Keeping Roses Healthy

Roses are attacked by various pests and diseases. To fight them, it is important to be able to identify them and know what action to take.

Regular spraying with a chemical compound is important. Fortunately most sprays that are used today, can be mixed together and can thus be used simultaneously, together, if desired, with a foliar feed.

There are several different types of sprayer available, but because of their cheapness and lightness, the pressurized plastic kinds are useful. For ease of handling, do not buy too large a one. Although it might have to be filled several times, a 2 pt. Size is adequate for up to 150 plants.capsid bug damage on roses

Pests

These can be divided into two categories – sap-suckers, which include greenflies and red spider mites and leaf-and-budeaters, such as caterpillars and sawflies.

Nowadays there are two types of insecticides: systemic insecticides, which enter the sap by absorption through the foliage and are used against the sap-sucking insects, and contact insecticides, which destroy leaf- and bud-eaters by direct contact with them. Systemics are of no value against the latter, because they do not imbibe sufficient sap to poison them.

The systemics are effective for about one month after spraying, whereas the contact types must be applied whenever the insects appear. A number of proprietary insecticides are mixtures of both.

Do not spray too soon, because the larvae of ladybirds, hover flies, lace-wing-flies and braconid wasps, which are the natural enemies of greenflies (aphids), appear at about the same time as the caterpillars. It is better to hand pick the latter and destroy them rather than spray them.

So that gardeners can detect hostile pests and diseases, their indications are given below.

Pests: Sap-Suckers

GREENFLIES (APHIDS)

These small green insects breed rapidly and soon engulf the rose trees; they are sometimes pink, red or brown in colour. By sucking the sap, they reduce the vigour of the plant, cripple the shoots and distort the leaves, which ultimately fall prematurely.

They must be dealt with expeditiously because not only do they damage the roses, they also exude a sweet, sticky fluid, honeydew, the natural food of the fungus, sooty mould, which by coating the leaves, interferes with their normal functioning. In addition, they are carriers of virus diseases.

THRIPS (Thunder-flies or Blackflies)

These small black or brown insects sometimes swarm all over roses, particularly in thundery weather. Their immature forms which are coloured pale pink to reddish, suck the sap. If they are present, leaves become mottled in appearance, young shoots are malformed and affected buds produce damaged blooms.

CAPSID BUGS

The small, green wingless nymphs of these insects distort the leaves and flower buds. Often dark brown areas, which are sometimes mistaken for black spot, appear on the leaves.

RED SPIDER MITES red spider mite on rose

These pests attack outdoor as well as greenhouse roses. They are very minute, red insects, that are so immobile that they may be mistaken for specks of dust. Their presence is shown by the development of a fine, silken web under the leaves, in which they live and breed. The leaves of infected roses become mottled and off-colour. If badly attacked, they turn yellow and fall. There is also loss of vigour.

Leaf and Bud-Eating Insects

CATERPILLARS

These, the larvae of moths and butterflies, manifest themselves in various ways, which include holes in the leaves, skeletonizing of the leaves, rolling of the leaves and injured buds. Sometimes they exude a sticky fluid.

LEAF-ROLLING SAWFLIES

The adults of these insects are like queen ants. They deposit their eggs in the margins of the leaves, accompanied by a toxic fluid, which causes the leaves to roll and hang down.

ROSE SLUG SAWFLIES

The larvae (grubs) of these insects usually devour the upperside of leaves and skeletonize them.

Diseases

The principal diseases of roses are due to parasitic fungi, and do damage by stealing vital foods from their tissues.

BLACK SPOT black spot on roses

This disease is well-known to all rose growers. Its attacks are at their worst in August and September, but it can strike at any time in the season.

It is recognized by the appearance of black or dark brown spots on the leaves, often initially the lower ones. They gradually increase in size and join together. The remaining part of the affected leaf becomes yellow and the leaves eventually fall off, leaving the plants bare.

No garden roses are immune from attack from this disease, but there are some that are less susceptible.

Hitherto fungicides used against black spot have been contact types, which form a protective coat, which must be renewed at intervals of a week or so, with more frequent applications in wet weather. None of the compounds recommended is a complete cure. Recently the first systemic fungicide has been introduced. It is claimed to be effective against both black spot and mildew.

MILDEW

This starts with white or greyish white spots on the young leaves and eventually spreads all over the plants, making them look as if they have been dusted with flour. The leaves become distorted. Dryness at the roots often makes plants susceptible to mildew. Thus by keeping roses well-watered, the incidence of the disease can be reduced.

Fortunately there appears to be a few garden roses that are very highly resistant to attack.

RUST

This has three characteristics: it only seems to attack roses in certain areas in this country, it often remains for two years or so and then disappears, and its spores need to be frozen to germinate and attack the following year. The last two might be connected, because a mild winter might prevent freezing. The disease appears in April as rust-coloured swellings on the back of the leaves. In June, orange-coloured spores develop. These germinate and the infection is widely spread. Later in August, they turn black and in this form they live over the winter.

DIEBACK AND CANKER DIEBACK AND CANKER on rose

A stem on a rose will often turn brown and die. This can be the result of several different things. Including frost damage, lack of water, careless pruning, causing jagged edges or being cut too high above a bud, the snapping of hardened shoots by high wind or rough handling, and by a fungus disease. The latter is true dieback; when the effect arises out of any of the other causes, it is commonly, but erroneously called `dieback’.

Another fungus disease, stem canker, infects wounds, the cut end of stems after pruning, especially when blunt cutters are used, and dormant buds, which ultimately leads to browning and dying back. The first signs are yellow or reddish spotted or streaked, pimply or water-saturated portions of the stem. The brown area eventually extends some inches down the stem, usually terminating in a reddish-brown border where it abuts healthy tissues. Whatever may be the cause, the only remedy is to prune an affected stem back to a healthy bud, or cut it out altogether, if it has reached the union, otherwise the tree might die.

CHLOROSIS

This is not a true disease, but an ailment, mainly caused by deficiency of iron and manganese. This can occur as far as the plant is concerned in soil in which there are ample quantities of these elements present, but where they are in a form in which the cannot be used by the rose, as often happens on alkaline (chalky) soil. The symptoms are yellowing of the leaves and stems in springtime. They often eventually shrivel up and drop off. Also growth is stunted.

This condition cannot be remedied by distributing ordinary, simple compounds of iron or manganese. If it is serious, the soil must be watered in spring with a proprietary formulation which contains chelates of iron and manganese in a form that can be absorbed by roses, together with active magnesium, which is another vital plant food.

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