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Greenhouse - Primrose

( Originally Published 1920 )



Cultural Considerations. Young seedlings are greatly injured if the compost contains unrotted manure. When the plants begin to grow rapidly a little bone meal may be worked into the soil. During the blossoming period a little weak liquid manure may be given, but only when the leaves are pale. In transplanting, the crown of the plant should not be planted too deeply in the soil lest it rot. Neither should it be planted too high lest it fall over. After transplanting primroses need shade. Later, however, they should be given the benefit of full light and ventilation. The soil should never be allowed to become dry.

DISEASES OF THE PRIMROSE

Primroses are subject to several serious diseases.

SPOT DECAY

Caused by Sclerotinia fuckeliana (De By.) Fckl.

Symptoms. The conidial stage, Botrytis vulgaris, of this fungus causes a spot decay on the foliage (fig. 63, a.). The fungus often thrives on old and faded blossoms. These, therefore, act as a source of infection. It is needless to add that cleanliness will form a part of the control method. Spraying with any of the standard fungicides is also recommended.

ANTHRACNOSE

Caused by Colletotrichum primula Hals.

Symptoms. Affected leaves become brown and spotted. The spots are more visible on the lower part of the foliage (fig. 63, c.). The acervuli with its black setae of the causal organism are especially conspicuous when looked at with a magnifying lens. At present little is known of the organism. The disease may be kept in check by spraying with a standard fungicide.

BLIGHT

Caused by Phyllosticta primulicola Dam.

Symptoms. On the leaves of the plant appear somewhat circular spots (fig. 63, b) that are brown or whitish in color with a light border, with which are found numerous pycnidia. The disease attacks various species of the genus Primula with different effects. On P. sieboldii and on P. obconia, the trouble is usually confined to the lower leaves. On P. sinensis the central part of the leaf is attacked be-fore the surrounding tissue loses its color. On P. sieboldii the leaves may often be blotched through-out, while on P. obconia one-half of the leaf is often destroyed before the other half shows any marked deterioration.

Little is known of the causal organism. The destruction by fire of diseased material and spraying with a standard fungicide is recommended.

LEAF SPOT

Caused by Ascochyta primulae. Wail.

Symptoms. The presence of the disease is shown by oval spots, which spread and often involve the entire leaf. This disease may often be mistaken for the spotting caused by Phyllosticta primulicola. However, a microscopical examination will distinguish the two organisms.

LEAF BLOTCH

Caused by Ramularia primula Thun.

Symptoms. The presence of the disease is shown by large yellow blotches in the ashen colored centers of which are borne the spores (fig. 63, d.).

The Organism. The conidiophores grow on both sides of the spots, and are rarely branched, continuous and somewhat denticulate. The conidia are thick but taper towards both ends. Their structure is continuous or one septate.

Control. This disease may be kept in check by spraying the plants with a standard fungicide. Infected material should be destroyed by fire.

ROSES (Rosa gallica chinensis).

Cultural Considerations. As soon as the cuttings form roots which are about one-half inch in length, they should be potted. At this stage, if left too long in the propagating bench, the wood tissue of the cutting will harden and the subsequent health of the plant will be endangered. A medium water-holding capacity is an indication of a good potting soil for roses. Most greenhouse (fig. 64) varieties prefer a heavy loam. Other varieties such as the Maryland thrive best in a soil which contains a large percentage of sand. Roses are very sensitive and readily become injured when given partly decayed organic matter. The case, however, is different when well rotted manure is used, for this latter food exerts a wholesome stimulating effect. The development of a good root system largely depends on the soil texture and on the plant food which it contains. It is necessary to stir frequently the surface soil of rose benches. This not only destroys weeds, but also provides aération. However, as soon as the surface soil becomes filled with feeding rootlets of the rose plants, the cultivating should be done very superficially or should cease altogether. During active growth, the plants require an abundance of ventilation and a comparatively low temperature.

DISEASES OF THE ROSE

Greenhouse roses are subject to the attacks of several important diseases.

THE BRONZING OF LEAVES

Cause, Physiological.

Symptoms. This trouble commonly affects grafted varieties of the Tea, Bride, and Brides-maid. By some growers the disease is often mistaken for a stage of the black spot caused by Diplocarpon rosez. Bronzing produces a mottled bronze coloring of the foliage. Later the mottling becomes more prominent in the form of spots, while the adjoining tissue turns pale yellow. Frequently the entire leaflet becomes bronzed with no yellowish color apparent. At times the affected leaflets and leaf stalks drop to the ground. The cells of the affected tissue contain an abundance of calcium oxalate crystals, a condition that indicates poor nutrition. Bronzing is usually confined to two places. First, where a stem has been cut and a new branch has started the leaf at the base begins to bronze; second, where an eye or an auxiliary bud has been rubbed off, the leaf generally becomes bronzed. From studies made at the Massachusetts Station there seems to exist a difference in susceptibility between young and old plants. Bronzing may be expected to occur on young plants. It is also prevalent both on plants which are forced too rapidly and on weak stock. The selection of strong, hardy stocks and care in feeding will prevent the trouble from becoming serious.

BLOSSOM BLIGHT

Cause, unknown.

Symptoms. The trouble manifests itself in the failure of the buds to open. At first the buds seem to develop normally. Soon, however, the outer petals wrinkle, turn yellow or straw colored, and stop growing. Occasionally the buds open partially, but fail to attain normal size. The true cause of the disease is unknown, although, as believed by Stevens and Hall, it may be due to some physiological disorder in the metabolism of the plant. No control method is known.

CROWN GALL

Caused by Pseudomonas tumefaciens Sm. and Towns.

This disease is a very dangerous enemy to out-door roses. It has, however, proved of little economic importance to indoor' roses (fig. 65, b.). For a description of the symptoms and of the organism, see p. 115.

DOWNY MILDEW

Caused by Peronospora sparsa Berk.

Symptoms. Downy mildew is more difficult to detect than the powdery mildew. It is also more difficult to control, because the causal organism lives within the tissue of its hosts. This mildew resembles that of the grape, potato, bean, etc. It usually appears in irregular spots. On the lower surface of the leaves, the fruiting of the fungus resembles a downy white to purple coating. It is fortunate that this disease is uncommon in the United States, and even more so under greenhouse conditions.

The Organism. The conidiophores are nine times branched; the branchlets are reflexed. The conidia are pale gray, subelliptic in form.

Control. The removal and burning of infected material and the spraying of the plants with a standard fungicide will keep it in check.

MECHANICAL SPOTTING

Caused by Pilobolus crystallinus (Wigg.) Tode.

Symptoms. The trouble, if such it may be called, is a small specking resembling fly speck on the leaves and flowers. There is but one case on record reported by Clinton. It appeared on two benches in a rose house. The infected benches were heavily mulched with cow manure, while the others did not receive this treatment. A careful inquiry revealed the fact that on the two manured beds the fungus Pilobolus crystallinus was very abundant. The spore heads of this organism (fig. 65, a) when ripe are shot off into the air and stick to any object on which they may alight, which in this case happened to be the foliage and blossoms of the roses. The mechanical spotting here referred to was caused by nothing more than the presence of the spore heads of the fungus. The trouble ceased when the fungus no longer produced spores. The specking on the rose blossoms was not serious enough to injure their market value.

ROSE RUSTS

Caused by Phragmidium species.

Rose rusts are more commonly found on plants growing in the open. These, however, may be introduced indoors with cuttings, or plants first started in the nursery.

Phragmidium subcorticum (Schrank) Whit. This fungus causes the true rust of roses. It is very prevalent in Europe, is of little importance in the United States. On the leaves this rust appears in small circular spots (fig. 65, e), and on the stems and petioles in large powdery masses. At first the sari or spore clusters are orange-yellow, but later turn brick red.

Phragmidium speciosum Fr. This fungus is the cause of a rose rust which affects the stems and which rarely appears on any other part of the plant. The sori are black and irregularly scattered. The causal fungus is carried over from year to year as viable mycelium in the affected host. Cutting out or burning the diseased stems will prevent the further spread of the disease.

Other Rose Rusts. There are other species of Phragmidiums which have been found by Mikio Kasai. Among them are the following: Phragmidium americanum (Pk.) Diet. found on Rosa dahierica; Phragmidium fusiforme Schroet on Rosa aricularis; Phragmidium Japonicum Diet. on Rosa multiflora, R. wichuriana, R. luciae; Phragmidium rose multiflora Diet. on Rosa multiflora, R. laevigata; Phragmidium rose rugosæ Kasai, on Rosa rigosa; Phragmidium yezoense Kasai on Rosa rugosa.

POWDERY MILDEW

Caused by Spharotheca pannosa Wallr.

Symptoms. Powdery mildew is a very trouble-some disease of greenhouse roses. The disease appears as powdery, whitish patches on the leaves, stems, and blooms. The affected foliage fails to develop normally, becoming uneven and twisted, curled and reddened (fig. 65, d.).

The Organism. On the rose the conidial or oidium stage is most frequent. The conidia are ovid, hyaline, and are borne on short conidiophores. The same fungus also causes the powdery mildew of the peach, in which case the ascus stage is most common.

Control. It is believed by many florists that drafts favor mildew. These statements seem to be borne out by actual observations. Mildew often starts first on rose plants facing broken panes. From these, the spores are then carried by the draft to other plants until the disease becomes thoroughly established in the house. It is, therefore, imperative that attention be directed to broken glass. While an abundance of ventilation is necessary, drafts of all sorts should be avoided.

Mildew may also be kept in check by boiling sulphur in the greenhouse for two to three hours, twice a week. The house is closed tightly during the operation, and ordinary flowers of sulphur is placed in a kettle over a small kerosene flame, as otherwise a big flame may cause the sulphur to catch fire. Mildew may also be controlled by spraying with potassium sulphide, at the rate of one ounce of the chemical dissolved in two gallons of water. The spray is only effective when used fresh. The chemical should be kept in a tightly closed bottle.

BLACK SPOT

Caused by Diplocarpon rosae Wolf.

Symptoms. Black spot is often very troublesome on greenhouse roses. Attacked plants lose their foliage and the general effect is a weakening of the plant and the formation of stunted blossoms. The spots are more or less circular, black, with a characteristic fringed border (fig. 66, a.). Frequently the leaf tissue adjacent to the spots becomes pale or chlorotic, long before the affected leaves drop off. As the spots become old, minute specks appear within. These are the fruiting bodies of the causal organism.

The Organism. The fungus of black spot has two spore stages. The summer stage (fig. 66, b) is known as Actinonema rose (Lib.) Fr. The pycnidia are tuberculoid in shape, scattered, black. The conidia (fig. 66, c) are oblong, constricted, and are borne on short conidiophores. The ascus or winter stage (fig. 66, f and g) was discovered by Wolf, who named it Diplocarpon rosie Wolf. The winter stage matures on dead and fallen leaves which have wintered over. The mature asci are oblong. The ascospores are discharged from an apical pore and pile up in whitish masses in the opened perithecia. The ascospores are not so strongly constricted at the septum as is the case with the conidia or summer spores, both of which are hyaline in color.

Control. There seems to be a difference in the susceptibility of some varieties to the disease. It seems that the bushy sorts are more susceptible than the climbing varieties. The thin-leaved varieties, too, seem to possess less resistance than those with thick leaves.

Spraying is often recommended for the control of black spot. The more recent investigations by Massey t show that ammoniacal copper carbonate is not as efficient as Bordeaux mixture for the control of the disease. Furthermore, the disease may be kept in check by dusting with sulphur-arsenate made of a mixture of ninety parts finely ground sulphur and ten parts powdered arsenate of lead. A lime sulphur solution composed of one part of the commercial concentrate solution to fifty parts of water is as efficient in controlling black leaf spot as is Bordeaux or sulphur-lead-dust. It is purely for the florist to decide whether he wishes to spray or to dust. The sulphur-arsenate may be applied with an efficient little machine known as the Corona hand duster. The same material is also very efficient for controlling the powdery mildew of the rose. In this case, too, the work of Massey has shown that sulphur-arsenate is even more efficient than lime-sulphur solution 1-5o, or Bordeaux mixture 5-5-50.

ANTHRACNOSE

Caused by Glæosporium rosie Hals.

Symptoms. The chief feature of this disease as observed by Halsted is a premature dropping of the foliage. Some stems may be entirely bare while others may have a few leaves still clinging to them. Infection may start on the leaves first, in which case they drop off, and soon develop the salmon-colored pustules on the dead spots. Generally, however, the trouble starts at the tender branches and works its way downwards. If infection takes place at a lower portion of the cane it will soon work its way up. The characteristic salmon-colored pustules are usually found in abundance on the affected canes. The rose anthracnose is very similar to that of the raspberry, although the latter is induced by a different species of fungus. Very little is known of the causal organism of the rose anthracnose.

Control. All dead leaves and canes should be re-moved and destroyed by fire. Spraying with a standard fungicide is also recommended.

LEAF SPOT

Caused by Mycosphaærella rosignea (E. and E.) Lind.

Symptoms. Purplish blotches appear on the leaves and later develop into sharply defined spots with brown centers and purplish borders. The perithecia of the fungus are found in large numbers on the dead tissue. It seems that the one-year-old plants are more susceptible to the disease than the two-year-old plants of the same variety. The disease usually occurs in the winter, but seldom causes serious damage.

The Organism. The perithecia are black and partly erumpent, while the asci are rather oblong and are arranged in two series in the ascus.

Control. The destruction by fire of all diseased material, and spraying with a standard fungicide is recommended.

SEPTORIA LEAF SPOT

Caused by Septoria rosa. Desm.

Symptoms. This disease is characterized by distinct spots on the foliage (fig. 66, h.). Spots with centers of creamy or dirty white surrounded by broad purple margins appear on the foliage. The pycnidia are formed in the center of the spots. Little is known of the nature of the causal organism. The destruction of all infected material by fire and spraying with a standard fungicide will keep the disease in check.

STEM CANKER

Caused by Coniothyrium fuckelii Sacc.

Symptoms. This disease usually produces cankers of the canes and branches. The lesions are brown in the center with a black border, limited by an outward reddish band or zone. The same disease at-tacks apples and raspberries.

The Organism. It has long been suspected that Leptosphæria was the ascus stage of Coniothyrium fuckelii. This Stewart seems to have verified, although definite evidences are still lacking. The perithecia are in groups, globose, black with fringed mouths. The asci are cylindric, eight-spored, one-rowed, three-septate. The pycnidia are similar to the perithecia; the spores are ovate, continuous, and grayish. Careful cutting and burning of affected material is suggested.

THE CROWN CANKER

Caused by Cylindrocladium scoparium Morgan.

Symptoms. Crown canker is perhaps one of the most important diseases of roses under glass. Although the percentage of roses that are actually killed is rather small the effect of the disease is to increase the financial loss by weakening the plants and reducing the yields in marketable blossoms. The disease first attacks the plants at the crown just above the surface of the soil, producing lesions both on the crown and on the roots. Infection often starts at the place of union of the scion and stock. The trouble is indicated by a slight discoloration of the bark, which soon deepens until the affected tissue becomes black and watersoaked (fig. 67, b.). As the lesions increase in number, the crown of the plant becomes girdled, and cracks appear in the bark of the infected area, which is sunken and in sharp contrast with the healthy bark surrounding it.

Another constant symptom of the disease is the punky consistency of the affected tissue. Although this effect is marked in the crown, especially of the diseased roots, the condition is further noticeable in the bark and sap wood of the affected crown or roots. The diseased roots send up suckers that are weak, spindly, and pale. A close examination will show that these, too, are affected at the point of attachment to the root. Affected plants linger for a long time, but they produce only a few stunted blossoms.

The Organism. The causal organism produces fertile and sterile hyphæ. The spores are borne in fascicles, are cut off from the short conidiophores by a constriction, and are held together by a sticky substance, but separate quickly when place- in water. The spores are cylindric, one septate, and hyaline (fig. 67, c to e.).

Control. Since the causal organism lives in the soil, steam sterilization is recommended. All dead or infected material should be destroyed by fire. The disease may also be avoided to some extent by placing grafted plants with the graft union above the soil. This will prevent infection at the wounded surface. Attention should also be given to the watering of the plants. Crown rot is worse in over-watered beds.

CERCOSPORA LEAF SPOT

Caused by Cercospora rosicola Pass.

Symptoms. This disease is characterized by roundish spots, gray in color, with a dark border separating the healthy from the diseased tissue.

The Organism. The conidiophores grow in tufts, are densely gregarious, small and dark colored. Conidia are straight, short, cylindric, and hyaline. Spraying with a standard fungicide is recommended.

THE RUBBER PLANT (Ficus elastica)

Cultural Considerations. Rubber plants generally adapt themselves to wide ranges of temperature. They may do as well when exposed to full sunlight as when kept under partial shade. In the summer it is advisable to plunge them out of doors to be hardened. The plants require frequent feedings with liquid manure.

DISEASES OF RUBBER PLANTS.

The rubber plant is considered very hardy. It is subject to but few diseases of importance.

LEAF SPOT OF INDIA RUBBER

Caused by Leptostromella elasticae Ell. and Ev.

Symptoms. The India rubber (Ficus elastica) is usually considered a hardy plant. Under green-house conditions, however, it is often attacked by a leaf spot which at times proves very disastrous.. The presence of the disease is first manifested by small spots (fig. 68, a) or streaks. These soon increase in number and in size until the entire leaf area becomes involved, resulting in a premature dropping of the foliage. The spots, which at first appear yellowish, soon turn brownish and finally become gray. The dead tissue is sharply defined from the living as it is banded by a narrow black margin. The pycnidia (fig. 68, b) appear scattered within the spots. The pycniospores are oblong, hyaline, and one celled.

Control. All de d and infected leaves should be destroyed by fire, and the plants should be sprayed with ammoniacal copper carbonate.

SCHIZANTHUS (Schizanthus pinnatus)

Cultural Considerations. Schizanthus is forced mostly as an early spring flower. It does not require a very rich soil, and thrives best under night temperatures of 45 to 50 degrees F.

DISEASES OF SCHIZANTHUS.

Schizanthi are very hardy plants. There is but one disease recorded in the United States on the greenhouse plants.

ANTHRACNOSE

Caused by Colletotrichum schizanthi Jen. and Stewart.

Symptoms. Anthracnose has been found by Jensen and Stewart to be a very destructive disease on greenhouse Schizanthus. The younger parts of the plant are usually more susceptible than the older ones. On infected stems, branches, or petioles, the disease causes watersoaked areas which extend in all directions. The spots become depressed, and grow deeper and deeper until the affected parts topple over and break. The lesions are light brown and dotted with the acervuli of the fungus. On the older parts of the plant the lesions take on the forms of cankers, although they do not sink as deeply. Occasionally, the disease attacks the leaves, forming light brown spots which are irregularly scattered.

The Organism. In structure Colletotrichum schizanthi is not different from any other Colletotrichum. The black setae are very numerous. The conidia are one celled, hyaline, oblong, and granular (fig. 68, c and d.).

Control. Diseased plants or parts of plants should be destroyed by fire. Spraying with a standard fungicide is also recommended.



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