Rose Pests and Diseases – Top Tips for Healthy Roses
Rose Pests and Diseases / Disorders that Affect Roses
Although the pests and diseases which may attack are fairly numerous, prompt action when trouble is noticed, or better still early preventive spraying or dusting where this is appropriate, will keep the plants clean and healthy.
The kind of aphids popularly known as greenfly may prove troublesome on roses from the beginning of May onwards, and will be first detected on the youngest and most tender leaves and shoot tips. The small green ‘lice’ can multiply very rapidly under favourable conditions so that shoots, leaves and buds become covered with them. These greenflies suck sap from the plant so weakening it and causing distortion. The bushes must be immediately syringed with BHC, derris, malathion or menazon.
During winter, a spray of 3% tar oil should be applied to bushes which are known to be severely infested with these gardeners enemies – the aphid rose pests.
Various caterpillars attack roses during the summer months. Though they vary in character they are all to be dealt with in the same way – poisoned with an insecticide such as DDT, BHC, derris or trichlorphon. These sprays should be repeated from time to time so that the young foliage is coated with the poison. Hand picking should also be carried out, and all curled leaves examined minutely for any caterpillars hidden within.
While other rose pests can usually be found on the damaged parts, the leaf-cutter bees are rarely seen by gardeners and yet the damage they cause is among the most spectacular. The bees do not actually feed on the leaves, but cut out regular oval or circular portions from them. These portions are used for lining their nests. Other than trying to follow the bees to their nests and destroying it (which is clearly somewhat extreme), or waiting by the plant to net them, little can be done to stop the damage which, I must say, reduces the foliage to comic shapes!
Leafhoppers are a common pest of the rose and other plants. They received their popular name on account of the habit of the adults of leaping when disturbed, but it is the much less active larvae that do most of the damage, sucking sap from the leaves and causing them to become pale and mottled. These insects change their skins as they grow and, apart from the foliage mottling, the most certain indications of their presence are the white moult skins attached to the under surface of the leaves.
Occasional spraying with BHC or DDT is the best safeguard-against injury.
The sawfly larvae of these are very destructive. The damage done by one kind is very distinctive, as only the surface of the leaf is eaten, a thin membranous ‘skeleton’ being left. The larvae are small, black, and have a superficial resemblance to slugs and so are often known as slugworms. Another kind of sawfly causes the leaves to roll up tightly. All may be destroyed by spraying the foliage with DDT or BHC but this must be done early with the leaf-rolling kind or it will be completely protected from the spray by the leaf tightly curled round it.
Thrips are minute and rapidly moving pests which frequently attack the flower buds, hiding themselves between the closely folded petals and causing browning of the buds. As they are completely hidden by the petals they are not easy to eradicate. Damagedshould be removed and burned and the plants sprayed with gamma-BHC (lindane) or DDT.
Mildew is one of the most common diseases affecting rose foliage. It first produces greyish patches on the stems or leaves and if unchecked can eventually look like a heavy coating of wet Hour. It is a difficult disease to keep in check especially on some varieties which are susceptible to it. Spraying with dinocap is most efficacious and Bordeaux Mixture or colloidal sulphur are also useful in its control. Like the majority of diseases, it is most easily controlled by preventive sprays before it makes its appearance. Four or five sprays throughout June, July and August will usually ensure a reasonable degree of freedom from, but in gardens that have a poor circulation of air it may be impossible to eliminate entirely.
Black spot usually appears about midsummer and lasts throughout the rest of the season. Affected leaves show circular-like black areas which extend until the leaves are completely covered and fall prematurely. Black spot can be checked to a certain extent by collecting all fallen leaves and burning them. ` Infected wood should be cut off and also added to the garden fire. In winter, after all leaves have fallen, plants and thebeneath them can be sprayed with copper sulphate, loz per gallon. Throughout the late spring and summer roses should be sprayed at least once a fortnight with captan.
Rust is not as common as mildew or, but occasionally proves troublesome. First the under surface of the leaves show small orange pustules which later turn to nearly black. All leaves bearing these black winter should be burned to prevent spring infection. Where rust occurred the previous years, colloidal sulphur, Bordeaux Mixture, thiram or zineb should be employed frequently in summer as a preventive spray. Ensure that the entire bush is sprayed, especially drenching the leaves. Apply the spray forcefully. Remove all weeds from around the bushes, and ensure that all prunings are gathered and burnt.
A trouble known as ‘die-back’ causes considerable winter losses among some roses. Dark brownish-purple patches appear on the branches and the whole stem above these dies. Constitutional weakness appears to have a good deal to do with die-back but it is actually caused by infection by the grey mould fungus (Botrytis cinerea) which attacks many other garden plants. All dead or discoloured growth should be cut out as soon as possible and the plants sprayed with Bordeaux Mixture or colloidal sulphur.
This is a trouble associated with some very full roses in wet weather, particularly in May and June. The flowers refuse to open properly. They turn brown and eventually decay completely or fall off. There is no known remedy and affected buds should be cut off and burned.
Chlorosis is a name given to a condition which causes leaves to lose their natural colour and turn yellow. It is due to lack of available iron in the soil and is most likely to occur on chalky or alkaline soils in which iron tends to be locked up in insoluble compounds. The remedies are to lower the alkalinity of the soil by liberal dressings of dung or acid peat and to apply iron sequestrols to the soil. This is a special compound of iron salts which can be purchased and should be used according to the manufacturer’s instructions.
Free your rose bushes of all of these rose pests and diseases and disorders for delightful, flowering healthy roses.