PESTS AND DISEASES OF THE ROSE

Insect Pests:

Although approximately 500 insect species have been recorded as attacking rose trees in different parts of the world, aphids, thrips and caterpillars are usually the only serious pests in this country. A few other pests — described later — occur sporadically but need not worry the amateur unduly. Greenfly or Aphids. The most important pests of roses. They suck the sap from the young growths, the undersides of the leaves, also the flower stems and buds. Some species are found on roses throughout the year, indeed in the greenhouse aphids are a constant menace to the well-being of the trees.

The results of aphid infestation are bud drop, premature leaf fall and a general reduction in vigour. Foliage is also disfigured by the sticky ‘honey dew’ excreted by the insects. This often encourages the growth of sooty mould.

Spring and early summer are the peak periods of attack. The complaint that rose trees become re-infested even after a really thorough spraying does not mean that the insecticide has been unsuccessful. Ants carry aphids from one plant to another and successive migrants arrive from other unsprayed plants and weeds, the pests moving their quarters after exhausting the food supply of a particular leaf or shoot. Dcrris, nicotine or gamma-BHC (lindane) may all be employed for aphid control. Liquids are preferable to dusts.

Always spray both sides of the leaves, as aphids, thrips, leaf hoppers and red spider mites tend to congregate on the undersides. Thrips or Thunder flies. These pests are more prevalent during hot, dry summers, appearing about mid-July. They infest shoots, leaves, flower buds and open blooms — especially the last two — causing mottled foliage and deformed buds which fail to develop properly. Roses with soft petals, such as Lady Sylvia and Talisman, are particularly susceptible to attack. Red varieties seem to escape.

In the open, both BHC and Pyrethrin give excellent control, while under glass Pyrethrin smoke generators are very effective.

Rose Caterpillars and Maggots. Attacks are usually more prevalent in early summer. The larvae of the buff tip, vapourer, winter moths and related species chew rose foliage, causing partial or complete defoliation. The leaves may also be skeletonised. They can be destroyed by the application of an insecticide dust or liquid, though hand picking is often sufficient, as in most seasons infestations rarely assume epidemic proportions. Tortrix moth caterpillars or rose maggots damage buds and partially opened flowers, causing a characteristic leaf roll. Spray with insecticide immediately attacks are noticed. Once leaf rolling has started dust with nicotine. The leaf-rolling rose sawfly causes a complete lateral folding of the leaflets, resulting in shrivelling of the leaves and a general reduction in vigour. Hand picking and burning the rolled leaves is the best answer. The rose slug sawfly or slugworm feeds, mainly on the under surface of the leaves, giving them a skeletonised and papery, scorched appearance. Dust or spray with insecticide or nicotine.

Green Capsid Bug or Lygus Bug. The type of damage is very characteristic. The nymphs feed on the sap from the foliage, giving it a brown, spotted look. As the leaves develop, holes are formed and the foliage exhibits an irregular, scratched appearance. BHC or Pyrethrin sprays give satisfactory control.

Red Spider Mite. Though red spider occurs chiefly under glass from April to October in hot summers, attacks often develop outdoors, especially against walls, where air does not circulate freely. This pest has a very wide range of host plants, including many well-known weeds such as bindweed, chickweed, docks and nettles. Damage is found on the undersides of the foliage and can generally only be detected with a magnifying glass, when the mites and their webbing will be observed. They suck the sap from the leaves, causing mottling, yellowing and ultimately leaf fall. Derris gives some control, but it is best to consult the literature of insecticide manufacturers to ascertain the most up-to-date treatment.

Less Important Pests. Cuckoo spit insects or frog hoppers are often troublesome on soft.stemmed plants, especially chrysanthemums, geums, pinks, lavender and to a lesser extent, roses. The larvae suck the sap from stems, flower buds etc., and in so doing emit the froth that covers them. Infested buds and shoots wilt, leading to a loss of vigour. As the nymphs must be wetted in their frothy mass, spraying with considerable pressure is always essential. Nicotine or gamma-BHC (lindane) insecticides are effective.

Leaf hoppers feed on the undersides of the foliage, giving it a pale, mottled appearance. The white cast skins of these pests are evidence of their handiwork. Leaf hoppers jump when disturbed and are easily destroyed by an insecticide spray. The mossy sponge-like excrescences which occasionally appear on rose stems are due to an attack by a gall wasp which lays its eggs in the leaf buds in spring, the plant reacting by the production of these characteristic galls. They should be cut away and burned directly they appear. Two of their names are the bedeguar gall and robin’s pincushion. Nineteenth-century north of England farmers used to collect specimens to cure cows afflicted with diarrhoea!

Cockchafer grubs are sometimes troublesome on light, sandy soils, especially near woods. They feed on rose roots and a gamma-BHG dust should be worked into the top spit. See SOIL PESTS.

Fungus Diseases. The three main rose diseases are powdery mildew, black spot and rose rust. The last-named is the least Hkely to worry gardeners, though it can be devastating when it does occur. Other minor troubles are covered. Note that purple spots on the leaves (frequently confused with black spot) are not caused by fungus diseases. Bad drainage or lack of one or more essential plant foods are the most likely explanations. A dressing of complete fertiliser, plus bonemeal in autumn, will usually help. Spring frosts can also cause similar symptoms, especially on soft, unripe shoots which were not removed when pruning — if these are left they are very liable to produce various markings, depilation and die-back, quite unconnected with any fungus disease. A damp autumn usually produces a large number of unripe growths. Powdery Mildew. This occurs wherever roses are grown. Although usually more serious in dry summers, it can be equally troublesome in town or country gardens. The curled and distorted leaves, likewise the flower buds and young shoots which never develop because of the debilitating effect of the white powdery covering that is the mildew growing on them, are unmistakeable. Mildew is, as it looks, mainly superficial but it sends ‘haustoria’ or suckers into the plant to extract food and it is these which cause the debilitating action.

Powdery mildew flourishes in damp places where the air is stagnant, also in draughty situations against walls and hedges where air does not circulate properly. A humid atmosphere is particularly favourable, as are sudden changes of temperature. It is more prevalent in late spring and early autumn than during the height of the summer. Dry weather is essential in the first place to facilitate dissemination of the spores, but prolonged night and early morning dews are required to aid germination. Some shelter from wind, such as that provided by trees growing in a corner of the garden, also assists the spread of the disease.

Infected branches should be cut away and burned. Some enthusiasts recommend drastic spring pruning, from the standpoint of the health of the tree. Obviously diseased growth of all kinds should be removed, but excessive pruning may debilitate the tree. Spraying must be preventive as well as curative, since fresh spores may arrive from outside and spread the disease. A wettable sulphur or Karathane fungicide can be used. Note that, generally speaking, varieties with glossy green foliage like Perfecta, Grand’mere Jenny and Sultane are usually resistant to mildew. The dark crimsons are mostiy susceptible. Susceptible varieties — Hybrid Teas: Anne Letts, Autumn, Comtesse Vandal, Crimson Glory, Glory of Rome, Josephine Bruce, Michele Meilland, Picture, President Herbert Hoover, Red Ensign, Spek’s Yellow, Tallyho, Tzigane, The Doctor, Virgo. Floribundas: Else Poulsen, Faust, Frensham, Our Princess. Wichuraiana Ramblers: Dorothy Perkins, Excelsa, Sanders’ White.

Hybrid Bourbon: Zephirine Drouhin. Some Resistant Varieties.

Hybrid Teas: Beaute, Cynthia Brooke, Eden Rose, Ena Harkness, Gordon Eddie, Grand’mere Jenny, Home Sweet Home, Independence, Karl Herbst, Margaret Amos, Mme Louise Laperriere, Monique, Mrs Sam McGredy, Peace, Perfecta, Shot Silk, Sultane.

Floribundas: Burning Love, Concerto, Dainty Maid, Irene of Denmark, Marchenland, Masquerade, Pinocchio, Red Favourite, Silberlachs, Vogue, Ycllowhammer.

Black Spot. In localities where this disease is a problem the efFects are often more damaging than with mildew, and it is a much harder disease to eradicate. Unlike mildew, it may on occasion cause complete defoliation of rose trees, which are liable to collapse in a severe winter or die back the following spring. In built-up, fairly heavily populated areas, especially where factories are in the immediate neighbourhood, black spot is less of a problem, as the soot and smoke are not favourable to the development of the fungus spores. It seems less troublesome in the Midlands and north of England.

Infection occurs chiefly on the leaves, though the stems and occasionally parts of the flower itself can also suffer damage. The first symptoms usually appear on the upper surface of the leaf as small, brownish-black spots. Black spot can be distinguished from other leaf spots by the darker colour of the markings and the fringed edge which can be seen with the naked eye. The spots frequently merge into one another. Sometimes the entire leaf turns yellow. Infected leaves often drop off, thereby weakening the tree, though diseased foliage remains longer on some varieties than others. Though the older leaves are usually attacked first, once black spot starts it spreads rapidly unless checked and the young foliage may exhibit symptoms before the older leaves. Infections of the stems, though significant, are not conspicuous like those on the leaves. They are usually very small. Their importance lies in their ability to produce spores which spread the disease.

Foliage must be wet for spread as well as infection, whereas with mildew wet leaves are only desirable for actual infection. Foliage only needs to be continuously wet for 6 — 7 hours for infection to develop. Thus in a wet season, black spot is far more serious. High humidity and areas notable for heavy morning dews encourage black spot. When infected foliage falls to the ground, the fungus over-winters on these leaves, fresh spores being produced the following spring. When the old leaves distintegrate, the fungus dies. It does not persist in the soil except when present on the leaf and spraying the soil with copper sulphate in January, as is sometimes advised, is therefore unnecessary. Black spot can over-winter on the stems.

Mulching with damp peat, lawn mowings etc., probably helps to prevent the spores being splashed up by rain. Collecting and burning diseased leaves in late summer and autumn, though a very tedious job, also helps to keep down infection but the main line of defence is preventive spraying. With mildew the mycelium of the fungus is external, but with black spot it penetrates to the tissues and it would therefore be impossible to destroy the mycelium without damaging the host plant at the same time. We must accordingly spray to prevent further infection. Ideally the spray should be on the plant in advance of expected wet weather and if torrential or prolonged rains follow, further applications will be essential, as the material may be washed off the surface.

Regular spraying is absolutely essential, especially in a wet season, if black spot is to be kept under control.

Use a captan or thiram fungicide immediately after spring pruning, again when leaves unfold and repeat as necessary at 10—14 day intervals in summer. More sprayings will be needed in damp weather. Note that in a season particularly favourable to infection, especially in country districts which are very liable to black spot, few varieties will escape completely. Some will, however, only show slight signs of infection. It is doubtful if any variety is absolutely immune. There is a number of strains or races of the black spot fungus. A variety which resists one strain may encounter another in a different garden against which it has no resistance, hence the conflicting reports from various different districts regarding susceptibility and relative freedom from attack. Susceptible varieties — Hybrid Teas: Burnaby, Caprice (Lady Eve Price), Claude, Comtesse Vandal, Crimson Glory, Doreen, Fred Howard, Golden Dawn, McGredy’s Ivory, McGredy’s Yellow, Mme L. Dieu-donne, Moonbeam, Phyllis Gold, The Doctor. Floribundas: Else Poulsen, Polly Prim, Siren. Hybrid Sweet Briars: All varieties.

Resistant varieties — Hybrid Teas: Betty Uprichard, Charlotte Armstrong, Gertrude Gregory, Home Sweet Home, Independence, Karl Herbst, Mme Louise Laperriere, Sutter’s Gold.

Floribundas: Concerto, Dainty Maid, Elsinore, Goldcup, Jiminy Cricket, Marchenland, Masquerade, Pinocchio, Paulsen’s Pink, Red Favourite, Silberlachs, Yellowhammer, Yellow Pinocchio.

Anthracnose. Of minor importance, sometimes confused with black spot. The spots are, however, rather smaller and are not uniform in colour like those of the black spot fungus. Attacks are largely confined to the leaves, though stems and flowers may be infected.

The spots develop on the upper surface of the leaves, and are dark brown, eventually turning blackish-brown with a brown or purplish rim — with black spot there is no rim. Infected portions of the leaves fall to the ground, leaving holes in the foliage. The disease does not appear to have any real debilitating effect on the tree and usually only occurs sporadically, mainly on climbers and ramblers.

Affected parts should be burned. If necessary spray with a copper fungicide to keep down future attacks.

Rose Rust. When this disease does occur it can spread with alarming rapidity. It is encouraged by heavy dews, high rainfall and a moist atmosphere, and is usually more serious on heavy land — light, rich soils are rarely touched. In the British Isles, Northern Ireland, the Midlands and the West Country are areas most liable to attack. In the south, whether in town or country, rust is at present uncommon, but the amateur should keep a careful watch for any signs of infection as in 1934—5 this disease was serious in the south and could easily be very troublesome again in the absence of proper control measures. Symptoms are small orange or yellowish-orange spots on the undersides of the leaves. They occur in early summer, turning black later in the season. In cool weather they develop a reddish hue. Infections may also be found on flower stems. With severe attacks the plant may be seriously weakened and eventually collapse.

Collect and burn infected growths. Mulching in summer is also helpful. Thiram has given outstanding results against rust. Note that the floribunda Fashion is very susceptible. Spartan is also liable to attack, though less seriously.

Chlorosis. The form of chlorosis affecting roses is very easy to identify. The leaves lose their characteristic green appearance and turn a pale yellow. The preliminary symptoms are usually noticeable in early spring. The leaves shrivel and fall to the ground, the tree producing stunted shoots, weak leaves and possibly a few mediocre flowers until it ultimately collapses. The reason for this is that owing to the absence of chlorophyll (green colouring matter in the leaves) the trees cannot assimilate food for growth.

Chlorosis occurs mainly on limy and chalky soils and results from iron ‘deficiency’. Iron is, of course, essential for the formation of chlorophyll. Whether there is an actual deficiency or the iron, though present, is not freely available, is uncertain. Sequestrene salts used according to the manufacturer’s directions are the most up-to-date way of controlling this disease, which is, of course, purely physiological.

Stem Canker. Provided it is checked in time, serious damage may be avoided, otherwise if the canker is allowed to persist, the tree may eventually die.

Stem canker affects the stems and twigs and is usually confined to trees 2 or more years old. It often occurs at soil level. Though infection may follow an attack of rust, it is usually the result of wounds after careless pruning cuts. Cankered areas are blackish-brown and have a sunken, cracked appearance. The canker may extend all round the stem, frequently killing all the growth above it. Die-back often follows on stem canker, as the stem is encircled with the canker, and water cannot reach the parts above it.

Cut back diseased growths immediately they are noticed. Chemical treatments have so far proved of little use.

Crown Gall. Probably more common on damp soils. Infections occur as the result of wounds caused by pruning, etc. The galls or swellings are usually found at or near soil level on the collar and sometimes on the branches. They are hard and woody, becoming cracked on the surface as they age.

Cut away the swellings and paint the cut surface with Stockholm tar. Leaf Scorch. The fungus is confined to the leaves and appears as very small yellowish-green patches on the surface. These patches ultimately turn a pale brown and are edged by purple or reddish lines. With young leaves the brown patches often fall out. Collect and burn diseased leaves. Honey Tuft. Armillaria root rot, honey tuft or ‘bootlace’ fungus is occasionally troublesome on roses and often comes from infected tree stumps. Black threads resembling flattened boot or shoe laces are found attached to the roots. Infected trees should be burnt and the surrounding soil dug over to a depth of 18 in. so that no traces of the fungus remain. See also HONEY FUNGUS.

Sooty Mould. A black, sooty layer covers part or the whole of the upper leaf surface, though the deposit is not usually very thick. It is caused by the mingling of the dark-coloured myeclia of several fungi. The mould is purely superficial and derives its nourishment from the ‘honey dew’ deposited by aphids and similar insects. If these are kept under control by gamma-BHC (lindane) or other sprays, sooty mould will give no trouble.

Grey mould or botrytis is sometimes a problem in cool districts. See BOTRYTIS.

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