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Nature Of Plant Diseases

( Originally Published 1920 )

THE successful greenhouse operator will realize the necessity of recognizing readily any plant disease. Very often this is overlooked and attention is attracted only when the trouble takes the form of an epidemic, and a large number of plants are thus carried off by it.

Plant diseases are usually of four kinds :

1. Those of a mechanical nature.

2. Those brought about by physiological disturbances or unfavorable environment.

3. Those brought, about by parasitic flowering plants, fungi or bacteria.

4. Diseases the cause of which is unknown.

A familiarity with the symptoms of diseases will enable us to determine the contagious nature of the trouble and often the methods of control to pursue. The following outline briefly summarizes the principal symptoms of disease in plants :

1. Discoloration or change of color.

a. Pallor, yellowish or white instead of the normal green.

b. Colored areas or spots.

White or gray, such as mildews, white rusts, etc.

Yellow, many leaf spots.

Red or orange, rusts, leaf spots.

Brown, many leaf spots.

Black, black rusts.

Variegated, leaf spots, mosaic

2. Shot hole, perforation of leaves.

3. Wilting, wilts, damping off.

4. Necrosis, death of parts such as leaves, twigs,

5. Atrophy, dwarfing or reduction in size.

6. Malformations or excrescences, galls, pus-

tules, tumors, cankers, rosettes.

7. Exudation, slime or gum flow.

8. Rotting, dry or soft rots.


Greenhouse plants, contrary to those grown out-doors, are open to but few injuries of a mechanical nature, for it is seldom, indeed, that indoor plants are injured by rain, hail, or frost.

Sunburn. While most greenhouse crops require a great deal of light, a few are injured by it. Some varieties of tomatoes, the Earliana especially, under the influence of strong sunlight are subject to sun-scald. Sunburn may be overcome by shading the glass. Of the various shading materials, the cheapest and quickest to use is air-slaked lime. The most expedient to use is air-slaked lime which has been slaked dry by sprinkling lightly with water. This is diluted in water and applied as a spray. If new lime is used it will be more difficult to wash off later. Moreover, it .seems that air-slaked lime sticks a good while, but rubs off easily. It is far more desirable to use shading material that must be applied twice in the summer than something that will stick hard and remain during the fall and winter season.

Smoke injury. As a rule large greenhouse establishments are situated near large cities which are centers of industrial production and manufacture. Greenhouse plants are often injured from the effect of smoke or gases which escape from the furnaces into the air.

The sources of smoke may be classified into three divisions : (1) Smoke from large buildings or from manufacturing plants; (2) Smoke from locomotives; (3) Smoke from chimneys of dwelling houses. Smoke is generally produced because of improper furnace construction, such as improper draft, over-loaded boiler, insufficient air space, insufficient air supply to boiler room, and also by carelessness of operation.

Smoke contains large quantities of carbon dioxide, steam and sulphur dioxide, besides its characteristic soot. The latter consists of carbon, tar, and mineral matter mixed with small quantities of sulphur, arsenic and nitrogen compounds which are of an acid nature. Soot adheres to plants, especially to foliage, giving them a burned, contorted appearance. Another effect of soot and smoke is to close up the stomata or respiratory openings of the leaf, so that asphyxiation results. The effect of smoke on plants is a loss of leaflets in case of compound leaves, and an abnormal curling and distortion. Lesions and spots may be formed on the foliage as a result of the sulphur dioxide which is present in smoke. The spots are at first small, but soon enlarge and finally involve the whole leaf, which dries and becomes gray. Smoke injury, although of a mechanical nature, may also be considered from a physiological point of view. The after effect of smoke on plants resolves itself into a question of insufficient food supply and assimilation. This is indirectly brought about by diminished illumination, interference with the normal transpiration and the reduction of leaf surface.

Methods of Control. There is as yet no definite method of control known, consequently all that can be done is to avoid the smoke belts. The greatest injury usually occurs in locations to the leaward of smoky districts and when the soil is wet. As far as possible, therefore, postpone irrigation during the windy days.


In this class are included disturbances which are due to unfavorable conditions of nutrition. There are numerous diseases of plants which are brought about by lack of, or by an excess of, certain food elements in the soil. The effect is an interference with the proper life functions of plants.


Symptoms. The symptoms of malnutrition are not always the same. They differ somewhat with the crop, the nature of the soil, and the fertilizer applied. In malnutrition the symptoms to be looked for are retarded growth, change of color in the foliage and root injury. Affected plants remain dwarfed at a time when maximum growth is expected. The color of the foliage turns a lighter green, especially in the spaces between the veins, which become yellowish green to brown. Roots of such plants are poorly developed, and secondary roots are often missing.

Causes of Malnutrition. The work of Stone*, and Harter f and others seems to have established the fact that malnutrition cannot be attributed to the work of parasitic organisms. Stone cites instances where constant watering with liquid fertilizers or manure would cause malnutrition in cucumber plants. The same is also induced when pig and cow manure are mixed, or when manure is worked into a soil already well fertilized otherwise. Harter records cases of malnutrition brought about by an excess of acidity in the soil. In soils where plants suffer from malnutrition, from 3,500 to 6,000 pounds of lime per acre area are often required to neutralize the excess of the soil acidity. This condition is apparently the result of intensive trucking and the heavy application of chemical fertilizers which leave the soil acid. Sulphate of ammonia, muriate and sulphate of potash and acid phosphate when used continuously will leave the soil in a very acid condition. On the other hand, nitrate of soda, carbonate of potash and Thomas phosphate tend to make the soil alkaline.

Another important cause of malnutrition is the exhaustion of humus. This is a natural result where commercial fertilizers are used instead of some form of organic manure.

Methods of Controlling Malnutrition. It is quite bvious from what has already been said, that the greenhouse grower is the loser if he uses his fertilizer injudiciously. Not only is malnutrition favored by such a course, but the yields, too, are considerably reduced. With acid soils, liming to neutralize the soil acidity will help control malnutrition.


This disease may be attributed to several causes. Greenhouse plants that receive too much shade will become yellowish, then whitish, and in time may lose all their green color and finally die. Chlorosis is often brought about when plants grow in soils that have become too alkaline. This is true for soils containing an excess of lime, wood ashes, or magnesia, and especially when nitrate of soda is used in excess.

Control. Chlorosis when brought about by the lack of available iron in the soil may be remedied by the application of small quantities of iron sulphate. If the disease is caused by the other factors previously mentioned, a cure may be effected by re-moving the cause.


This is another trouble which may be termed physiological and the cause of which cannot be attributed to the work of parasitic organisms. It is often noticed on tomatoes and various other plants. Various causes lead to it. Sudden drops of temperature at blossoming will induce many plants to shed their blossoms. Blossom drop may also be brought about when too much nitrogen is applied to the soil in the form of manure, especially hen manure. To overcome this, the fertilizer in the soil must be balanced by the addition of 600. pounds of acid phosphate and 150 pounds of muriate of potash per acre. Overacidity in the soil may also cause the shedding of blossoms. A sudden checking of the water supply, or overwatering may have the same effect. Finally, improper pollination is often one of the main causes for the blossom drop of greenhouse plants. In the field, pollination is favored by both wind and insects. In the greenhouse, these two agencies are practically shut out. 'With forced cu-cumbers, the difficulty is often overcome by installing beehives in the house. Bees are very active under high temperature conditions, and perfect pollination is the result. The usual practice is to sup-ply a beehive to every 200 feet of house. The hives should be placed on platforms several feet above the bed to protect the bees from becoming drenched during the watering or sprinkling of the beds. We should bear in mind that the hives must be taken out whenever the house is fumigated with potassium cyanide. Nicotine fumes do not seem to injure the bees, especially if the fumigation is carried on at night. Bees may be used to pollinate practically every crop grown in the forcing house. It seems, however, that bees refuse to work on tomatoes, perhaps because of a dislike for their nectar. In this case, then, it is necessary to pollinate by hand. The investigations of Fletcher and Gregg and others have shown that the setting of a good crop of smooth heavy tomatoes depends largely on the proper distribution of pollen over the stigma. A lack of pollination will of course result in no crop. An uneven distribution of pollen will result in too large or irregular fruit. During the winter and on sunny days, it will pay to go over the plants and tap each blossom with the finger or with a stick on which is fastened a small glass rod or spoon. This will shake out the pollen and enough of it will be liberated by this operation to insure complete fertilization. A high temperature will favor the maturing and the bursting of the pollen sacs even during cloudy weather. It is, therefore, advisable to run up the temperature of the house as high as is expedient on the days when the tapping of the blossoms is done. This should always be done during the day and never at night. The pollen sacks (anthers or male organs) do not burst freely until after the yellow petals have fully expanded and have begun to wither slightly. The pollen is discharged most freely in a hot dry atmosphere.


In this class of diseases may be mentioned those which are induced by parasitic flowering plants such as the dodder and the broom rape. These, however, as well as the diseases induced by bacteria and fungi, will be considered under their respective hosts.

Carriers of Diseases. In the greenhouse, disease producing organisms are often brought directly with infected soil or manure in the compost. Fusarium lycopersici Sacc., the cause of sleeping sickness of tomato, as well as large numbers of other parasites, are brought in that way.

Little as yet do we realize the importance of insects as carriers and disseminators of plant diseases, although we are becoming increasingly aware of their rôle in human and animal pathology. Acting as carriers of spores of parasitic fungi, which may adhere to any part of their body, they are responsible for distributing plant diseases. Insects, too, by feeding on plants or in searching for the nectar of the blossoms, are likely to come in contact with diseased parts. Their bodies may become coated with spores of parasitic bacteria or fungi, which are thus carried from plant to plant and from field to field. The striped cucumber beetle is known to carry the virus of cucumber mosaic, and the germ of cucumber wilt (Bacillus tracheiphillus Ew. Sm.). It is there-fore very essential that every effort should be made to keep insect pests out of the greenhouse.



In this class will be included those diseases which spread by contact, but the exact cause of which is unknown. Special emphasis will be given to that important disease known as mosaic, This trouble attacks a variety of greenhouse plants. It is especially severe on the tomato, cucumber, and sweet pea.

Symptoms. Mosaic is readily distinguishable by a yellow dotting or mottling on the foliage, presenting in some instances a beautiful mosaic structure, whence its name. Affected leaves linger and often curl

Cause of Mosaic. The cause of Mosaic is as yet a disputed question. Allard claims that mosaic is caused by an ultra-microscopic pathogen, that is, a parasitic organism which cannot be detected by our present technic in microscopy. Mosaic may be transmitted from plant to plant. The easiest way to prove this is to rub with the fingers a diseased leaf and then immediately rub a healthy one. The disease will appear on the inoculated host in about ten days. In the greenhouse, the green aphid and the white fly act as carriers of mosaic.

Control. Methods of control in mosaic lie in the direction of prevention. Diseased plants should be destroyed by fire, and all indoor insect pests kept in check.

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