Roses Common Diseases and Pests
Changes in cultural practices are often a sufficient control
Included here are some mild but labour-intensive disease and insect control measures. It is important to be aware of the potential impacts on ecosystems of using pesticides and fertilizers.
We appear to be in an insect decline, and 2 of our 7 planetary boundaries have been exceeded, biochemical flows, that is, overuse of nitrogen and phosphate fertilizers, and biosphere integrity, that is, loss of genetic diversity.
It is admirable to create beautiful gardens and flowers, but a flower can still be beautiful with marks and chewed edges if a natural ecosystem is protected.
Diseases
Crown Gall, caused by the bacterium Agrobacterium tumefaciens results in brown lesions or corrugated cankers which are woody tumour-like growths and can be several centimetres across. Crown Gall can cause a graft canker near the graft junction of the rootstock to the scion branch shoot growth, or on the roots or the stem near the soil surface.
Inspect rose grafts for crown galls when purchasing new plants, prevent damage to stems of existing garden plants, cut out stems with galls and burn if found, or remove the entire plant if the infection is extensive and do not plant in the same hole for several years.
Grey Mould fungus Botrytis cinerea attacks weak and soft growth, often seen in seedlings, but occurs less on vigorous shoots.
Downy mildew Peronospora sparsa a water mould may cause yellow to brown spots on leaves and may cause defoliation. More often a problem in glasshouses.
Powdery mildew fungus Sphaerotheca pannosa is common and has a distinct pale grey to white powdery growth.
Black spot fungus Diplocarpon rosae causes round black spots on the leaves.
Rose rust fungus Phragmidium mucromatum causes a yellow-orange powder on shoots in spring and the shoots may grow in a twisted distorted shape, and in summer may cause yellow spots on the underside of the leaves and black spots underneath in autumn/fall.
Disease control
The fungal diseases above can be partly controlled or reduced by spraying with lime sulphur, in a liquid mix of 1 part to 60 parts of water, but changes in cultural practices are also needed.
This includes ensuring adequate soil moisture and nutrition, avoiding any periods of soil dryness that may weaken a plant making it more susceptible to attack, also ensuring any dry spots, such as under the eaves of houses if growing climbing roses on a wall, are adequately watered.
Also, ensure enough sunlight, roses prefer full sun, and sufficient ventilation to reduce humid conditions which favour fungal attacks by pruning the roses to an open shape and pruning back any adjacent or overhanging shrubs.
Remove any infected plant parts from the rose shrub and collect and take away any fallen leaf and stem matter, these can be burnt, buried deeply, at least a spade depth, or removed off-site. This prevents any overwintering of the fungal spores, also wash any hand tools such as secateurs, including your gloves and hands, again to remove any fungal spores and reduce new infections.
Pests
Aphids
The most common and largest of the many aphid species is Macrosiphum rosae Green Fly. It overwinters in the egg stage of its lifecycle on roses or in sheltered places. The aphids are green or pink and collect at the plant’s new shoots.
Chaetosiphon tetrahodus small green rose aphid and Myzaphis rosarum lesser rose aphids appear to spend all their life cycle on roses, they are smaller than the green fly aphids.
Longicaudus trirhodus the rose columbine aphid overwinters in the egg stage on roses and on rose leaves in the spring and early in summer when winged forms appear and they move to other plants such as Aquilegia species columbine plants.
Natural aphid control with insect predators
There are many different types of insects beneficial to our gardens, some of these insects are predators of other insects during part of their lifecycle but are mainly nectar feeders.
They include ladybird beetles, lacewings, hoverflies, and parasitic wasps which will predate aphids and whitefly.
As predominately nectar feeders, to attract these insect predators we need to incorporate into our gardens, plants that flower in different seasons which together will provide an ongoing source of food, pollen, and nectar in flowers.
Plants with masses of small flowers, often held together to look like one flower, provide a more plentiful food source.
Daisy-type flowers are an example and garden plantings can include spring flowering marguerite daisies and paper daisies including the groundcover paper daisy Chrysocephalum apiculatum yellow buttons.
Some Summer flowering daisies are seaside daisies, calendula, and chamomile.
Other plants with many small flowers are chives, spring onions, garlic, flat-leaf parsley, fennel, carrot, and Queen Anne’s lace.
Herbs also provide flowers for insect pollen and nectar sources, such as basil, lavender, mint, rosemary, and thyme. Even saltbush in the garden will attract a range of insects.
To attract predatory insects, have a range of plants flowering throughout the year in your garden, particularly in spring and summer when most insects are more active, to provide flowering food sources for those predatory insects.
Caterpillars
Caterpillars of many moths species attack rose leaves, shoots, and flowers.
Some are commonly seen such as the garden rose or leaf roller tortrix moth.
Others making more isolated appearances are the brindled beauty (Europe), mottled umber (Europe), and Operophtera brumata the winter moth which can overwinter mild winters in the adult stage.
Larvae, the caterpillar, of the different species vary in their feeding habits.
Some can be seen openly eating the rose leaves, buff tip, dagger, gold tail, lackey, vapourer, and the Geometers with their characteristic looping gait Ω.
Others roll the leaves, tortrix. Some mine the leaves, tunneling into the foliage, such as the rose leaf miner Stigmella anomalella (Europe).
Rose maggots can do a lot of damage by burrowing into flower buds and rolling the leaves, such as Archips podana green caterpillar, A. rosana dark olive green, Epiblema cynosbatella reddish brown, Notocelia roborana reddish brown.
Control measures will depend on the severity of the infestation, those caterpillars feeding in the open can be left for the birds or hand-pick off the leaves.
If a spray is required try one of the biological insecticides containing the bacteria Bacillus thuringiensis.
Hand-picking the leaf or flower bud is effective against the rose flower maggots and leaf rollers, where a spray may not reach. Any infected plant material should be burnt or removed from the site.
Rose leaf hopper, which is a sucking insect, causes a pale mottled appearance of the rose leaves during spring and summer. Some varieties of rose with thin papery leaves are more susceptible than others.
Rose sawflies and their larvae can be a serious pest, eating the rose leaves, rolling the leaves, and eating into shoots. Hand-picking the affected leaves and shoots and burning the infected material is chiefly used.
Rose scale comprises two species Lecanium corni the European brown or peach scale, and the Aulacaspis rosae syn. Diaspis rosae scurfy scale. Generally, they only attack old wood, and pruning to allow new canes to replace old, together with sufficient soil water and nutrition should prevent infestations of scale.
Ensuring adequate soil nutrition will help the rose plants to be healthy and resist diseases and pests but beware of applying too much fertilizer at once, small amounts each season apart from winter should be sufficient.
A large amount of the fertilizer nutrient applied may leave your property, either dissolved in surface and soil water in the case of nitrogen or adhered to soil particles eroded from the garden soil in the case of phosphate. This nutrient will find its way into drains and waterways.
Eutrophication, low oxygen levels in rivers, lakes, and seas, is caused by excess algae growth eventually being decomposed by bacteria that use the dissolved oxygen in the lake or coastal waters, resulting in the death of some fish species and shellfish which cannot migrate to better water. This in turn can result in the death of seabirds and marine mammals which depend on fish and shellfish for food.
Further reading:
Insects decline, why have they gone? | by Peter Miles | Age of Awareness | Medium
References:
Chittenden, F.J., (1977). The Royal Horticultural Society. The Dictionary of Gardening (2nd Ed.) Oxford. Great Britain. University Press. Book.
GBIF Global Biodiversity Information Facility GBIF
