: DISEASES AND ABIOTIC DISORDERS

Integrated Pest Management for Home Gardeners and Landscape Professionals

A variety of plant pathogens can at- cycle yet allows time for foliage to dry tack roses. The most common problem before evening. in California is powdery mildew, but a number of other diseases including The pathogen requires living tissue in , black spot, botrytis, downy mil- order to survive, so pruning, collect- dew, and anthracnose may cause prob- ing, and disposing of during the lems where moist conditions prevail. dormant season can limit infestations although it may not entirely eradicate To limit problems, choose varieties and them, since airborne spores from other irrigation practices carefully, promote locations can provide fresh inoculation. air circulation by following appropri- ate pruning techniques and providing varieties vary greatly in resistance, sufficient space between plants, and with landscape (shrub) varieties among remove severely infested material the most resistant. Glossy-foliaged va- promptly. Although some rose enthu- rieties of hybrid teas and grandifloras siasts consider regular application of often have good resistance to powdery fungicides a necessary component of mildew as well. Plants grown in sunny rose culture, many gardeners are able locations with good air circulation are to sustain plants with little to no use less likely to have serious problems. of fungicides, especially in California’s dry interior valleys. Fungicides such as triforine (Ortho Rose- pride) are available, but generally you In addition to diseases that bacterial, must apply them to prevent rather than fungal, and viral pathogens cause, ros- eradicate infections, so timing is critical Figure 1. Powdery mildew on a rose. es may display similar damage symp- and repeat applications may be neces- toms resulting from chemical toxicities, sary. In addition to synthetic fungicides, mineral deficiencies, or environmental least-toxic fungicides are available, in- problems. Such problems are termed cluding horticultural oils, neem oil, jojo- abiotic disorders, and changing envi- ba oil, sulfur, potassium bicarbonate, and ronmental conditions often can correct the biological fungicide Serenade. With these symptoms. the exception of the oils, these materials are primarily preventive, although po- AND SHOOT DISEASES tassium bicarbonate has some eradicant AND DISORDERS activity. Oils work best as eradicants but Powdery mildew, caused by the fun- also have some protectant activity. Do gus Podosphaera (previously Sphaerothe- not apply oils to water-stressed plants or ca) pannosa var. rosae, produces white to within two weeks of a sulfur spray. Figure 2. Downy mildew on a rose leaf. gray powdery growth on leaves, shoots, , buds, and occasionally on See the Pest Note: Powdery Mildew on You occasionally may observe fruiting (Fig. 1). Leaves may distort and drop. Ornamentals for more details on manage- bodies of the on the undersides ment. of leaves. Powdery mildew doesn’t require free water on plant surfaces in order to de- Downy mildew, caused by the fungus To reduce downy mildew, increase air velop and is active during California’s Peronospora sparsa, requires a narrow circulation through pruning and avoid warm, dry summers. Overhead sprin- range of temperature and humidity to frequent overhead irrigation that re- kling, such as irrigation or washing, thrive. Interveinal, angular purple, red, sults in foliage that stays wet. Control during midday may limit the disease or brown spots appear on leaves, which with fungicides is very difficult; envi- by disrupting the daily spore-release then become yellow and drop (Fig. 2). ronmental management is much more

EST OTES Publication 7463 PUniversity of California N Agriculture and Natural Resources May 2009 May 2009 Roses: Diseases and Abiotic Disorders

likely to be effective. Because downy ing neem oil, potassium bicarbonate, mildew requires moist, humid condi- and sulfur, as discussed above under tions, it is most likely to cause problems “Powdery mildew,” or neem oil also has in coastal areas of California and, dur- been shown to be effective in reducing ing a narrow period of time in spring black spot. and fall, in the Central Valley. Anthracnose, caused by the fungus Rust, caused by the fungus Phragmidi- Sphaceloma rosarum, results in leaf spots. um mucronatum (formerly P. disciflorum), When first formed, spots are red or prefers cool, moist weather such as that sometimes brown to purple. Later the found in coastal areas of California but centers turn gray or white and have a also may be a problem inland during dark red margin. Fruiting bodies may Figure 3. Rose rust appears as reddish wet years. Infected plants have small, appear in the middle of the spot, and brown spots on the lower leaf surface (left and bottom) and as yellow patches orange pustules on the undersides of the lesion may fall out creating a shot- on the upper leaf surface (right). leaves while the upper sides of leaves hole symptom. may discolor and drop (Fig. 3). No information on management is Avoid overhead watering and prune available. Hybrid teas and old-fash- back severely affected canes. During ioned climbing and rambler roses are the winter collect and dispose of any the most affected. leaves remaining on the plants and those that have fallen off. Plants can and -like diseases occur tolerate low levels of damage without wherever roses grow, although dam- significant losses. You can use preven- age may be mostly cosmetic with little tive applications of fungicides, but it reduction in plant vigor. may require frequent applications to keep plants rust free, which may not be Rose mosaic disease (RMD) is named justifiable in garden or landscape situ- after the leaf symptoms infected roses Figure 4. Black spot causes dark blotch- ations. display. Ringspots, line patterns, mo- es and yellowing. saics, and distortion or puckering are Black spot, caused by the fungus Diplo- typical (Fig. 5). Leaf symptoms will carpon rosae, produces black spots with vary depending on which virus or vi- feathery or fibrous margins on the up- ruses are present, the rose , the per surfaces of leaves and stems (Fig. 4). time of year, and growing conditions. Small, black fruiting bodies often are Color break on flowers also can be present in spots on the upper sides of symptomatic of rose mosaic disease. leaves. No fungal growth occurs on the undersides. Visual symptoms also can be transient; for example, hot, bright days can cause This fungus requires free water to the symptoms to appear milder or reproduce and grow, so don’t allow disappear. The virus remains, and the leaves to remain wet for more than 7 plant becomes a symptomless carrier. hours. When hosing off aphids, do it in Figure 5. Rose mosaic disease has dam- aged this foliage. the morning or midday, so leaves have RMD is the result of an infection with a chance to dry before evening. Provide a number of different viruses, the most good air circulation around plants. common being Prunus necrotic ringspot Remove fallen leaves and other infested virus and mosaic virus. Arabis mo- mosaic were discovered in California material, and prune out infected stems saic virus also can cause RMD. These and other parts of the United States. during the dormant season. viruses may be present alone or in vari- These diseases include rose ring pat- ous combinations, accounting in part tern, rose spring dwarf (RSD), and rose Black spot usually is not a problem in for the array of symptoms observed on leaf curl. RSD causes rosetting or a most of California. Miniature roses infected plants. An accurate diagnosis balled appearance in the new growth are more susceptible than other types, may require laboratory tests and bio- following bud break. The leaves first although a few varieties are reliably logical indexing. emerging in the spring are recurved or resistant to all strains of black spot. Ap- very short and show conspicuous vein ply fungicides such as chlorothalonil or A group of diseases of unknown causes clearing or a netted appearance. These triforine as preventatives. Oils, includ- that mimic some of the symptoms of symptoms become less apparent as

◆ 2 of 5 ◆ May 2009 Roses: Diseases and Abiotic Disorders

shoots eventually elongate. Canes may cultivar without the virus pathogens. develop a zigzag pattern of growth as Both are slow, time-consuming pro- the season progresses. Recent studies cesses. Worldwide, plant material that have associated a new virus with this has been tested for and found free of condition; see the publication by N. viruses known to cause disease symp- Salem listed in References. toms is referred to as “clean stock.”

Viruses present many problems to com- For the home rose grower, no effec- mercial rose growers. Rose gardeners, tive method exists for eliminating the retailers and regulatory officials don’t viruses that cause rose mosaic disease. like the look of the symptoms. Cut Use of virus-indexed stock—plants that flower producers may see a significant have tested negative for these viruses Figure 6. Nitrogen deficiency has caused decrease in production and/or bloom by laboratory and field methods—for these lower rose leaves to develop in- quality, depending on the variety of field propagation is the recommended terveinal , a yellowing effect that occurs between the veins while the rose and type of virus. Nursery plant preventative practice. veins themselves remain green. The producers may face rejection of inter- virus-infested upper leaves exhibit a state shipments; destruction may be- Nutrient deficiencies cause specific condition known as pale vein clearing. come severe enough that large numbers symptoms such as leaves that yellow of plants become unsalable. and older leaves that drop (Fig. 6). Be- cause many California soils have low For homeowners, the problem largely percentages of organic matter, the ni- is unsightly foliage, with possible de- trogen reserve typically is low, so you creased plant vigor and smaller and/or should add this nutrient as inorganic fewer flowers. fertilizer or from organic sources.

The most common causes for the Micronutrient deficiencies, especially spread of these viruses are propagation iron and zinc, appear as interveinal procedures such as budding an in- chlorosis of new leaves. These elements fected scion onto a healthy understock may be deficient, because soils are too or a healthy scion to an infected under- wet or too alkaline or because the soil stock. Disease symptoms aren’t always type, such as sandy loam, is low in mi- obvious, which is why the use of virus- cronutrient content. Because inorganic tested planting stock is advantageous. forms of iron and zinc form insoluble Figure 7. Injury from a glyphosate (e.g., Roundup) herbicide has caused the rose Some evidence indicates rose mosaic precipitates in alkaline soils, you can on the left to develop a pale, underde- spreads in commercial rose plantings apply iron and zinc directly to foliage. veloped blossom and puckered, needle- via root grafts. You can apply iron and zinc in a che- like shoots. lated form to either soil or foliage. Many rose catalogs and books refer to “virus-free” roses. The science of plant Nutrient excesses may limit rose virology has shown in recent years growth if the total salt level becomes Herbicide damage may manifest it- that most horticultural plants have too high; a value of less than or equal self in a variety of symptoms, which cryptic viruses in them, the function to 2 dS/m (decisiemens per meter) is include cupped, curled, or yellowed and importance of which aren’t known. recommended. Plants may show a lack leaves, small leaves, or death of the As more sophisticated virus-testing of vigor and short shoots, although entire plant. The herbicide class and techniques have been developed, many no definitive leaf symptoms may oc- dosage to the plant determine which “virus-free” programs discovered cur. However, if salt concentrations are symptoms appear and their severity. their stock was not as free of virus as found to be very high (greater than 4 Injury from glyphosate (e.g., Roundup) thought. dS/m), you also may see browning of is relatively common. the leaves. Foundation Plant Services (FPS) at Damage symptoms may not appear UC Davis uses the term “virus tested” A few nutrients cause specific toxicities. during the application season, espe- or “specific virus tested,” meaning Boron, which can reach high levels in cially if the application occurred in tested for the specific viruses known some California soils, will cause stunt- autumn, but may appear the following to cause rose mosaic disease. FPS cur- ing of plants, chlorosis, and marginal spring as a proliferation of small shoots rently employs two virus-elimination browning of the newest leaves. A soil and leaves from buds (Fig. 7). The plant techniques, heat therapy and meristem concentration of less than or equal to 1 will outgrow the injury if the dosage tissue culture, to re-establish a rose part per million is recommended. was not too high.

◆ 3 of 5 ◆ May 2009 Roses: Diseases and Abiotic Disorders

SYMPTOMS ON However, in roses the most common PETALS AND BUDS cause of phyllody is environmental stress, such as hot weather when flower Botrytis blight, caused by the fungus buds are forming, or water stress. If Botrytis cinerea, thrives in high humid- environmental factors are the cause, ity (Fig. 8). Affected plants have spotted affected plants usually have normal flower petals and buds that fail to open, and abnormal flowers simultaneously often with woolly, gray fungal spores but otherwise look healthy. When the on decaying tissue. Twigs die back, weather cools, the bush resumes pro- and large, diffuse, target-like splotches ducing normal flowers. form on canes. Rose growers familiar with the char- Figure 8. Botrytis blight has infected Lessen humidity around plants by acteristics of individual varieties can these rose petals. modifying irrigation and pruning tech- assess if phyllody is caused by disease niques and reducing ground cover. Re- or environmental stress by carefully move and dispose of fallen leaves and examining plants. A lack of stunting petals and prune out infested canes, or yellowing and good overall growth buds, and flowers. Botrytis blight is a indicate a virus or likely problem usually only during spring isn’t the cause but instead an individual and fall in most of California and dur- flower probably is responding to spe- ing summer along coastal areas when cific environmental conditionals. the climate is cool and foggy. No management practices are suggest- Rose phyllody is a flower abnormal- ed other than pruning out individual ity recognized for more than 200 years blooms. in which leaf-like structures replace flower organs (Fig. 9). The fundamen- CANKERS OR GROWTHS ON tal cause seems to be changes in plant Figure 9. This rose exhibits symptoms CANES of phyllody in the center of its flower. hormone balance, brought about by abi- Botrytis blight, as described above, otic conditions such as environmental can cause twig dieback and blotches on stress or living infectious agents. Some canes. A number of different fungi can Sunburn appears as blackened areas, rose varieties such as floribundas are cause stem cankers and dieback. Can- especially on the south and west sides more likely to exhibit phyllody symp- kers are brown, often with gray centers of canes. Excessive temperatures on toms, probably due to genetic suscep- or small, black, spore-producing struc- rose canes cause sunburn, usually as tibility. In fact, one floribunda ancestor tures on dead tissue. is Rosa chinenis, from which came the an indirect result of drought stress or “Green Rose,” a curious variety that has spider mite pressure, which caused de- It is important to provide proper care foliation. Reflected heat from masonry, a stable causing phyllody in to keep plants vigorous to prevent vinyl siding, or rock mulch also can all its flowers. problems. Prune out diseased or dead cause canes to sunburn. tissue, making cuts at an angle in and viruses can disrupt healthy tissue just above a node, and Crown gall, caused by the bacterium normal hormone production, inducing avoid wounding canes. Cankers often Agrobacterium tumefaciens, affects many phyllody in many plant species, but develop after cold temperature injury, play less important roles in rose phyl- so early spring pruning may not effec- woody plants including fruit trees, or- lody. Although a few reports exist of tively eliminate them if late frosts oc- namentals, and roses as well as some rose phyllody caused by phytoplasmas, cur; additional late spring pruning may herbaceous plants including chry- the association is poorly documented. be necessary. santhemums and daisies. Crown gall Rose rosette disease, believed to be invade tissue after wounding. caused by a virus vectored by the erio- Winter injury from cold temperatures Galls, in the form of large, distorted phyid mite Phyllocoptes fructiplilus, also results in dead or dying flowers, twigs, tissue growth, form at the base of the is reported to cause phyllody of rose and stems. A thick layer of leaf mulch cane or sometimes on roots or farther blossoms. Insects—most often leafhop- may protect roses during the winter in up on stems. Infected canes can be pers—can spread these diseases, so the cold mountain areas. Stem canker dis- stunted and discolored. Do not plant appearance of phyllody often raises eases caused by pathogens that move susceptible plants in infested soil or concerns about possible disease spread into injured tissue may follow winter near infected plants. Purchase and through the garden. injury. plant only high quality stock.

◆ 4 of 5 ◆ May 2009 Roses: Diseases and Abiotic Disorders

REFERENCES Horst, R. K. 1983. Compendium of Rose Diseases. St. Paul: APS Press. Pemberton, H. B., ed. 2007. Proceedings of the Fourth International Symposium Karlik, J. F. July 2008. Pest Notes: Roses on Rose Research and Cultivation. Acta Horticulturae No. 751, International in the Garden and Landscape—Cultural Society for Horticultural Science. Practices and Weed Control. Oakland: Univ. Calif. Div. Agric. Nat. Res. Publ. Dreistadt, S. H. 2004. Pests of Landscape 7465. Trees and Shrubs. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3359. Salem, N., D. Golino, B. Falk, and A. Rowhani. 2008. Identification and par- Elmore, C. L., J. J. Stapleton, C. E. Bell, tial characterization of a new luteovi- and J. DeVay. 1997. Soil Solarization: A rus associated with rose spring dwarf Nonpesticidal Method for Controlling Dis- disease. Plant Disease 92: 508-512. ❖ eases, Nematodes, and Weeds. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 21377.

Flint, M. L., and J. F. Karlik. September 2008. Pest Notes: Roses in the Garden and Landscape—Insect and Mite Pests and Beneficials. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7466.

For more information contact the University of California Cooperative Extension in your county. See your telephone directory for addresses and phone numbers.

AUTHORS: J. F. Karlik, UC Cooperative Ex- tension, Kern Co.; and D. A. Golino, Founda- tion Plant Services, UC Davis. TECHNICAL EDITOR: M. L. Flint COORDINATION AND PRODUCTION: M. L. Fayard PHOTOGRAPHS: Figs. 1 and 3-8. J. K. Clark; Fig. 2. J. F. Karlik; Fig. 9. B. S. Fer- WARNING ON THE USE OF CHEMICALS guson. Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock. Produced by UC Statewide IPM Program, Pesticides applied in your home and landscape can move and contaminate creeks, rivers, and oceans. University of California, Davis, CA 95616 Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens containing fruits or vegetables ready to be picked. This Pest Note is available on the Do not place containers containing pesticide in the trash or pour pesticides down sink or toilet. Either use World Wide Web (www.ipm.ucdavis.edu). the pesticide according to the label or take unwanted pesticides to a Household Hazardous Waste Collection site. Contact your county agricultural commissioner for additional information on safe container disposal and for the location of the Household Hazardous Waste Collection site nearest you. Dispose of empty containers by following label directions. Never reuse or burn the containers or dispose of them in such a manner that they may contaminate water supplies or natural waterways.

The University of California prohibits discrimination or harassment of any person on the basis This publication has been anonymously peer of race, color, national origin, religion, sex, gender identity, pregnancy (including childbirth, and reviewed for technical accuracy by University of medical conditions related to pregnancy or childbirth), physical or mental disability, medical condition California scientists and other qualified profession- (cancer-related or genetic characteristics), ancestry, marital status, age, sexual orientation, citizen- als. This review process was managed by the ANR ship, or service in the uniformed services (as defined by the Uniformed Services Employment and Associate Editor for Urban Pest Management. Reemployment Rights Act of 1994: service in the uniformed services includes membership, applica- To simplify information, trade names of products tion for membership, performance of service, application for service, or obligation for service in the have been used. No endorsement of named products uniformed services) in any of its programs or activities. University policy also prohibits reprisal or is intended, nor is criticism implied of similar products retaliation against any person in any of its programs or activities for making a complaint of discrimina- that are not mentioned. tion or sexual harassment or for using or participating in the investigation or resolution process of any This material is partially based upon work supported such complaint. University policy is intended to be consistent with the provisions of applicable State by the Extension Service, U.S. Department of and Federal laws. Inquiries regarding the University’s nondiscrimination policies may be directed to Agriculture, under special project Section 3(d), the Affirmative Action/Equal Opportunity Director, University of California, Agriculture and Natural Integrated Pest Management. Resources, 1111 Franklin Street, 6th Floor, Oakland, CA 94607, (510) 987-0096.

◆ 5 of 5 ◆