, permethrin, pyrethrin or spinosad will control caterpillars. Granular soil applications of disulfoton will control caterpillars and last longer within the plant to prevent future infestations..


Grasshoppers


Grasshoppers are general feeders that feed on the foliage of many kinds of plants.


Control: Keep weeds and grass near roses under control because these are the breeding sites for grasshoppers. Insecticidal sprays with acephate, bifenthrin, carbaryl, cyfluthrin, esfenvalerate, lambda cyhalothrin, malathion, permethrin or pyrethrin will control grasshoppers. See Table 1 for specific products.

  

Black Spot


Black spot is a common and serious rose disease often reaching epidemic proportions in a season. The disease is caused by the fungus, Diplocarpon rosae. It is most severe after long wet, warm periods in the spring. Symptoms occur on rose leaves as circular, black spots surrounded by a yellow area. Infected leaves often drop from the plant. Infection continues throughout the summer months. The immature wood of first year canes develops raised, purple-red irregular blotches. Plants become stunted and produce fewer, paler flowers. By mid-summer severely infected plants may have lost all of their leaves.


Prevention & Treatment: The spread of black spot can be reduced and future infections minimized by following these cultural practices:


 Maintain Good Sanitation: Sanitation practices are critical in reducing future disease development. In the fall or winter remove all old leaves on the ground along with any mulch that has been contaminated with infected leaves. Replace with a fresh layer of mulch before new rose growth begins in the spring.
 Remove & Destroy Infected Canes: Canes affected by black spot have dark or reddish areas (lesions). Severely infected plants should be pruned back in the winter or early spring to within 1 to 2 inches of the bud union, according to variety and cultivar. During the growing season, remove and dispose of infected leaves as they appear.
 Keep Leaves Dry: It is best not to syringe plants with water, and do not use overhead irrigation, especially not in the late afternoon or early evening. Soaker hoses are an excellent way to water roses and to conserve water. Promote rapid drying of leaves by planting roses in the full sun. Space new plants far enough apart to allow for good air circulation.


Use fungicide sprays to control black spot effectively, even on resistant varieties. A rigorous fungicide program must be followed during conditions that favor disease development for susceptible cultivars. Select one of the following fungicide sprays, if disease is severe enough to warrant control: captan, chlorothalonil, mancozeb, myclobutanil, propiconazole, triforine or copper fungicides. Apply all chemicals according to directions on the label.


Powdery Mildew


Powdery mildew is another widespread and serious disease problem of roses. It is caused by the fungus, Sphaerotheca pannosa var. rosae and produces a grayish-white powdery substance on the surfaces of young leaves, shoots and buds. Infected leaves may be distorted, and some leaf drop may occur. Flower buds may fail to open, and those that do may produce poor-quality flowers. It can occur almost anytime during the growing season when temperatures are mild (70 to 80 °F), and the relative humidity is high at night and low during the day. It is most severe in shady areas and during cooler periods.


Prevention & Treatment: Rose varieties differ in their susceptibility to powdery mildew, thus resistant varieties are the best defense against this disease. A film of water inhibits infection, so in years when rainfall is high during spring and summer, control measures may not be needed until the drier months of late summer. Remove and destroy diseased leaves and canes during the growing season. Rake up and destroy leaves under the plant in the fall.


If the disease is severe enough to warrant chemical control, select a fungicide that controls both black spot and powdery mildew. Fungicide sprays recommended for use in the home garden include: propiconazole, thiophanate-methyl, myclobutanil, triadimefon, triforine, sulfur, neem oil (clarified hydrophobic extract), or baking soda mixed with horticultural oil. Apply all chemicals according to directions on the label.


The following roses have some disease resistance;


Black Spot - Resistant:


 Hybrid tea: ‘Pride N Joy’
 Floribunda: ‘Sexy Rexy’
 Grandiflora: ‘Prima Donna’


Black Spot & Powdery Mildew - Moderately Resistant:


 Hybrid tea: ‘Duet,’ ‘Eiffel Tower,’ ‘Grand Slam,’ ‘Jamaica,’ ‘Matterhorn’
 Floribunda: ‘Golden Slipper,’ ‘Saratoga’
 Grandiflora: ‘Camelot,’ ‘John S. Armstrong,’ ‘Pink Parfait,’ ‘Queen Elizabeth’
 Shrub roses: ‘All That Jazz,’ ‘Carefree Wonder’


Black Spot, Powdery Mildew & Cercospora Leaf Spot - Resistant


 Rugosa roses: ‘Blanc Double de Coubert,’ ‘Fru Dagmar Hastrup’ (‘Frau Dagmar Hartopp’) ‘Rugosa Alba,’ ‘Topaz Jewel’
 Alba rose: ‘Alba Semi-Plena’


Stem Canker & Dieback


Cankers usually appear as dead or discolored areas on rose canes and vary in color from light tan to dark purplish brown. They are caused by various species of fungi, including Botryosphaeria, Leptosphaeria, Coniothyrium and Cryptosporella. These fungi enter healthy canes through wounds caused by winter injury, improper pruning, wind, hail damage, or flower cutting. Cankers can enlarge until they entirely surround the cane, and/or reach the base (crown) of the plant spreading to other canes or killing the plant. They commonly occur on roses that have been weakened by black spot, poor nutrition or winter injury.


Prevention & Treatment: There are no fungicides specifically available to control stem canker. Keep plants healthy by controlling black spot, powdery mildew and insects. The following cultural methods can help minimize disease development.


 Avoid Injury to the Plant During Transplanting, Cultivating, Pruning, & Flower - Cutting: Wounds are a major way the fungus enters the plant.
 Prune Properly: To prune an outward facing bud. This will help to avoid too many branches growing into the center of the plant that may cross and rub together.
 Remove & Destroy all Infected or Dead Portions of Canes Immediately: Make all pruning cuts well below the diseased areas, and prune about one-fourth inch above an outward-facing bud node, without cutting the nodal tissue, at a 45-degree angle. Prune live canes in the spring, not fall. Disinfect cutting tools after use on a diseased plant in a solution of 1 part household bleach to 9 parts water.

 

Rust


Rose rust is a disease caused by the fungi Phragmidium species. It causes orange-colored spots to appear on stems and leaves. When rust is severe, an orange dust-like substance may be present on the plant surface and on the ground below the plant. Rose rust attacks all plant parts except the roots and petals. Severely diseased leaves of highly susceptible cultivars may turn yellow or brown and drop.


Prevention & Treatment: Provide good air circulation. Do not plant roses in crowded areas and prune plants to keep the centers open. Water plants before noon and avoid getting the leaves wet. Remove and destroy diseased leaves and plants. Fungicides containing myclobutanil, mancozeb or propiconazole are recommended for homeowner use. Apply all chemicals according to directions on the label.


Botrytis Blight


Rose flowers and buds are often infected with the gray-brown fuzzy growth of the gray mold fungus Botrytis cinerea. The fungus is most active when temperatures are 62 to 72 °F and conditions are moist. Infected canes have discolored sunken areas (cankers) and dieback that can extend down the stem from the flowers. Diseased flower petals have small, light-colored spots surrounded by reddish halos, which can quickly expand into large, irregular blotches. Buds fail to open and often droop. Thrips can cause similar damage to half-open buds, so inspect plants carefully.


Prevention & Treatment: Keeping the area clean is more important than anything else. Collect and discard all fading flower blossoms and leaves. Provide good air circulation, and avoid wetting the leaves when watering. Disease easily develops on canes that have been damaged, on canes that are kept too wet by the use of manure mulch, or on wet leaves. If chemical control is necessary, fungicides containing captan, chlorothalonil or neem oil (clarified hydrophobic extract) are available for homeowner use. Use neem oil on a trial basis, especially on open blooms and during hot weather. On dormant bushes copper sulfate fungicides can be used. Apply all chemicals according to directions on the label.


Rose Rosette Disease


Rose rosette disease is an untreatable rose disease caused by the Rose rosette virus (RRV), and is spread and introduced into the rose during feeding by the rose leaf curl mite (Phyllocoptes fructiplilus). This extremely small eriophyid mite feeds on cell sap of the tender stems and leaf petioles. The rose leaf curl mite alone causes little damage while feeding, but if it is a carrier of RRV, symptoms begin to appear in the rose typically within one to three months.


Roses exhibit reddened terminal growth on infected branches, and the stems become thicker and more succulent than those on unaffected parts of the plant. These stems exhibit an abnormally high number of pliable thorns, which may be either green or red. Rose leaves that develop on infected branches are smaller than normal and may be deformed similarly to herbicide injury by 2,4-D. Lateral branches may grow excessively from main stems and create a witch’s broom symptom quite like glyphosate (Roundup™) injury on roses. Flowering is reduced, and the petals may be distorted and fewer in number.

 

These symptoms generally become evident in the late spring to early summer and progress during the growing season. Once the rose becomes infected, RRV moves throughout the plant and the entire plant is infectious. By the time symptoms are evident in a rose, it already may have spread to adjacent plants by the movement of the eriophyid mites. Infected plants typically die within a couple of years.


Prevention & Treatment: The wild multiflora rose is very susceptible to the rose rosette disease, so any nearby wild plants should be removed and promptly disposed. Any infected, cultivated roses should be immediately removed, then burned or bagged. Also remove any roots, which might re-sprout later. Do not leave an uprooted infected plant in the garden, as the mites may leave this rose for other nearby plants. Always space rose plants so they do not touch.


Because RRV is systemic within the infected rose plants, grafting asymptomatic stems onto other rose plants will transmit the virus. Pruners used on diseased plants must be disinfested with rubbing alcohol or a dilute bleach solution before being used on uninfected plants, as sap on the pruners is contaminated with the virus.


To reduce the spread of the eriophyid mites from the site of an infected rose, nearby roses can be treated with a bifenthrin spray every two weeks between April and September. This may prevent additional plants from becoming diseased. Check product labels for the correct active ingredient. Follow label directions for use.

 


Rose Mosaic


The symptoms associated with Rose mosaic virus (RMV) are highly variable. Yellow wavy line patterns, ring spots and mottles in leaves will occur on some varieties of roses sometime during the growing season. In general, symptoms are most evident in the spring. Yellow net and mosaic symptoms on the leaves are also associated with RMV and detract from the overall quality of the plant. Infected plants become weakened and are more sensitive to damage caused by other stresses, such as drought or low temperatures.


Prevention & Treatment: Virus-infected plants cannot be saved. Rose mosaic spreads slowly, if at all, in established rose plantings through root grafts. Infected plants should be removed from highly prized plantings and destroyed. Buy only healthy plants from a reputable dealer; especially avoid purchasing plants showing any mosaic symptoms.


Crown Gall


This disease is caused by a soil-inhabiting bacterium, Agrobacterium tumefaciens, which infects many ornamentals in the home garden. The symptoms are rounded galls, or swellings, that occur at or just below the soil surface on stems or roots. The galls are light green or nearly white when young. As they age, the galls darken and become woody, ranging in size from small swellings to areas several inches across. The galls disrupt the flow of water and nutrients traveling up from the roots and stems, thus weakening and stunting the top of the plant.


Prevention & Treatment: To prevent crown gall, select disease-free roses. Once a plant is infected, nothing can be done since there are no chemical controls available for crown gall. Avoid injury to the roots and crown of the plant during planting and cultivating because the bacteria enter through fresh wounds. Remove infected plants as soon as galls are observed. If possible, remove and discard the soil from the area where the infected plant was located. Disinfect all cutting and pruning tools that have been used near crown gall. To disinfect tools, dip them for several minutes in a solution of 0.5 percent sodium hypochlorite (household bleach).