These articles discuss the major diseases of roses. As with insects, identification is key importance to treatment. If you are not sure, contact other Consulting Rosarians or your local Agricultural Department.
Powdery Mildew
Causal Organism: Sphaerotheca pannosa (Wallroth ex Fr.) Lev. var. rosae Woronichine
Powdery mildew is one of the most prevalent and serious diseases of roses. It is usually serious in dry climates like California but can occur as well in more humid regions over a wide range of temperate conditions. While no complete resistance is observed, some roses appear to be more susceptible than others.
Symptoms: The first symptoms appear as slightly raised blister-like areas on the upper leaf surfaces. Later, the young expanding leaves become twisted, distorted and covered with a white powdery mass of mycelium and spores. Young peduncles, sepals, petals and stems may also show distortion while growing tips and buds may be killed. Infected older leaves and stems may remain symptomless.
Disease Cycle: The fungus can overwinter as spores on infected stems or leaves. As conditions warm in the spring, dormant spore become active. The white fuzzy growth on the leaf surface contains thousands of fungal spores. Wind carries these spores to young leaves, causing more infection.
Optimum conditions for spore germination are 71°F and 98% relative humidity. Temperatures above 90°F will inhibit spore germination. After spores germinate they penetrate the surface of the leaves forming a fungus structure that takes the nutrients from the plant. Successful infection will result in further development of the disease.
Air movement is important in dispersal of the spores. Closely planted gardens with some air movement are ideal conditions for spread of this disease. Spore maturation and release usually occurs during the day when relative humidity is low. At night an increase in relative humidity favors spore germination and penetration of the fungus. Cool damp nights and warm sunny days favor the development of powdery mildew.
Control: Dormant pruning and cleaning up old leaves can remove substantial amounts of primary inoculum. Sanitation should always be the initial means of control. When roses are pushing new spring growth, the conditions for disease development are also favorable. At this time, protection can be gained from the use of fungicidal sprays applied at seven to ten-day intervals. Removable of diseased foliage can be useful under certain conditions. Fungicides in a wettable powder formulation may provide better coverage if used with a spreader sticker; most flowable formulations already have this property. While the old standby lime sulfur is a good protectant fungicide, it may burn foliage under warm conditions. Several new fungicide products now claim to have some systemic and eradicative properties, but don’t expect to cure a severe disease situation. Fungicides will work best in the initial stages of disease development, not after massive infection has already taken place. Remember, fungicides are chemicals that have a degree of toxicity and should only be used when needed and strictly according to the label.
Black Spot
Causal Organism: Diplocarpon rosae
This disease may also be called leaf spot, leaf blotch, star sooty mold and several other names. It is the most common or prevalent disease of outdoor roses on a worldwide basis. The fungus has probably been widely distributed with cultivated roses and no really complete resistance is believed to exist.
Symptoms: Small black spots of 2 mm (1/125-inch) diameter can be found on upper leaf surfaces as well as immature canes. These spots are black and sooty enlarging to 12 mm (1/20-inch) with generally circular appearance having a feathery edge. Further development involves the appearance of yellow margins around the spot and a yellow condition that can extend into the entire leaf. Late in the growing season, defoliation can occur along with purple red, raised irregular blotches on immature wood.
Disease Cycle: Black Spot is spread by splashing water. Infection occurs after leaves are wet for several hours. Therefore, the disease is more serious during periods of rainfall. The Black Spot fungus overwinters as spores on infected canes and leaves that have fallen and been left on the ground. Autumn cleanup is crucial, otherwise the entire cycle can repeat itself the following spring and summer. In the spring, overwintering spores are splashed up onto newly emerging foliage during rains or irrigation and cause primary infections on the new leaves. Within about two weeks after the primary infections, structures form within lesions and produce spores which cause secondary infections throughout the growing season. The fungus produces infectious spores throughout the year, causing repeated infections in warm, wet weather. Leaves less than two weeks old are the most susceptible to this disease. Defoliation brought on by Black Spot is worse during wet weather, especially humid weather. The fungus becomes active in a wet environment with a temperature of about 75 degrees Fahrenheit. It needs about 7 hours of these conditions to germinate and then symptoms will begin to appear on rose foliage within three to ten days. Usually the lower and inside areas of the plant are more likely to be infected because these areas have longer time periods when water is available on leaf surfaces allowing the germination and penetration of the fungus.
Control: Dormant pruning, removal of attached leaves and cleaning up old leaves under the bushes can do much to reduce the overwintering inoculum. A well-aerated site will do much to reduce disease incidence by preventing the occurrence of free water on the foliage for long time periods. Pruning to allow air movement and watering without wetting the foliage could also be important means of prevention. Some roses may be more resistant than others, but specialized races of the fungus quickly arise. When conditions are favorable in early spring, protectant sprays can be used on a one-week interval or after periods of rain to prevent infection. Surfactants may allow the active materials to spread more evenly on plant surfaces and persist longer during periods of precipitation.
Downy Mildew
Causal Organism: Peronospora sparsa
Downy mildew is a very common disease of roses which occurs under moist cloudy conditions. The disease is found throughout the United States, Europe and South America. All species of cultivated and wild roses seem to be susceptible.
Symptoms: Under cool and moist spring conditions, young leaves, stems and flowers may manifest purple to red or brown irregular spots. As the disease advances, lesions on leaves become angular and black with the possible appearance of white mycelium on the underside of the leaf. Advanced infections will have yellowing of leaves with brown necrotic areas and noticeable leaf abscission. The blotches tend to form between the interveinal areas. As the scientific name indicates, the production of spores is sparse and therefore this disease is difficult to diagnose. Along the coast of California where moist cloudy conditions persist through summer, the disease is severe and can occur on large canes as purple irregular blotches. One of the primary symptoms of downy mildew is that leaf drop begins at the top. No other disease seems to damage roses in this manner.
Disease Cycle: The fungus over-winters as spores in or on plant parts. Germinati spores swim in free water on the plant surfaces until re-infecting the plant The infection progresses toward the development of new outbreaks. Cool, wet, high humidity weather favors downy mildew development. Dense canopy growth and tight plant spacing encourages downy mildew spore production on the lower leaf surface. Splashing water from overhead irrigation or from rain fall not only splash downy mildew spores from leaf to leaf and from plant to plant, but also provide the wet leaf surface required for downy mildew spore germination. Again meticulous garden cleanup is necessary to reduce reinfection of plants.
Control: This disease can be especially severe on greenhouse-grown roses where humid conditions exist for long time periods. Ventilation and reducing humidity below 85% will reduce disease development. Sanitation in the garden will reduce the primary source ofinoculum. Where damp, humid conditions exist, systemic fungicides containing metalaxyl will give some control. Other fungicides can also be used, but the persistence of the material through wet periods or rain is important to maintain protection.
Anthracnose
Causal Organism: Sphaceloma rosarum
Not much is known about this disease; however, it can be severe under cool, moist spring conditions. Wild roses, climbers and ramblers seem most susceptible, but hybrid tea and bush roses also get the disease.
Symptoms: Initially leaf spots are about 0.5 cm (1/ 4-inch) diameter and black, which makes them easily confused with blackspot. As the development progresses, the spots become purple to brown and finally light brown or tan with a red or purple margin. Stems, hips and pedicles can also be infected and as lesions mature small black dots will appear in the papery tan centers. These black dots are the spore producing acervuli (a tiny cushionlike or blisterlike structure produced by certain fungi on a plant host consisting of a mass of spores) of the fungus. Spotting, yellowing and severe defoliation and shot hole can result under moist spring conditions.
Disease Cycle: The fungus overwinters in old lesions on leaves and canes. Warm spring conditions promote the development and release of spores from the old lesions. The spores are believed to be carried by water or rain to new leaves and stems. Not much is known about the sexual stage of the fungus or conditions favorable for spore germination. It seems that cool moist conditions are favorable for this disease as for Black Spot.
Control: Removal of old leaves from around the base of plants and pruning out canes that have infections will do much to reduce the inoculum levels in spring. Generally, the same spray program that is also used for blackspot should work for anthracnose.
Botrytis Blight
Causal Organism: Botrytis cinerea
Botrytis blight is common in all parts of the world. This fungus is not a specific pathogen and can take advantage of many situations to produce a blight or rot condition on many hosts. It is an opportunist on cut or pruned rose canes and will infect flowers and buds.
Symptoms: The most common symptoms usually are seen on young flower buds which droop, turn black at the base and later produce the cottony grey-black mycelium of the fungus. Flowers can also be affected in the same way, and cut ends will have the black canker-like symptoms with presence of mycelium. Any time conditions are cool and wet, a grey-black mycelial growth will indicate botrytis.
Disease Cycle: Botrytis is a gray fungus that generally lives on dying tissue. With the right conditions, any dead plant tissue can release thousands of Botrytis spores. Botrytis infection occurs when water remains on leaves or buds for an extended period of time. This fungus is not specific to roses, but will grow on many different plants and plant debris. Under cool wet conditions, profuse spores are moved / to roses by air currents or blowing rain. A minor wound in a bud or flower, or perhaps a pruning cut will provide the initial point of entry. As the infection progresses, additional sites become infected. fungus is a low -level parasite and will colonize wound sites as well as dead plant materials.
Control: Prevention is the best means of control. This can be accomplished through intense sanitation procedures. By elimination of opportunistic colonization on dead plant material, the amount of infection can be reduced. Good ventilation is also essential in reducing disease incidence. Some sprays may give short-term relief, but the fungus usually becomes quickly resistant. In greenhouse conditions, special covers are used to reduce the levels of ultraviolet light required by the fungus for infection. In most cases, removal of infected plant parts and protection of wounds by chemicals is all that can be done until warmer and dryer conditions prove too unfavorable for continuing the disease.
Rose Cankers
Causal Organism: Coniothyrium spp.
After a long, cold, wet winter, roses can be expected to have some dead and cankered canes. While these pathogenic fungi are not very sophisticated, they can cause considerable damage. The problems can be especially acute on old, established roses that have lost some vigor and on young bare root roses emerging from cold storage.
Symptoms: In early spring, pruned stems provide wound sites which can be colonized by canker causing fungi. The stems will yellow, often have red spots and later become brown or black Black spots bursting through the tissue can often be found in discolored tissue which is the fruiting structures of the fungus-containing spores.
Disease Cycle: The fungus overwinters in infected canes and spores can be spread to healthy canes by splashing water, wind, and pruning tools. However, the pathogen can only enter plant tissue through wounds. The canker fungus is most active during the cold time of the year when roses are not actively growing. Pruning cuts or wounds on stems provide sites of entry for germinating spores. The fungi are not high -level pathogens and cannot produce the disease when conditions are favorable for plant growth. During the dormant months, fungi colonize the tissues, and are spread to other pruning or wound sites. The disease can be extensive and severe under the ideal conditions for development.
Control: Promoting vigorous growth and removal of dead canes and stubs will help to reduce the primary source of infection. When pruning before winter, always make an angular cut close to an active bud so the callus can form a protective layer. A dormant spray could be used to protect pruning cuts and wounds during the cold and wet winter. In spring, removal of infected canes and a general spray program for fungal diseases should reduce canker problems and protect the plants until they can become vigorous growers once again.
Rust
Causal Organism: Phragmidium spp.
Rose rust is one of the most commonly found and easily identified diseases of rose. Severe outbreaks occur where cool temperatures and high moisture are found during the prime growing season. Where foliage is heavy, the first symptoms usually are found close to the ground and on the insides of plants. Nine species of the rust fungus are known to occur. The fungus is complex, having up to five different spore stages in its life cycle. The resistance of various cultivars is usually well documented because rust is a very common disease in some parts of the country.
Symptoms: In early spring, powdery pustules of light orange to yellow spores appear on the underside of leaves. Soon after, these aeciospores can be found on the upper side of leaves where they are more obvious. As we move into summer, the spore-producing organ of certain fungi is predominate forming the typical red brown to orange spores. This stage can repeat several times during the growing season usually in 10- to 14-day intervals. Finally, as fall approaches and evenings become cool, the overwintering clusters of spore cases develop black spores of rust fungi. Leaves, succulent canes and flower parts are all susceptible to the rust fungus.
Disease Cycle: The fungus overwinters as spores of rust fungi on fallen leaves or on infected canes. The spores are wind borne and germinate to infect leaves through the surface of the leaves. As the infection proceeds, the various spore stages develop on roses. There is no alternate host for rose rust. Reinfection and spread occurs through spores. Spore germination requires cool summer temperatures and continuous moisture for at least two hours so the spores can enter the leaf surface. Spores serve as a means of overwintering on leaves and infected canes and careful removal and disposal of infected debris is important.
Control: In areas where rust is severe, sanitation should be practiced to reduce inoculums and prevent early season infections. Infected canes should also be spring pruned to reduce the initial levels. Pruning very dense bushes will help to reduce the moisture levels inside of plants and prevent some infections. Preventative fungicidal sprays should be applied every 7 to 10 days when conditions are favorable for Rust development.
Bacterial Crown Gall
Causal Organism: Agrobacterium spp.
This is really the only serious bacterial disease of roses. The bacteria is found worldwide wherever roses are grown and is especially severe in loose sandy or sandy loam soils.
Symptoms: On roses, the symptoms of overgrowths can occur at the crown, bud union or on the roots. Galls are usually round to irregular in appearance and may have a rough exterior. Upon cutting across a gall, a disorganized callus type of tissue is commonly found. The portions closest to the exterior usually contain the actively growing bacteria. However, once the tumor-inducing plasmid is introduced into a plant, disease can occur without the presence of the vectoring bacteria.
Disease Cycle: The bacteria causing the disease are soil borne and can survive in soils for many years. When roses are planted in infected soil, any wound sites on the roots or crown offer points of entry for the bacteria. Once in the host, the tumor-inducing principle carried on a small circular portion of DNA is incorporated into the plant cell and an overgrowth begins to form. In some plants, the bacteria can be systemic and galls may begin to form at many sites on the plant. As the galls develop, callus tissue is laid down which is susceptible to other types of breakdown, decay or sloughing. In this way, the soil around a plant can become infected with the bacteria. The bacteria survives many years in the soil and can be moved with water or other infected plant parts.
Control: Always inspect new plants thoroughly before placing them in your garden. Since wounds can be infected at the time of digging, some plants may have latent infections which could become evident several years after they were planted. If crown gall is detected, the plant may survive many years but could serve as a reservoir for the bacteria. Thus, removal of the plant as well as adjacent soil is recommended. In some cases, soil fumigation is used to kill the bacteria in the soil, but this is only effective where relatively porous and dry soils are involved. Plant surgery is also an option on specimen roses, but precautions should be taken to sterilize cutting equipment before and after use. These plants should probably be removed from the garden and watched closely for further development of disease. Any plants propagated from infected material should also be watched for the development of galls resulting from systemic contamination.
Rose Mosaic
Causal Organism: Prunus Necrotic Ringspot Virus (PNRSV) or occasionally, Apple Mosaic Virus (ApMV)
Mosaic is probably the most commonly found virus disease in roses, but many other virus diseases also exist. It is of far less concern than rose rosette, which is contagious and kills the rose, but is of some concern, in that it reduces growth and productivity of the bush, size of flowers, stem length, cold-hardiness, and overall longevity of the bush.
Symptoms: Rose mosaic usually appears in spring as a distortion of growing tips or expanding leaves. Later the leaves can appear to be wavy and have yellow lightning bolt-like patterns, splotchy yellow or white spots, oak leaf patterns or simply gold to yellow veins. Plants infected with mosaic may be slower to develop in spring than healthy plants and usually produce fewer good quality blooms. During the warm summer, typical symptoms can disappear only to come back as fall and cooler temperatures arrive.
Control: Since there is no cure for the virus diseases, it is important to purchase only quality materials which have no symptoms of the disease, preferably from a nursery that uses certified virus-indexed stock. While there have been rumors for many years that the disease spreads via pollen or other natural means, after more than 40 years of research, there is no evidence that it ever spreads in rose gardens. So the old concept that infected bushes must be removed and destroyed, to prevent spread, appears to be completely unfounded. The situation is exacerbated by the fact that symptoms may not show up for several years, depending on the climate and weather, so rose growers mistakenly think their plant has “caught” the disease in the garden; in reality, it was infected when they bought it. Propagation of buds from infected roses, or cuttings made from such plants, nearly always results in transmission of the disease to the new plants.
Rose Rosette
This is a serious disease that is showing up in different parts of the country, in particular the middle section of the United States. The exact cause of rosette is unknown, and it has been suggested that it could be a virus that is carried from plant to plant by mites, or the reaction of the plant to substances injected by blister mites. In either case, it is noticed on ‘Rosa multiflora’, which is a common rootstock, and in some species like ‘Rosa woodsii’. The foliage on an affected plant will look like witch’s broom; the leaflets looking distorted and wrinkled. Not to be confused with Spring Dwarf Virus.
Disease Cycle: Rose rosette disease is caused by a virus or virus-like pathogen yet to be characterized. Because the exact causal agent has not yet been identified, there is currently no laboratory test for confirming rose rosette disease. The disease is diagnosed based on characteristic symptoms or by grafting suspect plant material onto known healthy roses and demonstrating transmission of symptoms after a period of weeks to months.
The disease is known to be transmitted by the eriophyid mite or by grafting. The wild multiflora rose (R. multifiora) is very susceptible to the disease and is a common source of infection. Cultivated roses planted downwind of infected multiflora roses are especially at risk because the mite vector travels on wind currents from infected plants to healthy plants. Some growers have observed symptoms on previously healthy plants within four weeks of being planted downwind from diseased multiflora roses.
The causal agent of rose rosette disease is not soil-borne, so it is possible to successfully plant healthy roses in beds where diseased plants have been removed; however, the pathogen may persist in old root pieces that remain in the soil from previous diseased roses. If plants regrow from these old root pieces, as multiflora rose is apt to do, they can serve as an infection source for healthy plants. Therefore, it is important to remove old plants thoroughly and ensure that infected plants are not allowed to regrow from old, infected root pieces.
Control: There is no control once rose rosette is established in a plant. It is best to remove the plant before it spreads to other roses in your yard. Do not toss into a compost heap. There is no proof at this time that miticides help to control Rose Rosette Disease. Credit: Dr. Mark Windham
Source: American Rose Society, Consulting Rosarian Manual, 3rd Edition, July 2016
Rose Bacterial Cane Blight
Cause: An undetermined pathovar of Pseudomonas syringae, a bacterium. This disease is different from the various symptoms on roses previously attributed to P. syringae and/or P. syringae pv. morsprunorum in the literature. An aggressive cane blight has been observed in the Treasure Valley of Idaho since 1996. Symptoms were common under cool, wet conditions in spring (March to May) and the level of incidence and sever ity of the disease varied from year to year. Several cultivars of climbing, floribunda, grandiflora, hybrid tea, hybrid perpetual, miniature and shrub roses can show severe symptoms.
Symptoms: The symptoms usually started at the base of a vegetative bud or at leaf scars or wounds, as reddish-brown areas on the bark that later turn dark-purple-to-black and necrotic. The necrotic areas expanded around and along the cane, often involving a major part or even the entire cane. Vegetative buds on the affected parts of the cane turned brown and dried. The surface of the necrotic areas of the bark was glossy, and the tissue beneath the epidermis was brown to dark brown, and moist in the early stages. Often confused with winter injury.
Cultural control: The following is suggested in the absence of specific research to control this problem.
- Remove and destroy infected stems.
- Disinfect pruning shears before cutting more stems.
Chemical control: Spray before fall rains begin and again when half the leaves have fallen. Repeat in spring to protect new growth.
Products available:
- Bonide Liquid Copper Fungicide at 0.5 to 2 fl oz/gal. May result in purple spots on some cultivars.
- Monterey Liqui-Cop at 3 Tbsp/gal water. May discolor blooms.
Source: https://pnwhandbooks.org/plantdisease/host-disease/rose-rosa-spp-hybrids-bacterial-cane-blight