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Greenhouse - Cucumber

( Originally Published 1920 )



Cultural Considerations. Cucumbers are extensively forced for the winter or early spring markets. (fig. 20.) The houses generally used are either two-thirds or even-span and are provided with ground beds instead of benches.

Unlike lettuce, cucumbers are not so sensitive to variations in soil texture. A great variety of soils may be used if, however, they are well provided with organic matter. Cucumbers require a night temperature of about 65 degrees F. and about 85 degrees during bright weather. In cloudy weather, how-ever, the day temperature should be about 10 to 15 degrees lower, otherwise the plants will become weak, spindly and susceptible to disease. Extreme care is required in watering the plants. Overwatering during cool, wet weather will greatly injure them by encouraging numerous diseases.

Of the varieties which lend themselves to forcing are the Telegraph (English), and all strains of Dark Spine and White Spine among the American varieties.

LEAF CURL

Cause, physiological.

Symptoms. The trouble is often manifested as a wilting of the edges of the leaves which curl into a spherical form. The wilted area soon dies, thus preventing any further development of the affected leaf. As the inner part of the leaf continues to grow and as it is restricted by its outer dead area, it assumes a convex form and a contorted margin, so that it curls up and assumes the shape of a ball. In advanced cases the stems, too, curl. The disease was studied by Stone * who believes that the cause of the trouble is overmanuring. Abnormal modifications in the light, soil texture, and moisture conditions may frequently induce the same trouble. To prevent this disease the use of excessive manures should be avoided. As far as possible, conditions which favor weak soft growth should be eliminated.

MALNUTRITION OF CUCUMBERS

Cause, physiological.

Malnutrition generally results from an overfertilization of the soil. This trouble has been carefully studied by Haskins t who found it very prevalent on indoor cucumbers.

Symptoms. At first, the plants have a vigorous appearance, but soon turn yellow and fail to form fruit. The leaves of affected plants become spotted,. resembling somewhat mosaic.

Cucumbers are often grown in the same beds for a number of years. Each year as new manure and fertilizers are added, the salt content of the soil becomes higher and more concentrated. Although there is an abundance of available food in the soil, the cucumber suffers because it is unable to stand a concentrated form of food. Chemical analysis of a normal and an overfed soil throws much light on this important subject.

From Table 17 it is seen that an overfed soil has a marked increase of soluble plant food. If we should express this in terms of fertilizers it would make 3 3/4 tons of nitrate of soda, 6 1/2 tons of high grade sulphate of potash, 2 tons of 16 per cent. phosphate, and 1 ton of hydrated lime. To express it in terms of a mixed fertilizer it would amount to 1472 tons of a formula testing 4 per cent. nitrogen, 23 per cent. actual potash and 2.25 per cent. available phosphoric acid. The injurious effect on cucumbers is not due to the excess of any one element but rather to the toxicity of the combined excess of soluble salts.

PHYSIOLOGICAL WILT

Symptoms. Cucumber growers often find that their plants wilt badly when subjected to the intense rays of the sunlight. This is especially true when bright weather follows a continued cloudy spell. This trouble is common in houses which are poorly ventilated, and where the plants are weak. Too high a temperature and poor lighting will greatly favor wilting. The removal of these causes will effect a cure.

LACK OF COLOR IN FRUIT

Cucumber fruit, especially of the White Spine variety, often becomes very white, thus commanding a lower market price. This discoloration is probably due to a lack of available nitrogen in the soil. It may be remedied by the application of one part liquid nitrate of soda to one thousand parts of water.

MOSAIC OR "WHITE" OR "LITTLE PICKLE"

Cause unknown.

Symptoms. The first sign appears as a yellow mottling near the stem end of the fruit. Later the light areas are found all over the cucumber, and the darker portions frequently form protuberances. Some fruits retain their green color and show the disease only by being distorted. The leaves become mottled light to dark green (fig. 21, a), and some-times wrinkled, while the stems and petioles are dwarfed and distorted. Affected leaves die prematurely and are replaced by others, which in turn con-tract the disease. The trouble is spread principally by the melon louse, Aphis gossypii Glov., and to a lesser degree by striped cucumber beetle, Diabrotica vittata Fabr. Satisfactory methods of control are still wanting. Affected plants should be destroyed to prevent further spread of the disease.

BACTERIAL WILT

Caused by Bacillus tracheiphillus Ew. Sm.

The symptoms and the damage caused by this wilt will be found discussed under the muskmelon, p. 155. Recent investigations by Rand and Enlows have shown that seeds from diseased plants fail to reproduce wilt. This is true not only for the cucumber, but also for all the other cucurbit hosts which are subject to this trouble. Of the numerous varieties of cucumber, none shows promise of resistance to the disease.

ANGULAR LEAF SPOT

Caused by Pseudomonas lachrymans Sm. and Bry.

Symptoms. The trouble is characterized by angular brown spots which tear or drop out when dry, giving a ragged appearance to the infected leaves. In the early stages of the disease a bacterial exudate collects in drops on the lower surface of the spots. This usually dries up and becomes whitish. It seems that angular leaf spots usually attack only the foliage, rarely the fruit.

The Organism. The parasite is a short rod with rounded ends, occurring singly or in pairs with a decided constriction, and occasionally in chains of twelve individuals or more. It is motile by means of polar flagella, produces capsules on agar and milk; no spores, and no gas is formed. The organism completely liquefies gelatine in about three or four weeks. Little is known of methods of control.

DAMPING OFF, see Pythium, p. 28.

DOWNY MILDEW

Caused by Pseudoperonospora cubensis (B. and C.) Rost.

Symptoms, The disease appears on the leaves as yellowish spots, which have no definite outline. In a warm, moist atmosphere numerous spots coalesce, and soon the affected leaves turn yellow and die. With cool temperatures, the spots seem to spread less rapidly. The disease appears to work on the older leaves, beginning on those at the center of the plant and working outwards. With infected plants the center of the hill is clearly marked by a cluster of yellow leaves. Diseased plants may flower profusely, but set no fruit. The few cucumbers which are set are small, deformed, and unfit for the market.

The Organism. The fungus derives its food from the host cells by means of suckers or haustoria. The mycelium is hyaline, non-septate; the conidiophores arise in small clusters through the leaf stomata and are branched and flexuous. The zoosporangia are hyaline but slightly violet, tinted in mass. Germination of zoosporangia is by means of motile zoospores. The zoospores or sexual fruiting stage was first found on the host by Rostovtsev.

Control. Downy mildew seems to be most prevalent on greenhouse cucumbers planted in August. Those set in October seem to be free from it. Where the disease makes its appearance, it is advisable not to syringe the plants, but on the other hand to keep the foliage dry. Diseased plants or parts of plants should be destroyed by fire. Late planting in September or October wherever practicable is also advised.

TIMBER ROT

Caused by Sclerotinia libertiana Fckl.

Symptoms. The disease seems to attack the stem end of the plant nearest to the ground line. Affected stems at first water-soaked, then become invaded with a cushion of white mycelial growth. Rapid wilting, with no recovery, immediately fol-lows. As the affected plant dies, the shriveled stem becomes covered with black masses of fungous bodies, sclerotia. The same fungus also causes lettuce drop. For a description of the causal organism and methods of control.

POWDERY MILDEW

Caused by Erysiphe cichoracearum D. C.

Powdery mildew of cucumbers is not a serious greenhouse trouble. Like all powdery mildews, the causative fungus grows on the surface of the leaf, giving it a white mealy appearance. From the mycelium are produced erect threads which bear the summer spores of the fungus. The ascus or winter stage appears as minute dark-brown, rounded capsules enclosing a group of spore sacs within which are formed the ascospores.

Control. The conditions which favor mildew are overwatering, lack of ventilation, lack of light, and too high a temperature. Proper attention to these factors will help to remove the cause and to effect a cure.

ANTHRACNOSE

Caused by Colletotrichum lagenarium (Pass.) Ell. and Hals.

Symptoms. This disease is often serious on green-house cucumbers and muskmelons. It is seldom so in the fall and winter, but is most frequently met with in the spring of the year. Affected plants dry up and present a parched appearance. The disease also attacks the cucumber leaves, forming round spots (fig. 21, b.), and on the fruit, deep cankers, thus ruining its market value. It is claimed by practical greenhouse men that the great difference in temperature between day and night, which is unavoidable in the spring when the fires have gone out, favors infection.

The Organism. In structure, Colletotrichum lagenarium resembles the organism of bean anthracnose. The cucumber fungus has a peculiar ability to remain dormant during the dry weather; but it is easily revived by moisture. The fruits of the fungus are borne in masses on the pustules which take on a salmon color. The spores are typical of all Colletotrichums—that is, oval, one-celled, and hyaline. The setæ in C. lagenarium are not very plentiful. In pure culture it resembles C. lindemuthianum; however, pathologically it is distinct from the latter, since numerous attempts by the writer and by others have failed to infect growing bean plants with the watermelon anthracnose or the watermelon with that of the bean.

Control. As soon as the disease makes Its appearance the foliage should not be syringed, but kept dry. Spraying with 3-5-5o Bordeaux is recommended.

ROOT KNOT, see Nematode, p. 28.

EGGPLANT (Solanum melongena)

Cultural Considerations. Eggplants require as much heat as cucumbers. The night temperature should never run down below 6o degrees F. The day temperature may safely be maintained at 8o degrees or more, provided, however, sufficient ventilation is allowed. The best soil for greenhouse eggplants is a light sandy soil containing plenty of organic matter. Raised benches with bottom heat is preferred for winter forcing. The eggplant does not thrive under an excess of water. To obtain marketable fruit, the blossoms must be hand pollinized to insure fertilization.

DISEASES OF EGGPLANtS

Eggplants are subject to numerous diseases.

SOUTHERN WILT, see Tomato, p. 185. DAMPING OFF, see Pythium, p. 17.

FRUIT ROT

Caused by Phomosis vexans (Sam and Syd.).

Symptoms. Fruit rot attacks all parts of the plant except the roots. On the seedlings it causes a damping off. Young plants are attacked at the stem end or an inch or two above the ground line as indicated by a constricted area at that place. On the leaves the trouble is manifested as large brown round spots which later become irregular and ragged (fig. 23, a.). The older spots are light purple in the center and are surrounded by a black margin. As they en-large the spots also invade the veins, midribs, and petioles, forming depressions. Diseased fruits are at first soft and mushy, but later become dry, shriveled, and mummified (fig. 24, a.).

The Organism. Pycnidia are usually found on all parts of the attacked plant. Within the body of the pycnidia and intermixed with the conidior phores (fig. 23, c.) and pycniospores (fig. 23, b.) are found filiform hooked-shaped bodies termed stylo-spores (fig. 23, d.). Phomosis vexans has been erroneously referred to as Phoma solani Hals; Phoma vexans Sacc. and Syd., and Aschochyta hortorum Speg.

Control. The seedlings in the seed bed should be sprayed with Bordeaux at least once before trans-planting. The plant in the house should be sprayed from four to eight times with either Bordeaux mixture or ammoniacal copper carbonate.

ANTHRACNOSE

Caused by Gloesporium melengonea E. and H. Anthracnose on the eggplant attacks only the fruit. The trouble is characterized by numerous deep pits which later become covered with salmon colored acervuli (fig. 24, b.). The latter are made up of myriads of spores of the fungus. Spraying for fruit rot will also help to control anthracnose.

ROOT KNOT, see Nematode, p. 28.



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