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Greenhouse - Cauliflower

( Originally Published 1920 )



INDOOR cauliflower seems to be subject to less diseases than that grown out of doors. The troubles which attack this plant are practically the same as those which are found on the cabbage.

CLUB ROOT

Caused by Plasmodiophora brassicae Wor.

Symptoms. Affected plants show a wilting of the foliage in the day, although recovering in the evening or during cloudy weather. Diseased plants are dwarfed, pale, and sickly looking. The seat of the trouble is at the roots. The latter swell considerably in size, often taking on the form of a hernia (fig. 17, a-c.). The disease is more severe on seedlings in the seed bed, from whence it is carried to the field or to the greenhouse.

The Organism. Club root is caused by a slime mold. The spores of the parasite (fig. 17, d.) are nearly round and possess a transparent and refractive cell wall. The first signs of germination are a swelling of the spores, followed later by a bulging at one side. The inner pressure exerted splits the spore wall, thus permitting the protoplasm (swarm spores) to ooze out. The latter is without a cell. wall, and moves by means of a thick flagellum at the small end. The germination of the spores is improved by exposing them for a short time to cold and drying. The best medium is water which has been filtered through muck soil.

Infection of the hosts takes place through the wall of the root hair while the organism is in a uni-nucleate stage. Entrance of the parasite is evidenced by the browning and shriveling of the root hair.

Control. If this disease becomes introduced into the greenhouse, the safest course would be to sterilize the soil in the benches and in the seed bed. Sterilization with steam or formaldehyde is recommended (see pp. 32-43).

BACTERIAL LEAF SPOT

Caused by Pseudomonas maculicolum McC.

Symptoms. The disease is characterized by numerous small brownish to purple-gray spots. When the small spots coalesce, the entire leaf surface may be involved. Practically all parts of the leaves are affected. When the midribs and veins are attacked, the tissue becomes shrunken, and the leaves have a puckered appearance. In the early stages of infection, the spots on the leaves are watersoaked, later they become dry and turn dark merging into purplish gray. In transmitted light, the centers of the spots are thin, almost colorless, and are surrounded by a dark border. The diseased leaves become yellow and drop off prematurely. The trouble apparently does not attack the cauliflower head. The same disease may also attack the radish.

The Organism. The disease is produced by Pseudomonas maculicolum, a rod-shaped organism, with rounded ends, usually forming long chains in certain media, but producing no spores. The organism is actively motile by means of polar flagella. Involution forms are produced in alkaline beef bouillon; and pseudo-zoogloeĉ occur in acid beef bouillon. No gas is produced and the organism is aerobic. It is killed by drying and exposure to light.

Control. Badly diseased plants should be pulled up and destroyed. Spraying with 4-4-50 Bordeaux is recommended. In spraying cauliflower with cop-per compounds, and especially if the latter are in a concentration somewhat stronger than the plant can stand, numerous warts will appear on the leaves in about three days after spraying. These warts should not be mistaken for a disease induced by a parasitic organism. The wart formation is apparently due to a stimulation from the salts absorbed by the host cells.

BLACK ROT

Caused by Pseudomonas campestris (Pammel) Ew. Sm.

The disease is known both as stem rot and black rot. The latter perhaps is the more common name.

Symptoms. Black rot has distinct symptoms which cannot easily be confused with other diseases. On the leaves, the symptoms are manifested as a burning appearance on the edges and a yellowing of all the affected parts except the veins, which remain blackened. From the margin of the leaves, the disease works downwards to the stalk. From there it travels up again to the stems and leaves. The parasite works in the fibrovascular bundles of the leaves and main stalk, causing a premature defoliation. Occasionally, the disease enters one side of the stalk, the latter becoming dwarfed and the cauliflower head grows one-sided. In severe cases of attack, there is a total lack of head formation. Upon splitting open a stump of an affected plant, one finds a black ring which corresponds to the places of the fibrovascular bundles invaded by the organism. Infection takes place through small openings naturally found on the leaves and known as water pores which are scattered over the teeth of the leaves. Infection by means of insect bites is also a very common occurrence. Outbreaks of black rot may undoubtedly be traced back to the use of infected manure. Black rot also attacks greenhouse radish.

The Organism. Pseudomonas campestris is a rod-shaped organism, slightly longer than it is broad. When young it is actively motile by means of long polar flagella. It is found single or in pairs and produces no spores. It liquefies gelatine completely in about fifteen days. On agar plates the colonies are round, yellow in color, and the margin entire.

On potatoes a liberal growth is produced with no odor and no browning of substance.

Control. Before planting, cauliflower seed should be disinfected for fifteen minutes in a solution of pint of pure (40%) formaldehyde diluted in seven gallons of water. In making the seed bed, manure known to be free from cabbage refuse should be used. All insect pests should be kept in check by spraying with arsenate of lead. The disease can-not be controlled by merely cutting off diseased foliage. If anything, this operation aggravates the trouble. Diseased plants should be pulled out and destroyed.

SOFT ROT

Caused by Bacillus caratovorus Jones.

Soft rot, although a field trouble, causes considerable damage to greenhouse cauliflower.

Symptoms. The disease is characterized by a soft, mushy to slimy decay of the entire plant. The disease works very rapidly under favorable conditions of moisture and temperature. The causal organism can gain entrance only through a wound or bruise.

The Organism. Soft Rot is caused by a bacillus that is rod-shaped, of varying length, and usually formed in chains. It moves about by peritrichous flagella. It completely liquefies gelatine in about six days. Gas is produced with a majority of strains.

Control. Diseased plants should be destroyed by fire. To check further spread, water should be withheld and plenty of ventilation allowed. During watering unnecessary splashing of soil -particles on the plants should be avoided.

DAMPING OFF

Caused by Olpidium brassicae (Worr.) Dang.

The symptoms of damping off for cauliflower are similar to those produced by Pythium de Baryanum, p. 17. The sporangia of the parasite may be found singly or in groups in each infected host cell. The zoospores are globose, uniciliate. The resting spores are globose, wrinkled, and star-like in appearance.

The disease is found mostly in seed beds, where it does considerable damage. For methods of control, see pp. 32-34.

DOWNY MILDEW

Caused by Peronospora parasitica (Pers.) De By.

Symptoms. Downy mildew, while a common field disease, causes considerable damage to young seedlings in the seed beds. It is characterized by whitish downy patches on the underside of the leaf. Seen from above, the affected areas are angular, pale yellow, and somewhat shrunken. The spots seem to be limited by the veins of the leaves. The disease is common in damp houses. Besides the cauliflower, the radish, and numerous other cruciferous hosts are known to be susceptible to downy mildew.

The Organism. The sporophores of the fungus are stout and numerously branched, each of these repeatedly forked. The tips of the smaller branches are_ slender and curved. The conidia are broadly elliptical, and the resting spores are globose and smooth, becoming wrinkled with age.

Control. In the seed bed, spraying with 4-4-50 Bordeaux will control the disease. The first application should be given as soon as the disease makes its appearance. Later the application will be governed by disease conditions. Care should also be taken to avoid sowing the seeds too thickly. Overwatering, poor ventilation, and high temperature favor the disease.

DROP

Caused by Sclerotinia libertiana Fckl.

Drop is a disease fairly common on cauliflower. The trouble may be recognized by a drooping and wilting of the leaves. The bases of the affected foliage are covered with a white weft of mycelial growth, later by sclerotia. For a more extended discussion of the disease, see lettuce drop, p. 150.

RING SPOT

Caused by Mycospharella brassicaecola (Duby) Lind.

Symptoms. On the leaves, the disease appears as numerous small spots and the affected foliage turns yellow. Most of the spots are formed on the laminĉ, but others are also formed on the large midribs. The spots are definite in outline, round and visible on both surfaces of the leaf. The color is light brown to gray, with dry centers surrounded by olive green or blue green borders which shade off into the natural color of the leaf. The outer edge of the spot is covered with the fruit of the fungus. Spraying with 4-4-50 Bordeaux is recommended.

BLACK MOLD

Caused by Alternaria brassicae (Berk.) Sacc.

Affected leaves are covered with spots which are nearly black on the under side of the leaf. The spots are composed of a series of rings, the smaller ones enclosed within the larger. There is no distinct border separating the diseased portions from the healthy; the spots gradually shade off into the healthy tissue. Little is known of the causative fungus or of the control of this disease. It is probable that spraying with 4-4-50 Bordeaux will be of value.

ROOT KNOT

Caused by Heterodera radicicola (Greef) Mull.

Root knot is characterized by small swellings on the lateral feeding roots. For a description of the parasite and methods of control, see p. 28.



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