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Pests and diseases of roses - fact sheet
Powdery mildew caused by the fungus Sphaerotheca pannosa: The disease is characterised by white powdery fungal growth over the upper surfaces of the leaves. Some distortion of leaves can often be observed. This disease is particularly exacerbated by warmer, humid weather (but not necessarily wet weather). The fungus rapidly spreads as it produces masses of conidia that are wind and water (splash) borne.
Black spot caused by the fungus Diplocarpon rosae: Disease symptoms are black blotches on the leaf surface. In some cultivars the spots may be very small but in the majority they rapidly expand and can be surrounded by yellowing. In these cases the extent of defoliation may be severe. This disease is favoured by wet weather and is best controlled by the use of resistant varieties such as Iceberg. Leaves must be wet at least 7 hours for infection to occur and so the timing of irrigation is important to minimise the activity of the fungus. It is better to irrigate in the morning so that there is a reasonable chance that the leaves dry out during the day. Again the use of sodium bicarbonate is recommended for trialing.
Stem canker: caused by a variety of fungi. The main symptom of the disease is a dieback from cut stems and occasionally from dead flowers. If the disease gets into the crown of the rose it can kill the whole plant. This disease (along with viral diseases) can be most effectively controlled by hygiene techniques where secateurs etc are sterilised between each plant. The fungus enters via cut surfaces and so it is essential that the cutting implements are free of fungi.
Viral diseases: there are a number of viruses that effect roses but the most important are rose mosaic virus and rose wilt.
Rose mosaic virus: Symptoms include yellowing of leaves, blotching etc. but are most often observed as a mosaic pattern of yellowing that looks like an oak leaf pattern. Infected plants may have some mottling and distortion of flowers, although there is a viral disease, rose flower break, reported to be different to rose mosaic virus which causes these symptoms. In most cases the initial effects of the virus are not significant but build up over a period of time to produce effects that can become quite considerable. There are only two effective strategies to control these diseases. The first is to ensure that the roses that are purchased come from a reputable producer who can guarantee that the roses are free of virus (ie. certified). The second and most important is to ensure that there are very stringent hygiene procedures in place to minimise the spread of the virus from plant to plant. Sterilisation of cutting implements between plants is essential. These viruses do not spread by insects or any other vector.
Rose wilt: this is thought to be caused by a virus but this is not fully understood. Symptoms are a downward curling of leaves, epinasty and premature leaf drop. The colour of the leaves is often very weak and the whole growth of the plant is very poor. Plants with these symptoms should be removed and disposed off in the rubbish. Purchase plants only from certified growers.
Botrytis blight caused by the fungus Botrytis cinerea: This fungus most commonly causes a rot and premature senescence of flowers. Grey brown fungal growth may be evident on affected flowers. The fungus is favoured by still moist conditions. Prompt attention to dead heading can help to limit the spread of the disease.
Other diseases: downy mildew, rust diseases. These will cause problems on occasions but would not be expected to cause significant problems regularly.
Two spotted mite (Red spider mite) Tetranychus urticae: Damage by mites causes silvering and mottling of foliage and distortion of flowers and growing points. The presence of webbing on the underside of the leaf is also distinctive. The mites themselves are just barely visible to the naked eye. Control of this arachnid is best achieved via the use of predatory mites. These are species of mites that have been imported into Australia for this purpose. There are two main species that are used, Phytoseiulus persimilis and Typhlodromus occidentalis. Phytoseiulus persimilis is more useful in warmer climates as they cannot survive colder winters, although there are strains being selected for cold tolerance. Typhlodromus occidentalis is able to survive colder winters and is used successfully in apple orchards in colder parts of Australia.
Rose Aphid Macrosiphum rosae: The presence of aphids is fairly obvious as the aphid is easily observed. They can cause deformities to leaves and flowers. Control may be achieved via the use of the parasitic wasp Aphideous rosae. The wasp lays its eggs into the aphid and the wasp larvae eat out the inside of the aphid causing it to swell (becoming mummified) and turn a straw colour. The adult wasp emerges from a hole in the back of the aphid mummy. The wasps are tiny and difficult to see. This wasp can give effective control provided that the use of pesticides harmful to the wasps are avoided.
Thrips Thrips imaginis: This insect will be seasonal causing more problems in mid to late summer. Damage is seen as silvering of the foliage and distortion of growing points and flower buds. The underside of leaves may be covered with the excreta of the insect which is like brown tar droplets. There are no effective specific biological controls and thus control can be difficult to achieve. Thrips are preyed upon by ladybirds and lacewing but the control achieved is not great enough in plague situations.
Caterpillars: caterpillars will cause occasional (although severe) damage, especially where they eat out the flower buds.
White fly Trialeurodes vaporariorum: White fly is generally associated with changes in seasonal conditions and dry periods. The insect itself is a small white triangular fly while immature stages are flattened whitish-green scale like creatures. It is a sap sucking insect that is important on a variety of host plants other than roses including fuchsia, geranium, tomatoes, dahlias and many species with softer foliage. It is best controlled by the parasitic wasp, Encarsia formosa, which has been shown to very effective provided night temperatures are above 20ºC.
Roses require quite high levels of nutrients and this plays an important role in the ability of roses to resist pests and diseases. In particular it is important to ensure that levels of potassium and trace element nutrients are adequately supplied. Potassium in particular is important in the resistance of plants to fungal diseases.
Plant Disease Diagnostic Unit