Chapter 20

Diseases And Their Control

Ranked in order of relative importance, the greatest factors in the incidence of disease in plants and animals are feeding, environment, heredity, and infection. Consideration of the first, second, and third helps in prevention; consideration of the fourth involves treatment. Rather than being forced into the position of needing to fight disease, every possible preventive measure should be adopted. Each rose plant has some degree of inherent resistance to every disease.

Health and life are intimately co-related, and plants have no greater ability than animals to live on chemical foods. The protoplasm of every cell of every creature still hides the secret of life and inherent resistance to disease. It is something more than genes, hormones, vitamins, and atomic energy. The ideal, in supplying plants with all requirements in balanced proportions and available forms, is most nearly approached by decomposed vegetable matter. Burning not only alters the chemistry of its constituents, but probably destroys or greatly diminishes the undiscovered factor so vital to life.

Overfeeding with any one element can lead to increased susceptibility to disease just as can underfeeding. Drainage and ample soil moisture are imperative.

Inherent resistance to black spot infection was greatly lessened by hybridization with R. lutea. Pedro Dot and Pernet-Ducher have sent us many such hybrids, and a large percentage of them have failed in our coastal areas because of marked susceptibility to black spot with consequent defoliation and die-back. Yet those same varieties grow well in the raisers' nurseries on the Mediterranean littoral, which has a climate resembling that of our coasts. Most of these varieties of roses grow well behind our mountain ranges. The difference in behaviour appears to be due to altered soil foods as well as climatic conditions.

Rose diseases and pests, when listed, may appear a terrifying and discouraging array of troubles for a beginner in rose-growing, but he should remember that these are only some of the plant diseases present in Australia; that the rose overseas is attacked by diseases unknown to us here; that the rose is one of the most disease-resistant of all garden plants; and that, although it is helpful to be able to recognize each disease, none is of any serious consequence except rose wilt, mildew, black spot, aphis, thrips, the die-back-symptom group, the scales, and the caterpillars. These should be controlled to the best of one's ability, and it is important to know which sprays or dusts to apply to get best results. It would be useless to apply a spray that may be very good in controlling mildew, believing that it is "good for roses", and that it will, therefore, kill thrips, black spot, scales, and every other disease and pest that may or may not be affecting the roses. It is equally useless to spray at wrong times, or to fail to realize that some diseases can be avoided by prophylactic spraying, whereas no curative measures for them are of any value. Many rose-growers waste time, energy, and money on sprays, often doing more harm than good. Of course, others err in never spraying, but plants of other types in those gardens will be diseased as well as the roses.

In general, sprays are most effective if applied with the nozzle pointing upwards through the plants, so that the liquid reaches the under-surface of each leaf. Spraying in windy weather is wasteful, and, if the material contains any ingredient that is poisonous to man, the danger is increased greatly. It is best to spray in the early morning, for there is usually less wind at that time. Many sprays will cause severe damage to foliage if used on hot days.

Mildew (Plate 42)

Mildew, caused by the fungus Sphaerotheca pannosa, is a very common rose disease. It is entirely superficial,and most common on young growth. Frequent changes of temperature during the growing season predispose to mildew, making it a serious disease in some seasons in southern Victoria, Tasmania, and the South Island of New Zealand.

In the spring it appears as greyish-white areas on the leaves, stems, and buds of young shoots. Later it has a white, powdery appearance. It is least prevalent during midsummer, but reappears with the advent of autumn's warm days and cool nights. Spores are present on all roses throughout the year. Tea Roses and Pernetianas are fairly resistant to mildew.

Control is relatively easy, especially if early preventive measures are adopted. Heavy feeding with wood ashes or one of the inorganic compounds of potassium increases resistance to mildew, but can never cure infection. Sulphur became the time-honoured fungicide for mildew. The various wettable and colloidal sulphur preparations, such as Wetsul and Spersul, are great improvements on the flowers of sulphur formerly used. Shirlan and Clensel are useful and safe at any time. Bordeaux mixture, Bordinette, and lime-sulphur kill many mildew spores when used after winter pruning. The more recently introduced preparations of TMTD are better than any of the older sprays, and, in addition, they are very efficient in controlling black spot. There are several brands of TMTD sprays available; Thiotox is the one of which I have had most experience.

For established mildew some people have had considerable success with a quarter of an ounce of washing soda, plus a little soft soap, to each gallon of water. It is simple and safe. In the past, the best remedies in my hands have been white-oil emulsions. They should be mixed in slightly greater strength than advised by the manufacturers, and not used on hot days. If a mild infestation of mildew is sprayed twice at an interval of two days the fungus clears away quickly. These emulsions have the advantage, too, of not marking foliage. Never mix preparations of white-oil with sulphur in any form; the combination is unnecessary and very damaging to almost all types of plants.

Karathane is the most recently introduced fungicide. It was used first as a miticide and was very effective in controlling red spider, but it was found that it would eradicate established mildew. It must be applied to every affected part of any plant so that the fungus and the foliage are quite wet. Spraying should be done on good drying days, when it is expected that the temperature will not exceed 8o° F. Karathane will cause damage if the leaves remain wet for too long after spraying, or if the temperature is too high, or if the spray mixture is too concentrated. Both the powder and the water must be measured accurately. It renders prophylactic spraying for mildew unnecessary. Thus only a few plants need to be sprayed at all, and not many applications on each of those plants in the course of the year will be necessary. This marks a great reduction in labour.

Several new sprays, with zinc compounds as their basis, are promising too.

Even though one may cure a very bad infestation of mildew, the plant suffers considerably from the effects of the fungus. Such attacks result only from planting roses too much in the shade, or choosing varieties that are very susceptible to mildew, plus a failure to realize, well in advance, the likelihood of this occurring, and neglecting to use an appropriate preventive spray. Plants affected to this extent produce very poor blooms, if any; those that are only slightly affected may give roses of almost usual quality.

Downy mildew affects roses, but is distinct from the form described, and does not exist in Australia or New Zealand. Mildew of other plants is due to slightly different fungi and cannot be transmitted to roses.

Black Spot (Plate 42)

Black spot, a fungus disease due to Diplocarpon rosae, is apparent only on mature leaves and stems, though it may gain entry while the growth is young. Black or brown spots appear on the foliage, then the remainder of each leaflet turns yellow, and it soon drops. Frequently, whole plants of susceptible varieties become defoliated; many then die back badly, and in some instances the whole plants die. Black spot infestations reach their maximum in mid- or late summer, and are much worse in some years than in others. The disease is most common in humid coastal areas, and least common in our inland districts, where the atmosphere is dry. It seems probable that soil conditions are important too, but we do not know yet whether this is so, and, if so, in what way. Therein may lie the explanation of why roses raised in Barcelona and on the French Riviera do not get much black spot there, whereas they are so badly affected here that they will not grow in our coastal areas, where the climate is similar. Some rose-growers state that black spot is worst in acid soils, and support their claims by pointing out that the soil round Barcelona is alkaline. They overlook several other considerations, especially the very efficient natural drainage of that area. Other people consider that roses do better in every way in slightly acid soils.

Tea roses are almost immune to black spot; Pernetianas are least resistant. Very susceptible varieties should be avoided.

The spores of black spot shelter during the winter in bark crevices and in the soil. The fungus penetrates into the substance of the leaves and stems, rendering it quite inaccessible. Hence cure is impossible and prevention is the only hope of control. Maintenance of adequate soil-moisture, especially by mulching, helps considerably in reducing the incidence of black spot; and so does avoidance of overhead watering after midday, for if rose foliage remains wet for six hours or more the spreading of all fungus diseases is helped.

The first leaves seen to be affected with black spot should be clipped off and burnt. At any time of the year all rose stems and leaves should be burnt as soon as possible after cutting, for they carry at least a few spores of black spot and mildew.

For over a hundred years copper fungicides afforded the best means of control of black spot. We advanced from Bordeaux mixture to Bordinette, and then to Cuprox, and we applied them after winter pruning, the spring blooming, and the summer trimming. By doing so, we decreased the infestation greatly, but recently TMTD has been made available and it excels and supersedes all copper sprays. There are several proprietary preparations of it for example, Thiotox. By applying it to the under-surfaces of the foliage every ten to fourteen days we can almost banish black spot and all other fungus diseases from our roses. The introduction of TMTD and the practice of frequent spraying mark a tremendous advance in efficient growing of roses. The health and vigour of the plants is amazing; they retain their leaves, and so are able to manufacture vast additional quantities of food.

Rose Wilt (Plate 43)

Rose wilt is due to infection with a filterable virus, quite unrelated to those causing wilt in other plants. Inoculation of filtered sap from affected roses into healthy roses will reproduce the disease. Budding from affected plants can be a source of transmission.

In most years wilt in any type of plant is only endemic, but rises from time to time to epidemic proportions. This increase is frequent in the forms affecting Iceland poppies, stocks, antirrhinums, tomatoes, and other plants. We know of no explanation of this sudden increase in virulence and incidence of such diseases in plants and animals.

Rose wilt is peculiar to Australia and New Zealand, and was recorded first in 1908. A somewhat similar disease occurs in Italy. Our first epidemic occurred in 1915, and there was a very serious recrudescence in 1927-9. We have had no widespread epidemic since that time. The disease is most common in the spring, and early stages may easily escape notice. The first obvious symptom is a recurving of the leaflets at the tips of young shoots. If touched they fall off and are noticeably brittle. Defoliation spreads down the shoot, with the leaves becoming light in colour, or even yellow, before falling. Very soon the stem-tips lose colour, become almost translucent and die; they are then black. Later the base of the shoot turns yellowish-green and soon becomes dark-brown. The inter- vening part of the stem remains green for a while, but gradually the whole shoot dies. Older wood may be affected later.

i. Rose aphis on rose bud. 2. Top winged viviparous female aphis. 3. Apterous oviparous female aphis. 4. Apterous viviparous female aphis. 5,6,7. A typical cutworm (5. Larva,6.Pupa, 7. Moth). Plate 44 White scale, i. Scale on rose twig. 2. Larva just escaped from egg. 3. Larva fully developed. 4. Larva, colour changed from first moult. 5. Larva after moult. 6. Pupa (male). 7. Pupa (male), dorsal view. 8. Puparia of adult female, ventral view, disclosing female and eggs. 9. Male perfect insect. (After French.) Plate 45

An infected rose may produce apparently healthy growth after losing some stems. The new shoots may repeat the symptoms in the following spring, or the whole plant may appear to recover for even a year or two. All varieties seem to be susceptible.

Probably the virus of rose wilt is transmitted from plant to plant by aphis or secateurs. Very infectious and absolutely incurable, it is the most dreaded of all rose diseases. One infected plant can ruin a whole garden within a very few years. Dead wood does not seem to be infectious.

Control of aphis and quick recognition of early symptoms are extremely important. No time should be lost in digging out and burning an infected plant. At least two or three barrow-loads of soil from round its roots must be removed from the garden. Leave the hole open to sunlight for a week or more, lightly sprinkle it with lime, and refill it with a mixture of one part of old cow manure or compost and six parts of virgin top-soil, to which a little bone-meal has been added. Usually this treatment will be given in November. Allow the soil to rest until the following planting season, for it seems possible that the disease can be transmitted through infected soil.

There are advocates of sterilization of secateurs as one moves from plant to plant in the everyday gathering of rose-blooms, or removal of faded flowers. Such a procedure makes rose-growing very tedious. The early symptoms of rose wilt appear before any blooms are ready to cut, and any plant that comes under suspicion should be watched carefully, for one desires neither to dig out an unaffected plant nor to allow the disease to spread to other plants by delaying unduly the burning of a plant with rose wilt. During this period of suspicion it is easy and safe simply to avoid that plant. If you fear that other members of your household may be less careful, mark the plant by tying a large, easily seen cloth on a stake driven in beside it.

Die-Back

Die-back in roses has never been recorded authoritatively in Australia as a distinct infectious disease. Overseas it has been described as a fungus disease due to Gnomonia rubi. So far, in this country it is always merely a terminal symptomatic phase and secondary to some other factor. The most common forms are:

1.	Constitutional die-back, such as in Pernetianas and Wichuraianas. In the former, almost every year, part of the plant dies but strong new branches grow. It is not infectious, is unavoidable, and varies in degree with soil and atmospheric conditions. In most Wichuraianas nearly all of the preceding year's wood dies.
2.	Die-back that is secondary to defoliation caused by heat, drought, spray-burn or black spot.
3.	Die-back as the terminal phase of rose wilt.
4.	Die-back during winter or early spring due to the effects of cold conditions on immature wood.
5.	Die-back due to deficiency of one or more plant foods, especially potassium, nitrogen, calcium, phosphorus, copper, boron, and zinc.
6.	Die-back due to senility, injury, inefficient drainage, canker, sunburn, and other minor causes.

It is useless to attempt to treat the symptom, die-back; one must discover the cause, and then adopt appropriate measures.

Rose Rust

Rose rust is caused by Phragmidium mucronatum, a fungus which lives in the tissues of the leaves, stems, and calyces. It appears in spring as orange or yellow spots, made up of spore masses, varying from one-twelfth to one-sixth of an inch in length. They form hypertrophies, and often cause curvatures of the affected parts. In midsummer the spores change in character and the colour of the spots alters from orange or yellow to red. As winter approaches the spores undergo a further change and develop black hairlike tufts on the under-surfaces of the leaves. The affected leaves turn yellow and fall prematurely, spreading the disease. Any visibly affected part should be burnt promptly; the rest of the plant should be sprayed with Thiotox, or one of the colloidal or wettable sulphur preparations. Rose rust is most troublesome in hot and humid areas. Adelaide nurserymen regard it as one of the major diseases affecting their young plants.

Scales

White scale (Plate 45) derives its name from its appearance on the plant. The causative insects, Aulacas-pis rosae or Diaspis rosae, begin life as very small winged creatures. After mating they become sedentary, lose their wings, and degenerate into enormously distended bags of eggs, protected by a shield, or scale, that is impenetrable to water, frost, and sprays. Shade and overgrowth by weeds or annuals predispose to infection. The more sheltered, older parts of the roses are the most frequent sites of attachment.

Close examination will reveal the large number of component scales in what at first will have appeared to be one scale. Each scale is only about one-twelfth of an inch in diameter, but the colonies average about one-third of an inch in length and about one-sixth of an inch in width. The colonies in bad infestations cluster so closely that they overlap, making large areas of greyish-white colour.

From beneath each scale comes a young insect. Wingless females increase the spread of the disease on any already affected plant; winged females carry the scale to other roses. Males are all winged, and mate with either type of female.

The best mode of attack is to aim at sealing the scales and so prevent the insects from breathing. Red oil may be used as a spray or paint in winter, but it burns foliage at other seasons. White-oil preparations, kerosene emulsion, glue, or thin boiled starch may be used at any time. The under-surfaces of leaves and branches must not be missed; small patches can be scrubbed.

Red scale, due to Chrysowphalus aurantii, is most common on old plants in hot, dry weather. The female varies from yellow to reddish-brown, is round and has a small pedicle giving a ringlike central marking. The male is ovoid in contour, much smaller, and winged. Red scale is much less common than white scale; it feeds in the same way, by sucking, and can be controlled by the same means.

San Jose scale (Aspidiotus ferniciosus) infects roses only occasionally, but can be very destructive. The adult female scales are round and grey, with raised golden centres. Young scales are smaller and darker. The male scale is oval and only about half as long as the diameter of the female scale. Control measures are the same as for white and red scale.

Aphides

Aphides, or green-flies, are among the best-known garden pests. There are many varieties, and all have come to us from overseas. The rose aphis, Macrosiphum rosae (Plate 44), is usually green, but is sometimes pink or red when fed on shoots of those colours.

In winter the shiny black aphis eggs lie in crevices of the bark and round dormant buds. With the first warmth of spring the eggs hatch; the aphides of this first brood are all females. They are remarkable in reproducing without fertilization, that is by parthenogenesis. They do not lay eggs, but give birth to living young. The second generation is identical with the first, and well within another forty-eight hours a third generation is produced in the same viviparous manner. Each female may live many weeks reproducing at the same phenomenal rate. One investigator claims to be conservative in stating that the progeny of one aphis may reach, within three weeks, the astounding figure of one quintillion that is, 1 followed by 30 ciphers. The weight would exceed that of five hundred million stout men! These calculations ignore the activities of the natural enemies of the aphides.

As the aphides hatch they migrate to young growth and cluster round the sappy tips. Most of the early aphides are wingless, followed soon by winged specimens that fly to other plants. The appearance of young males seems to be in response to bad weather conditions. Subsequent reproduction is by the laying of eggs that can withstand extreme climatic changes  the aphis has become oviparous.

In early summer the winged insects fly to other plants, and very few aphides are to be found on roses again until March, or later. Then they return to feed, breed, and deposit their eggs, which hatch in the following spring.

MOULIN ROUGE. A Floribunda seedling from Alain and Orange Triumph that has won many liish awards. Plate 46 Top: Painted apple moth and caterpillar. Centre: Light-brown apple moth and caterpillar. Bottom left: Caedicia olivacea. Bottom right: Looper caterpillar. Plate 47

With curiously constructed beaks, aphides do tremendous damage by withdrawing great volumes of plant sap. They will distort flowers, cause bud-drop, mar foliage, and even stunt the bushes. We believe, but have not proved it, that they can transmit rose wilt. They secrete a colourless sticky substance called honey-dew {Vumagine), which attracts ants and serves as a medium on which "sooty mould" grows.

In controlling aphides the attack can be made on either the egg or the insect. Tar distillate or E605 will destroy every egg it reaches, and so all spring aphides can be eliminated, but there is no such easy means of control for the autumn multitudes. Spraying with organic phosphates, a product containing the gamma isomer of benzene hexachloride (BHC), or a twenty to twenty-five per cent DDT preparation with a white-oil or wax or "mayonnaise" base is effective at any season. DDT in a water base is not effective against aphides. Pespruf Emulsion (No. 20), Rulene and Rucide are all DDT preparations that have given me good results. The first and second are in the form of white-oil or "mayonnaise" emulsions with the insecticide added; the third has an advantage over other DDT preparations in being less easily washed off by rain. It has a proprietary base other than white oil, and needs to be heated in preparing for use, but do not boil it. In many preparations, other emulsifying agents are used with the white oil, and the base is then called a "mayonnaise" type.

Enthusiastic references to organic phosphate preparations (Systox, HETP and E605) are unavoidable in discussing control of insects of all kinds, caterpillars, and other rose pests, for they are, undoubtedly, the most potent sprays, for such purposes, that have ever been produced.

Unfortunately they are extremely toxic to human beings, and it has been found impossible to rely on people being sufficiently cautious, in using them, to avoid tragedies. In consequence, legislation has been enacted recently, in some Australian States, imposing conditions that render the use of organic phosphates practicable only for users of large quantities, but virtually impossible for home gardeners. However, we now have other sprays available that partly compensate for the prohibition of the use of organic phosphates. These new preparations contain Chlorparacide (p-Chlorobenzyl p-Chlorophenyl sulphide), and it is compatible with both DDT and the gamma isomer of BHC. By mixing Chlorparacide with either of these other products one obtains a spray that is superior to the organic phosphates in some ways, and inferior to it only in that it is slower in killing aphides. As the gamma isomer of BHC kills quicker than DDT, I would recommend it instead of DDT. Lindane is a name given commonly to the gamma isomer of BHC, and it is not merely a proprietary name for a commercial preparation.

There is, already, on the market at least one proprietary preparation (called Spraymate) wherein DDT and Lindane are combined with ovicides similar to Chlorparacide. This spray is compatible with TMTD and Zineb, and, when mixed with either, gives control of fungi as well as insects and arachnids. Spraymate is, of course, not nearly as toxic to human beings as E605, yet it does the same work almost as effectively, and so it is the best safe insecticidal and miticidal spray available for most gardeners. In addition, it is not very expensive.

These sprays supersede all preparations such as nicotine sulphate, Clensel, and soapsuds. Washing off aphides with the hose jet and killing them by squeezing between finger and thumb have been widely advocated. Such methods require no apparatus and no spraying material, but are so comparatively ineffective as not to warrant consideration when spraying is
so easy, so simple, so inexpensive, and so infinitely more successful.

Are You Ready To Move Onto The Next Lesson? Click Here...