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Greenhouse - Lettuce

( Originally Published 1920 )



Cultural Considerations. Lettuce is extensively grown as a greenhouse crop (fig. 25). The best results are obtained where the soil contains considerable sand, especially where head lettuce is produced. This is also true for Coss lettuce. However, the Grand Rapids variety will thrive in any soil. Lettuce is a heavy feeder, hence stable manure is often used exclusively. If the crop does not make rapid headway, nitrate of soda at the rate of one pound to 100 square feet of space may be applied. In using acid phosphate or potash only small quantities should be applied for fear of burning the plants. Lettuce requires an abundance of water and good drainage. High temperatures and humidity will produce weak, spindly plants. Careful ventilation may be the cause of preventing numerous diseases. In mild weather the ventilators may remain open even at night. This, however, should not be done during freezing weather.

It is fortunate that lettuce seed retains its vitality for three or four years. This enables the grower to test out carefully the strains which he uses.

DISEASES OF LETTUCE

TOP BURN

Cause, physiological

Symptoms. In plants suffering from top burn a withering of the leaves is followed by the curling back of the tips and margins of the outer leaves, the affected areas becoming blackened to a distance of an inch or more from the margin. The condition greatly disfigures the lettuce head and reduces its market value.

Causes. The trouble may originate on a bright day following a cloudy spell. The greenhouse air is then saturated with moisture and consequently the lettuce leaves transpire very little. With the sudden appearance of strong sunlight, there is a rise in temperature, and a loss of moisture in the greenhouse air. Under these conditions, the foliage will transpire heavily. If the water given off by the leaves is greater than the roots are able to replace, the leaves will wilt, and if this is continued for any length of time, the tissue along the leaf margin will wither and die.

Control. A practical remedy for top burn is to saturate the greenhouse air with moisture on bright days which follow cloudy spells. This will prevent the undue transpiration of the leaves and its subsequent bad effect.

BACTERIAL BLIGHT

Caused by Pseudomonas viridilividum Br.

Symptoms. The disease seems to attack only the outer leaves of a head. The affected foliage is first covered with numerous watersoaked spots which en-large, fuse together, and involve the entire area of the affected leaves. The latter either soften or dry up, opening up the way for the entrance of other decay organisms, which may now attack the other-wise sound head.

The Organism is rod-shaped, occurring singly, in pairs, or in chains, and it moves about by means of polar flagella. On agar, the young colonies are round with entire smooth margins; they are translucent, cream white in reflected light, but bluish in transmitted light. The older colonies are not always uniform in color, but may take on yellowish bands or become mottled. The organism does not form gas and it liquefies gelatine slowly. It is not especially sensitive to sunlight.

Control. Since the disease may be introduced with infected soil, soil sterilization is recommended.

THE SOUTH CAROLINA DISEASE

Caused by Pseudomonas vitans Br.

Symptoms. The disease may attack the stems or leaves. At first, diseased plants become pale and lose their normal green. Later the head wilts and rots (fig. 26, a.). The rot may be confined to the outer leaves, or involve the whole head. Affected stems become brittle and may be readily broken. At first they are a blue green afterward becoming brown. The disease is met with in the field, but may also be introduced in the greenhouse.

The Organism. Nellie Brown has definitely proved that Pseudomonas vitans is the cause of the trouble which seems so prevalent in South Carolina. The causal organism is a short rod with rounded ends, motile by means of polar flagella (fig. 26, b.), one at each end; produces no spores, but capsules and pseudozoogloeae. It liquefies gelatin slowly and produces no gas.

Control. The disease may be introduced with infected seedlings or soil. For the latter, soil sterilization with steam or formaldehyde is recommended. Diseased seedlings should be discarded.

THE KANSAS DISEASE

Caused by Pseudomonas marginale Br.

Symptoms. The disease seems to appear when the plants are half grown. At first the leaves wilt slightly in restricted areas at the margin. On the older leaves wilting starts at the tips. The affected areas fall over and gradually dry up. The vascular tissue then becomes affected and brown, then reddish, and finally black. The tissue of the dead parts on the leaves becomes dry and papery.

This disease does not cause a rot of the leaves. The trouble is confined mainly to the edges of the foliage, marring the appearance and the market value of the product. The varieties most affected seem to be the Black-seeded Simpson, the Improved Hansen, and the Big Boston. Of the varieties less susceptible may be mentioned the Early Curled Simpson, and Vaughan's All Season. The variety Grand Rapids seems to be immune.

The Organism. Pseudomonas marginale is a short rod, rounded at both ends and motile by means of polar flagella. It forms capsules but no endospores, liquefies gelatin quickly and produces no gas.

Control. Infected material should be destroyed by fire. In watering, splashing should be avoided. Soil sterilization with steam or formaldehyde (see pp. 32-43) is recommended.

DOWNY MILDEW

Caused by Bremia lactucae Reg.

Symptoms. Affected leaves lose their natural green color and turn yellow. A careful examination will disclose a delicate downy web on the under side of the foliage which will have a wilted appearance. The downy web consists of the conidiophores of the fungus. These appear singly and are much branched. The conidia germinate by means of a germ tube. Downy mildew is a disease which is more troublesome in Europe than in the United States, and it is more serious on greenhouse lettuce than on that grown in the open. In the field it usually attacks fall lettuce. Downy mildew attacks not only lettuce, but also chicory and numerous other Composite.

Control. This disease is controlled in the same way as lettuce drop (see p. 151).

GRAY MOLD

Caused by Sclerotinia fuckeliana De Bary.

Symptoms. Gray mold attacks grapes in Europe but in the United States it is commonly met with on lettuce plants which are fully developed and somewhat overgrown. The disease is manifested by soft, watersoaked spots on the foliage, causing a wilting. The spots soon become coated with the fruit of a gray mold. The fungus has two stages, the Botrytis cinera Pers. stage, which is commonly found as a gray mold on wilted lettuce leaves (fig. 26, c.) ; the other is the winter or apothecial stage, known as Sclerotinia fuckeliana. American botanists have not as yet been able to connect these two forms. It seems, however, that Istvanffi was able to confirm the work of De Bary, who first indicated the relationship of Botrytis cinerea with Sclerotinia fuckeliana.

Control, see Lettuce Drop, p. 151.

LETTUCE DROP

Caused by Sclerotinia libertiana Fckl. Symptoms. The term drop best describes the symptoms of the disease. The first sign is a wilting of the lower leaves, which is immediately followed by a drooping of upper ones until the entire plant is involved. The affected plant has a sunken appearance as if scalded with boiling water (fig. 27, a.). In examining a dead plant, a white cottony fungus growth is found on the under side of the lower leaves, and near the moist regions at the stem end.

When the plants are fairly rotted, there appear on the cottony mycelial growth mentioned above, black bodies, or sclerotia, which vary in size from a pinhead to a grain of corn. The three definite symptoms of the disease may be summarized : (I) drooping, (2) cottony-like mycelial growth on the under surface of the affected leaves, (3) the appearance of sclerotia. The latter help to carry over the fungus during the winter. After the sclerotia have been in the soil over winter, they germinate in the following spring by sending out small mushroom-like fruiting bodies known as apothecia. The latter contain small sacs or asci which bear the spores.

Control. Lettuce drop is favored by high temperature, overwatering, and poorly drained beds, leaky roofs, and insufficient ventilation. To check the disease a low night temperature of 50 degrees F. should be maintained. The water should also be withheld and an abundance of ventilation given especially during cloudy weather. Soil sterilization with steam or formaldehyde is also recommended.

LEAF SPOT

Caused by Septoria lactucae Pass. and Septoria consimilis E. and M.

This disease is induced by two species of Septoria fungi. The symptoms produced by both are so nearly alike that it is difficult to distinguish one from the other, except by microscopic examination. Pale brown discolored spots appear on the older leaves with numerous black pycnidia in the center (fig. 27, b-e.). The disease is of little economic importance, as it usually occurs late in the season, on plants which have nearly passed their usefulness. The Boston variety is considered resistant, while the Salamander and the Wonderful are more susceptible to leaf spot.

SHOT HOLE

Caused by Marsonia perforans E. and E.

The disease is of little economic importance. Affected leaves are covered with dry spots which drop out, leaving irregular perforations. Along the border of these holes, the causative fungus may be found abundantly fruiting. The disease attacks the mid-ribs of the leaves as well as the stem of the plants. It seems to be more prevalent under conditions of surface irrigation.

With sub-irrigation, on the other hand, it is not found to cause any damage.

CERCOSPORA LEAF SPOT

Caused by Cercospora lactucae Stev.

This disease is as yet of no importance in the United States. The causal fungus attacks the older and lower leaves forming numerous irregular spots.

ROSETTE

Caused by Rhizoctonia solani Kuhn.

Symptoms. The disease attacks young seedlings by causing a damping off. Transplanted seedlings show infection at an early stage. Unlike healthy plants, they fail to send out new leaflets. The general growth takes the form of a rosette. The axis bearing leaf remains stunted. The roots show numerous deep lesions, and in an advanced stage are considerably rotted (fig. 28, b.). For a description of the causal organism, see p. 20, and for methods of control see LETTUCE DROP, p. 151.

RooT KNOT, see Nematode, p. 28.

MINT (Mentha sp.)

Cultural Considerations. Mint is often forced on a small scale. The plants are easily grown, and re-quire about the same indoor conditions as the lettuce.

DISEASES OF THE MINT

Indoor mint may be subject to but one disease of importance.

RUST

Caused by Puccinia menthae Pers.

This disease attacks about thirty-five members of the mint family. All the three stages of the fungus, i.e., ścidiospores, uredospores, and teleutospores, occur on the same host. The disease is characterized by brown sori which are at first cinnamon colored and later become chestnut brown. Diseased leaves curl and dry up. The infected parts of plants should be destroyed by fire.

MUSKMELON (Cucumis melo)

Cultural Considerations. Muskmelon culture is little different from that of the cucumber (fig. 29.). However, the former is very sensitive to cold drafts and sudden changes in temperatures. For forcing, the heavier types of soil seem to be more desirable than the lighter ones. The fertilizer requirements for muskmelons are practically the same as those for cucumbers. Muskmelons require an abundance of soil moisture, but are sensitive to overwatering. It is also essential to maintain a high humidity in the house during the period of active growth. During pollination and the ripening of the fruit the above condition is unnecessary. The temperature should be about 70 to 75 degrees F. at night and from 8o to 85 degrees Fahrenheit during the day. Muskmelons, like cucumber blossoms, must be pollinated artificially, since both male and female flowers are distinct. The pollination is done by means of a camel's-hair brush, where the pollen from the male blossoms is rubbed on the stigma of the female flowers.

DISEASES OF THE MUSKMELON

Like cucumbers, greenhouse muskmelons are subject to numerous diseases.

BACTERIAL WILT

Caused by Bacillus tracheiphillus Ew. Sm.

Symptoms. The symptoms of bacterial wilt are very striking. At first a few leaves of the plant are wilted. Soon after, the entire plant wilts and dies. Upon cutting through an infected stem, one observes a whitish viscid exudate that oozes out from the vascular bundles of the cut surface. If one places his finger on the viscid substance and then gently removes it, the bacteria will be strung out into numerous delicate threads resembling cobwebs. The disease works quickly, and the change of leaf color from bright to dull green is also sudden. Musk-melons, unlike squash, show no tendency to recover temporarily from wilt.

Bacterial wilt is spread about through the bites of leaf-eating beetles, such as striped cucumber beetle (Diabrotica vittata).

The Organism. B. tracheiphilus is a short straight rod with rounded ends. The organism occurs singly, in pairs, or rarely in chains of four; it is motile by means of flagella. It grows slowly on gelatine which is not liquefied. On potato cylinders growth is vigorous, resulting in a gray-white film with no changes manifested in the substratum. There is no gas production and the organism is aerobic.

Control. Infection begins at a place of injury that has been produced by the bite or puncture of insects. Hence, any attempt to control wilt should first aim to control insect pests (see pp. 381-410).

DOWNY MILDEW, see Cucumber, p. 138.

POWDERY MILDEW

Caused by Erysiphe polygoni D. C.

This disease is the same as the mildew which at-tacks cucumbers and numerous other hosts. Mildew is prevalent on greenhouse melons. It is characterized by powdery white patches on the leaves. For control, see p. 323.

MYCOSPH∆RELLA WILT

Caused by Mycosphaerella citrullina (Sm.) Gr.

Symptoms. This form of wilt is often a serious greenhouse trouble. Grossenbacher * found that infection is localized at the nodes and not at the internodes. The injury from Red Spider or from other sucking insects is perhaps responsible for opening the way to this disease. A characteristic of the trouble is that the edges of the infected areas are covered with an oily, green to raisin-colored gum. The older parts of the spots are either dark and gummy or gray and dry, bearing numerous brown pycnidia.

The Organism. The perithecia are roundish, rough, dark brown to black, and almost superficial on the surface of the spots. The necks of the perithecia are hairy, the ascospores are cylindrical, two-celled, hyaline, and slightly constricted at the center.

Control. Spraying with Bordeaux mixtures when the plants are about half grown and before the disease appears is recommended. Spraying should be continued so that the growing parts are kept covered with the fungicide.

ANTHRACNOSE, see Cucumber, p. 141.

LEAF BLIGHT

Caused by Alternaria brassicae var. nigrescens Pegl.

Symptoms. The disease begins as small round spots which gradually enlarge. These spots are dry, brown in color and made up of concentric rings or zones (fig. 30, a and b.). Usually the spots are very numerous and their presence causes the leaves to curl and dry up prematurely, leaving bare vines and unprotected fruit. As a result, the melons ripen early and have an insipid taste, and are very poor shippers. Leaf blight is most serious under field conditions.

Control. The disease may be kept in check by spraying with a weak solution of Bordeaux mixture.

SOUTHERN BLIGHT

Caused by Sclerotium rolfsii Sacc.

Southern blight, a disease that attacks a large variety of hosts, is a serious melon disease in the South-ern States. The injury in most cases is confined to the foot of the stem, the girdling and rotting of which finally causes the death of the affected plant. In the case of the cantaloupe, the fruit itself is attacked, infection usually taking place at a point where it touches the ground. The disease appears first as a slight soft spot which enlarges quickly, changing the entire mass of the fruit to a mushy pulp. The exterior of the affected melon is rapidly covered with a white cottony growth consisting of the mycelium of the fungus. Later there appear numerous whitish bodies known as sclerotia which turn yellowish and then brown. They help to carry the fungus over the winter. For methods of control, see pp. 32-43.

ROOT KNOT, see Nematode, p. 28 (fig. 31).



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