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Roses: Diseases and Abiotic Disorders ROSES: 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 rust, black spot, botrytis, downy mil- order to survive, so pruning, collect- dew, and anthracnose may cause prob- ing, and disposing of leaves 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 Rose 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- LEAF 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, sepals, buds, and occasionally on petals 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 fungus 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 Viruses and virus-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 cultivar, 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 Apple 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 chlorosis, 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.
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