Amazing articles on just about every subject...

Greenhouse Diseases - Methods Of Control

( Originally Published 1920 )

FROM a practical consideration the greenhouse grower is directly concerned in finding control methods to keep the various plant diseases under check. Fortunately, there are numerous methods which if followed out intelligently may be the means of reducing losses to a minimum. These methods may be enumerated as follows :

1. Soil sterilization. This method has been fully discussed under chapter 3, pp. 32-43.

2. Seed treatment, taken up in chapter 8, p. 104.

3. Spraying.

4. Hygiene.


While the orchardist has learned the necessity of spraying, it is doubtful whether greenhouse growers have sufficiently realized its value. Spraying has two aims : to kill the insect and animal pests, and to control fungous diseases. The substances which are used for the one purpose are without effect on the other.


All animal and insect pests are best controlled by the use of poisonous mixtures applied in the form of liquid sprays or in powders. Insecticides may be classified as internal or stomach poisons, and external or contact poisons.

(a) STOMACH POISONS. Paris green is one of the oldest stomach poisons. When chemically pure, it is composed of copper oxide, acetic acid, and arsenious acid. It destroys cutworms, caterpillars, beetles, grubs, slugs, etc. It should be applied preferably as a liquid, using one pound of the poison and two pounds of lime to two hundred gallons of water. The Paris green tends to sink to the bottom of this mixture, unless constantly stirred while being applied. This chemical is often adulterated with white arsenic, causing it to scorch the treated plants badly. Therefore for greenhouse crops the use of arsenate of lead is to be preferred, since it is less liable to scorch the foliage, and also because it adheres better. Its chemical composition consists of acetate of lead and arsenate of soda. It is applied to the best advantage as a liquid, composed of about three pounds of powdered arsenate or five pounds of paste arsenate to one hundred gallons of water.

Arsenite of zinc may also be used. It is a very finely divided fluffy white powder which can be thoroughly distributed and which adheres well to the foliage. It is intermediate between Paris green and lead arsenate in strength, and costs less than either.

It is essential when arsenicals are used to see that they are correctly labeled, and kept under lock and key, as they are poisonous to both man and animal.

Hellebore or white hellebore is somewhat less dangerous than the arsenicals. However, it loses its insecticidal value by being exposed to the air. It is a specific against slugs.

(b) CONTACT POISONS. These are used most extensively in the greenhouse.

Water. Every florist appreciates this simple and effective remedy. To be effective in dislodging and destroying soft-bodied insects, it must be thrown with considerable force on all parts of the plants. For this purpose a good nozzle attachment is very necessary. This simple remedy is effective in destroying red spider, white flies, and mealy bugs.

Tobacco. There are few substances which are more extensively used than the many commercially prepared tobacco products. Finely ground tobacco is extensively used as an insecticide, especially for. Aphids. The product known as nicofumes is extensively used for indoor fumigation.

Tobacco Decoction. This may be prepared by boiling for one-half hour one pound of tobacco stems in two gallons of water. It is then strained and more water added to replace that which was lost by evaporation during boiling. The liquid is used as a spray against plant lice.

"Black Leaf" and "Black Leaf 40." These are two commercial products. The latter is the more concentrated of the two and is extensively used in the control of sucking insects. The following table by McCue * shows the dilutions that should be used to give a nicotine content varying from 0.03 to 0.01 per cent.

Soap. Any good soap may effectively be used as a contact insecticide. The best soap is that which is made with caustic potash rather than with caustic soda. Soda soap washes are apt to gelatinize when cold and are made difficult or impossible to be used as a spray. Fir tree oil soap may be used at the rate of three ounces to each five gallons of water without injury to plants. Whale oil soap should be used at the rate of only one-fourth pound to each gallon of water. A stronger solution may injure tender plants.

Sulphur. This is used not only as a fungicide, but also as an insecticide as well. When used as a fumigant, sulphur at the rate of one-third of a pound to each i,000 cubic feet of greenhouse space will be effective. When ready to fumigate the house is closed tightly, the required amount of sulphur weighed out and divided into four equal parts on clean paper. The sulphur is then placed in wide, deep metal pans at the bottoms of which are first placed chips which have been soaked in kerosene. The pans are solidly placed at equal distances on raised bricks in the center aisle of the house. When all is in readiness fire is set to the chips. When these begin to burn well the sulphur is spread evenly on the burning chips of each pan. As soon as the sulphur ignites, the operator should run out and shut the door of the house as quickly as possible. Sulphur fumes have an irritating and suffocating effect on man. The sulphur fumes should be allowed to act for at least twelve hours before one opens the house. The sulphur fumigation may be started at any convenient time during the day or night. This treatment will destroy red spider and mildew.

Hydrocyanic-Acid Gas. There seems no doubt that fumigation with hydrocyanic gas offers the cheapest and most efficient method of controlling white flies, aphids, thrips, scales, and mealy bugs. However, this method has not yet gained general popularity because of the deadly nature of the gas and its injury to plants when overdone. The best generators for the gas are one-half or one gallon glazed earthenware jars. When ordering generators it should be indicated that tops are not desired.

Before fumigating it is essential to see that all broken glass is repaired, and that all cracks are carefully stopped up. It is very essential that the cubical contents of the greenhouse be accurately determined. To secure the cubical contents of an even span house compute the number of square feet in the rectangle and in the right angles, and multiply the sum of the three by the length of the house. To secure the cubical contents of a three-quarter span house multiply the sums of the areas of the rectangles, and the areas of the right angle triangles by the length of the house. In estimating the cubical contents of a greenhouse it is not necessary to make allowances for the space occupied by pots or benches. Fumigation should never be attempted during high winds. It should never be done during the day and not earlier than one hour after sunset. It is never wise to fumigate when the outside weather is near the freezing point. Nor is it well to fumigate during humid nights. The best time to fumigate is when the temperature ranges from 55 to 68 degrees F.

The chemicals required for fumigation are either sodium cyanid (NaCN) or potassium cyanid (KCN), sulphuric acid (H2SO4), and water (H20). Sodium cyanid is preferred. It should be free from chlorin and contain not less than 51 per cent of cyanogen. Cyanid is a violent poison. It should be stored in airtight cans and carefully labelled "Violent Poison." Commercial "sulphuric acid of 66 degrees Baumé or 1.84 specific gravity will answer the purpose. Upon referring to Tables 20 and 21 we see at a glance the amount of cyanid per each 1,000 cubic feet necessary to kill the particular insect and the amount each plant can stand. For example, if one-half ounce of cyanid as indicated in Table 21 is used per 1,000 cubic feet of space, and if the greenhouse to be fumigated contains 15,000 cubic feet, then multiply the number of cubic feet contained in the greenhouse by the amount of cyanid to be used per 1,000 cubic feet, 1,500 times 1/2 equals 7.5' ounces cyanid. If there is the least doubt as to the amount of gas the plant can stand without injury the initial dose should not exceed one-fourth ounce for each 1,000 cubic feet of house space.

Mixing the Chemicals. The chemicals should be mixed as follows : For each ounce of sodium cyanid use 1% fluid ounces of sulphuric acid and 2 fluid ounces of water. The water is first placed in the generators, then the sulphuric acid. The cyanid is then dropped into the warm acid, and the manipula-tor must at once leave the greenhouse and shut the door tightly behind him.

Short exposure with a greater strength of gas is more desirable than overnight exposure with a weaker gas. Better results are obtained when the fumigation lasts about two hours. After fumigation the house is opened at the top or at the side doors to allow the escape of the gas. During cold weather the ventilators should be opened for a short time only at several intervals.

It is not advisable to fumigate if the house temperature is below 52 degrees F. or above 70 degrees F. As already stated, fumigation should never be done while the sun shines. Hydrocyanic acid gas is soluble in water. It is, therefore, evident that neither the plants nor the benches should be wetted before fumigation.


The following don'ts laid down by Sasscer and Borden will be of value to the greenhouse man :

"Do not guess the amount of chemicals to be employed or the cubic contents of the house.

"Do not fumigate plants in a greenhouse in daylight.

"Do not fumigate when the temperature in the greenhouse is below $2 degrees or above 70 degrees F.

"Do not leave the chemicals within reach of those unacquainted with their poisonous nature. Always have them properly labeled.

"Do not handle the chemicals any more than is absolutely necessary. It is well to have a pair of old gloves for this, and to use them for no other purpose. Always wash the hands thoroughly after handling the chemicals whether gloves have been used or not.

"Do not allow the acid to splash or drop on the clothing or skin.

"Do not stay in the greenhouse any longer than is necessary to place the cyanid in the jars, and never enter a greenhouse charged with the gas until it has been thoroughly aired.

"Do not fail to post danger signs at all entrances before setting off the charge, and to see that the greenhouse is closed tightly.

"Do not attempt to fumigate a large greenhouse alone.

"Do not fumigate a greenhouse adjoining a dwelling without notifying the occupants before fumigation.

"Do not pour water on the acid; pour acid on the water.

"Do not become negligent in any of the precautions; to do so may cause serious results."


Kerosene Emulsion. The formula for kerosene emulsion is as follows :

Kerosene 2 gallons
water 1 gallon
Hard soap 1/2 pound

Dissolve the soap in the water by heating. Re-move the soapy water from the fire and add the kerosene and the liquid, violently mixing until a stable milky emulsion is formed. This emulsion should afterwards be mixed with water without the kerosene separating from it. As a spray for soft-bodied sucking insects, the above stock solution should be diluted ten to twenty times. Kerosene emulsion is not extensively used in greenhouses.

Linseed Oil Emulsion. Linseed oil emulsion has been recommended by Vinal for red spider on green-house cucumbers. The emulsion according to Vinal is made as follows :

(a) The necessary articles for preparation are:

1. Bucket pump.

2. Container or mixing tank. This should hold at least eight or nine gallons. For this purpose a small washtub is perhaps the most available. Pails may be used, provided the materials are mixed proportionally.

3. Ivory soap.

4. Raw linseed oil.

5. Hot water.

(b) The following proportions of materials for 100 gallons of spray are used:

1. Five gallons of hot, water.

2. One and one-half pounds of Ivory soap. (Six 5-cent cakes or three 10-cent cakes.)

3. One gallon of raw linseed oil.

(c) Steps in the preparation of stock solution follow:

1. Put the required amount of hot water in the container.

2. Shave the Ivory soap into this and stir until completely dissolved.

3. If at this time the temperature of the soap solution is too hot for the hand to bear, dilute with one gallon of cold water and let it stand until about body temperature or lukewarm. The cooling of this solution is necessary in order to prepare a permanent emulsion;. otherwise the oil will come to the surface on standing (see No. 6), It also prevents the chemical and physical killing properties of the linseed oil from being changed by heat.

4. Add slowly, while stirring vigorously, one gallon of linseed oil.

5. Completely emulsify by using the bucket pump and turning the stream back into the container again, keeping the nozzle below the surface of liquid. Five minutes' vigorous pumping should completely emulsify this solution.

6. Set aside for a few minutes while preparing spray tank in order to see that oil does not come to the surface.

(d) The following are directions for the preparation of spray tanks and spray:

1. Fill the loo-gallon spray tank about one-half full of water. If the water used is too cold, upon the addition of the stock solution the soap will solidify into small lumps, thus spoiling the emulsion. This may occur early in the spring, when the water is very cold, but later in the season ordinary top water may be used without danger of the soap solidifying on the addition of the stock solution.

2. Add stock solution made above. (See (c) 1, 2, 3, 4, 5, 6.)

3. Agitate. (If lumping occurs, the addition of a few pails of hot water will remedy this.)

4. Fill the ioo-gallon spray tank.


These poisons are used to control fungous diseases. As previously stated, some parasitic fungi live on the surface of the leaves and stems and are therefore easily controlled. An example of this is the powdery mildew. Other fungi, and these are in the larger majority, are those which live parasitically within the tissue of the host, and therefore cannot be reached by any spray. Fungicides are helpful only in pre-venting entrance of the parasite in the host. They are as ineffective in controlling insect pests as are insecticides in controlling fungous diseases.

The author has often referred in this work to the term "standard fungicide." In reality there is no one standard fungicide. The term as employed here, however, refers to any effective fungicide that is best adapted to each particular case. For instance, Bordeaux mixture may be termed "standard" if used to spray greenhouse muskmelons or cucumbers. In this case the staining of the Bordeaux would not injure the marketable product since the Bordeaux film may be readily washed or wiped off. However, Bordeaux mixture could not be termed standard for spraying roses in bloom. At that time the Bordeaux stain may injure the market value of the bloom more than would the disease that we wish to control. In this case, therefore, ammoniacal cop-per carbonate or some other colorless fungicide may be termed standard.

BORDEAUX MIXTURES. The strength used for tender plants is three pounds of copper sulphate—also known as blue stone, six pounds of lime, and fifty gallons of water. The easiest way to prepare it is to dissolve the blue stone thoroughly in twenty-five gallons of water. The best quality of unslaked lime should be used and slaked in a little water, care being taken, however, not to flood it while slaking, nor to let it become too dry. When the slaking is completed, enough water is added to make twenty-five gallons. The lime water and the blue stone solution are then mixed, pouring in first one part of lime water, then another part of the blue stone; the mixture is then strained and used at once.

For crops with less delicate foliage, the standard Bordeaux mixture is 4-4-50; that is, four pounds

Methods of Control 403 copper sulphate, four pounds unslaked lime, and fifty gallons of water. With greenhouse crops it is not always necessary to prepare stock solutions. Only enough for immediate use is prepared at one time.

In preparing Bordeaux the following points should be kept in mind :

(1) Copper sulphate solutions must be kept only in vessels of wood, fiber, brass, bronze, or copper. They must not be kept in iron or tin vessels, as they corrode them.

(2) It is necessary to use fresh lime, as air-slaked lime is useless.

(3) Bordeaux mixture can be used only when freshly mixed. If allowed to stand twelve hours after making, it loses all fungicidal value.

(4) Bordeaux mixture or lime should never be strained through burlap. The lint of the burlap is likely to work up into the nozzles and clog them.

(5) Undiluted solutions of copper sulphate or lime should never be mixed together.

(6) Bordeaux mixture should not be prepared with hot water.

Ammoniacal Copper Carbonate. The objection to the use of Bordeaux is that it stains the leaves and foliage.

To avoid staining, colorless ammoniacal copper carbonate may take the place of Bordeaux. It is prepared as follows :

Copper carbonate 5 ounces
Ammonia (26° Baumé) 3 pints
Water 50 gallons

The best results are obtained when the copper carbonate is first made into a paste with a little water. It is then dissolved by adding the ammonia, which is diluted with four quarts of water. If ` three pints of ammonia fail to dissolve all the cop-per carbonate, more may be used. Ammoniacal cop-per carbonate is only effective when used fresh. It loses its fungicidal value by standing, as the ammonia evaporates quickly.

Sulphur. Flowers of sulphur are often used to control powdery mildew or asparagus rust. It may be applied either by hand or with a duster. There are a number of other fungicides on the market which are not mentioned here. They should be thoroughly tested before they are used.

Combination Sprays. For purposes of economy, it is advisable to control both insect pests and fungous diseases at the same time. Spraying, if properly done, is effective in controlling or in keeping in check all the pests which attack greenhouse crops. In combining a fungicide with an insecticide, we may accomplish two aims in one operation. The various spray mixtures which may or may not be combined are indicated by Cooley and Swingle

Each of these preparations is mixed and applied just as if it were used alone. A combination of the ammoniacal copper carbonate with an arsenate would be unsafe, since the ammonia renders the arsenic more soluble, and hence may result in the burning of the foliage. However, it may be safely mixed with the tobacco products.

Recent investigations by Professor Safro, Entomologist to the Kentucky Tobacco Products Co., indicates that "Black Leaf 40" may be used with-out soap in combination with such spray chemicals as lime-sulphur, arsenate of lead, arsenite of zinc, and iron sulphate, for controlling sucking and chewing insects and fungous diseases. Professor Safro's work further claims that "Black Leaf 40" may be safely combined with Bordeaux, and the desired results obtained. He writes as follows : "For purposes of spraying, add to every one hundred gallons of Bordeaux three-fourths of a pint of `Black Leaf 40.' As far as safety to the foliage is concerned, much greater strength of nicotine may be added to the Bordeaux, but no additional effectiveness will be given to the mixture as an insecticide. Any nicotine solution which contains four hundredths of one per cent nicotine will be effective in controlling plant lice if the spraying is thoroughly done."


Bordeaux and Paris Green

Paris green 1/2 pound
Bordeaux mixture 50 gallons

Bordeaux and Arsenite of Soda

Arsenite of soda 1 quart
Bordeaux mixture 50 gallons

Bordeaux mixture must never be combined with kerosene emulsion, carbolic acid emulsion, and mis-cible oils.

(d) Potassium Sulphide. Like sulphur, this is a valuable fungicide for the control of the powdery mildew. The following strength is recommended:

Potassium sulphide 4 ounces
Water 10 gallons

Potassium sulphide is effective only if used immediately after it is prepared. It loses its value by being exposed for any length of time.

Stickers. It is well known that with some plants, such as cabbage, spray mixtures cannot be made to stick. The use of an adhesive added to the spray mixture will largely overcome this difficulty. An adhesive may be prepared as follows :

Resin 2 pounds
Sal soda (crystals) 1 pound
Water 1 gallon

The resin and the sal soda should be added to one gallon of water and boiled in an iron kettle for one and a half hours until clear. For plants which are hard to wet, such as cabbage or onions, the amount of the solution given above should be used for each fifty gallons of Bordeaux or ammonia-cal copper carbonate. For other plants, this amount is added to each one hundred gallons of the spray mixture.


It should be remembered that to destroy chewing insects, such as caterpillars, etc., the stomach poison must be evenly distributed all over the plant. This thorough spraying should be done as soon as the presence of the pest is suspected. Intelligent and observant growers will remember the time of appearance of the pest every year, although this date depends somewhat on the climate of each season: In destroying the green aphids, the contact poison, should be distributed as evenly as possible on the insect itself. It is, therefore, best to spray for aphids when they are actually found working on the plants. To check chewing insects and fungous pests, how-ever, the applications are made before the parasites appear. Before spraying it is necessary to have well in mind which organism is to be destroyed, and the proper ingredients to be used. To keep fungous pests in check it is necessary to have the plant covered with the fungicide all the time infection is feared or suspected. This spraying is preventive, protecting the plant from becoming infected. When the parasite has penetrated the host, spraying is of little value in saving the infected plant, although it will protect others which are as yet healthy. It is essential that the gardener be always ready to spray. Sometimes delay for even a day may prevent the attainment of positive results. The timely destruction of one insect, or of one spore, means the elimination of countless generations of these pests.

Thoroughness is as important in spraying as it is in everything else in life. Especially is this true for the control of fungous diseases.


Success in spraying often depends on the sprayer, and, especially on the nozzle. In small scale such as under greenhouse conditions, it is next to useless to invest in elaborate expensive machinery. A small bucket pump with long nozzle (fig. 81) as used by Professor Paddock of the Texas Experiment Station has given good satisfaction. The Auto Spray No. 1 is a very desirable spraying machine for indoor plants.


Since plants are endowed with life they readily respond to intelligent hygienic treatment. This is especially true with indoor plants, which at best are growing under abnormal conditions. Every effort should, therefore, be made to create indoors as nearly normal conditions as possible. The effect of proper sunlight, heat, moisture and ventilation has already been discussed under pages 53-85. Cleanliness is also an important consideration. The walks, interior walls and glass should be kept as clean as possible. Old and used pots should be scrubbed and washed at least once a year. Dead or infected plants should never find their way on the manure pile. Such manure is bound to find its way back and will contaminate the soil in benches and involve later extra expenses of soil sterilization. Insects and diseases should never be allowed to get a strong foothold. It is easy enough to destroy a few aphids for instance, but it becomes a matter of greater difficulty to handle a greenhouse which has become thoroughly infested. No definite rules can be laid down, but every greenhouse man must study his crops and his conditions in order to succeed in keeping his plants in the best condition of health.


With forced crops, perhaps more than with any others under glass, the success of cuttings is largely dependent upon proper selection. This is true for instance with carnations, roses, violets or chrysanthemums. It would scarcely seem possible that the nature of the cutting could materially influence the future plant. This, however, is a fact which has been aptly mentioned by Galloway. It must be remembered that plants, like animals, are influenced by inheritance as well as by environment. In selecting cuttings the object should be to procure those parts of the plant which will transmit with the greatest vigor the ability to flower or to fruit as the case may be. Experienced growers will appreciate this. Frequently in starting with two-rooted cuttings from the same plant, grown under the same conditions, dissimilar plants are produced. The one may be vigorous, blooming freely, while the other may be dwarfed and sickly, and produce no flowers, or merely a few of an undesirable type. In the selection of cuttings, appearance alone should not constitute the main guide. A cutting may appear vigorous, yet be immature or too old. Violet cuttings made from old wood will generally pro-duce inferior plants, which will run out within a year or be carried off by disease. Violet cuttings made from soft, immature wood, will result in weak, spindly growth and in plants susceptible to damping off. On the other hand, a cutting may be made of the proper material, yet if it is too short it will also be useless. A violet cutting that is too short will not have sufficient anchorage. Each time a flower is pulled it will roll around or its roots will break (fig. 82, a to c.).

Not only is care necessary in the selection of cuttings, but it is also necessary to provide proper conditions for growth. A setback at this time may result in disappointing returns later. Heat, air, light and water should be carefully attended to in dealing with soft-wooded cuttings.

Home | More Articles | Email: