Hydrocyanic acid gas, properly used, is also an excellent remedy for aleyrodes, aphides, "mealy-bug," and other soft-bodied insects which are sometimes troublesome on greenhouse tomatoes.
For a complete account of the methods of making and handling hydrocyanic acid gas, see Professor Johnson's book entitled "Fumigation Methods," published by Orange Judd Company, of New York. Sent postpaid for $1.--[AUTHOR.
CHAPTER XX
Tomato Diseases
By W. A. ORTON U. S. Department of Agriculture
DISEASES NOT CAUSED BY FUNGI OR INSECTS
The health of tomato plants is to a large extent dependent on the conditions under which they are being grown. The character and physical condition of the soil, the supply of water and plant food, the temperature and amount of sunlight, are all factors of the greatest importance in the growth and development of the crop. When there are variations from the normal in the case of any of these the plant adapts itself to the change as far as possible, but its functions may be so disturbed as to result in ill health or disease. It is in many cases difficult to draw a line between a natural re-action of the plant to its environment and a state of disease. For example, the trouble described in the next paragraph seems to fall into the first class.
=Shedding of blossoms.=--The tomato is very liable to drop its buds and blossoms and in some instances partial or total crop failures have resulted. The principal causes are an over-rapid growth, due in many cases to an excess of nitrogenous fertilizers, unfavorable weather conditions, especially cold winds, continued rainy or moist weather, which hinders pollination, lack of sunlight, or extremely hot weather.
Such shedding can be partially controlled by pruning away the lateral branches as soon as formed and topping the plants after the third cluster of fruit has set, and by a reduction in the use of nitrogenous fertilizers. A failure to set fruit in the greenhouse is often due to lack of pollination, which must be remedied by hand pollination.
=Cracking of the fruit.=--The formation of cracks or fissures in the nearly mature fruit is due to variations in the water supply and other conditions affecting growth at this stage. If after the development of the outer portion of the fruit has been checked by drought there follows a period of abundant water supply and rapid growth, the fruit expands more rapidly than its epidermis and the latter is ruptured. Some varieties of tomatoes are much less subject to this trouble than others and should be given preference on this account. The grower, so far as lies in his power, should seek to maintain an uninterrupted growth by thorough preparation of the land, by cultivation or by mulching. If the half-grown fruits are enclosed in paper bags, cracking may be prevented, but at the risk of loss of flavor in the ripened fruit.
=Leaf curl.=--The effect of pruning is to stimulate growth and to increase the size of the leaves, the effort of the plant being to maintain a balance between roots and foliage. With rapidly growing plants, especially in the greenhouse and garden where both high manuring and pruning have been practiced, more or less curling and distortion of the leaves may result without developing into serious trouble if the grower takes the hint and modifies his methods so as to permit a more balanced growth. On the other hand, the ill effects of over-feeding and pruning may reach a point where the plant is seriously crippled.
=Edema.=--Under certain conditions plants in greenhouses or even in the open field, may absorb water through the roots faster than it can be transpired through the leaves, with the result that dropsical swellings or blisters occur on the leaves and more succulent stems. There is also a deformation of the foliage, much like the leaf-curl produced by over-feeding. This trouble, known as edema, occurs when the soil is warmer than the air, or during periods of moist, warm, cloudy weather, which checks transpiration. The grower should cease pruning, and withhold water, and in the field cultivate deeply. In the greenhouse, adequate ventilation should be given, keeping the house dry rather than moist.
=Mosaic disease.=--The tomato is occasionally subject to a trouble allied to the mosaic disease of tobacco. It is characterized by a variegation of the leaves into light and dark green areas, usually accompanied by distortion and reduction in size. In severe cases a whole field may become worthless. While the nature of this malady is not fully understood, it is known to be due to a disordered nutrition of the young leaf-cells. It can be produced by severe pruning or by mutilation of the roots in transplanting, both of which should be carefully avoided. It is more likely to occur in seedlings that have made a soft, rapid growth on account of an excess of nitrogenous fertilizer or too high temperature.
Close, clayey soils, on account of their poor physical condition, also favor the development of the disease after transplanting.
=Western blight (Yellows).=--In the North Pacific and Rocky Mountain states, serious losses are annually caused by a disease apparently unlike any eastern trouble. It is marked by a gradual yellowing of the foliage and fruit. Development is checked, the leaves curl upward and the plant dies without wilting. The nature and cause of this disease is as yet unknown. It appears to be worst on new land. Experiments that have been made indicate that in older cultivated fields thorough preparation of the soil, manuring and cultivation, combined with care in transplanting to avoid injuring the roots and checking growth, will greatly restrict the spread of this blight.
DISEASES CAUSED BY PARASITES
There are several fungous parasites of tomatoes, which, for the readers convenience, may be briefly mentioned and the treatment of all discussed together. The first three are indeed somewhat difficult to tell apart without a microscope, as they produce a similar effect on the leaves and all yield to the same treatment--thorough spraying with Bordeaux mixture.
=Leaf spot= (_Septoria lycopersici_ Speg.) has been widely prevalent and injurious during recent years. It produces small, roundish dark-brown spots on leaves and stems. The lower leaves are attacked first and gradually curl up, die and fall off. The vitality of the plant is reduced and it is only kept alive by the young leaves formed at the top.
The fungus that causes early blight of potatoes (_Alternaria solani_ (E.
& M.) J. & G.) occurs on tomatoes also, sometimes doing much injury. The spots formed are at first small and black, later enlarging and exhibiting fine concentric rings.
A somewhat similar leaf-blight results from a species of _Cylindrosporium_, and other fungi may occur on diseased leaves.
=Leaf mold= (_Cladosporium fulvum_ Cke.) is quite distinct from the foregoing in appearance. It does not cause such distinct spots but occurs in greenish brown, velvety patches of irregular outline on the under side of the leaves. The lower leaves are first attacked, and as the disease progresses they turn yellow and drop off. This is the principal fungous enemy of greenhouse tomatoes, but also does injury in gardens, particularly in Florida and the Gulf region. It is readily controlled by spraying. In the greenhouse care should be taken to ventilate well, without, however, allowing cold drafts to strike the plants.
=Downy mildew= (_Phytopthora infestans_ DeBy.), the cause of the late blight of potatoes, will attack tomatoes during cool and very moist weather, which greatly favors its development. Such outbreaks sometimes occur to a limited extent in New England and serious losses are reported on the winter crop in southern California, but the disease has never been troublesome in other sections of the country, as it cannot develop in dry or hot weather. It affects the tomato as it does the potato, forming on the leaves dark, discolored spots, which spread rapidly under favorable conditions, killing the foliage in a few days. The fruit is also attacked and becomes covered with the mildew-like spore-bearing threads of the fungus. Bordeaux mixture properly applied is an efficient preventive.
=Spraying tomatoes.=--It should be the invariable practice of the tomato grower to spray with Bordeaux mixture to prevent injury from any of these leaf-blights. This should be done while the plants are still healthy, as if put off until the disease appears the battle is half lost. Make the first application to the young plants in the seed-bed a few days before transplanting. Spray again within a week after the plants are set in the field, and repeat at intervals of ten days or two weeks until the fruit is full grown. Success in spraying depends mainly on the thoroughness of the work. The aim should be to cover every leaf with a fine mist. Do not drench the foliage but pass to the next plant before the drops run together and off the leaf. Use a nozzle that gives a fine spray and maintain a high pressure at the pump.
_Preparation of Bordeaux mixture._--Formula: Copper sulphate (bluestone), 5 pounds; lime, 5 pounds; and water, 50 gallons. The copper sulphate may be either in crystals or pulverized. Dissolve by suspending the required amount in a coarse sack near the top of the water a few hours before it will be needed. The lime must be fresh stone lime of good quality. Slake thoroughly by the addition of small quantities of water at a time as needed, stirring until all small lumps are slaked.
Strain both the lime milk and the copper sulphate or bluestone solution through a brass strainer of 18 meshes per inch and dilute each with half the water before mixing together. Do not use Bordeaux left over from the previous day. An old mixture or one made from the concentrated solutions has a poor physical condition. It settles more quickly, tends to clog the nozzle and does not adhere so well to the foliage. Failure to use the strainer results in endless trouble in the field from clogged nozzles.
[Illustration: FIG. 42--PROPER WAY TO MAKE BORDEAUX (From W. G.
Johnson)]
When much spraying is to be done it is more convenient to keep the bluestone and lime in separate permanent stock solutions, as shown in Fig. 42, containing 2 pounds to the gallon of their respective ingredients. These will keep indefinitely, if the water evaporated is replaced, and may be used from as needed.
_Spraying apparatus._--Tomato growers having only a small area to spray may use one of the numerous forms of hand-pumps or bucket sprayers now on the market. For larger fields it will be necessary to employ a barrel sprayer. This consists of a hand-pump mounted in a barrel or tank and equipped with two leads of 3/8 inch hose 25 feet long, each with a four-foot, extension made from 1/4 inch gas pipe, and a double Vermorel nozzle. The barrel should be carried in an ordinary farm wagon. Three men do the work. One is expected to drive and pump, while the other two manipulate the nozzles. The outfit is stopped while the plants within reach are sprayed, then driven forward about 30 feet and stopped again.
On an average in actual field practice 3 to 4 acres a day can be sprayed in this way, applying 100 to 200 gallons of Bordeaux per acre. To keep the long hose off the plants two poles about 10 feet long may be pivoted to the bed of the wagon so as to swing at an angle over the wheel and carry the hose. The pump for this outfit should be of good capacity, with brass valves. A "Y" shut-off discharge connection on the pump is a convenience for stopping the spray at any time. The most satisfactory nozzles are those of the Vermorel type. It is cheapest in the long run to buy the best grades of pumps on the market. This outfit is excellently adapted for spraying small fields of potatoes and for general orchard work, and is invaluable on the average farm.
=Phytoptosis.=--This disease is known to occur only in Florida, where it is sometimes common enough to require remedial treatment. The affected portions of the foliage are more or less distorted and covered with an ashy white fuzz. The general vigor and fruitfulness of the plants are greatly reduced. The name applied to this trouble denotes its cause, an extremely small mite (_Phytoptus calacladophora_ Nal.), which by its presence on the leaves or stems so irritates them as to result in the abundant development of modified plant hairs, which shelter the mites and form the fuzzy covering characteristic of the disease. A remedy for phytoptosis is available in the sulphur compounds. The following one is particularly recommended by Prof. P. H. Rolfs, to whom our knowledge of the disease is due:
_Preparation of sulphur spray._--Place 30 pounds of flowers of sulphur in a wooden tub large enough to hold 25 gallons. Wet the sulphur with 3 gallons of water, stir it to form a paste. Then add 20 pounds of 98 per cent. caustic soda (28 pounds should be used if the caustic soda is 70 per cent.) and mix it with the sulphur paste. In a few minutes it becomes very hot, turns brown, and becomes a liquid. Stir thoroughly and add enough water to make 20 gallons. Pour off from the sediment and keep the liquid as a stock solution in a tight barrel or keg. Of this solution use 4 quarts to 50 gallons of water. Apply with a spray pump whenever the disease appears, and repeat if required by its later reappearance. The use of dry sulphur is also recommended.
DISEASES OF THE FRUIT
[Illustration: FIG. 43--POINT-ROT DISEASE OF THE TOMATO (Redrawn from N. Y. Expr. Sta. No. 125)]
=Point-rot.=--This trouble, called also "blossom-end rot," and "black-rot," occurs on the green fruit at various stages of development, as shown in Fig. 43. It begins at the blossom end as a sunken brown spot, which gradually enlarges until the fruit is rendered worthless.
The decayed spot is often covered in its later stages by a dense black fungous growth (_Alternaria fasciculata_ (C. & E.) J. & G. syn.
_Macrosporium tomato_ Cke.), formerly thought to be the cause of the rot, but now known to be merely a saprophyte. Point-rot sometimes occurs in greenhouses, but is more common in field culture. It is one of the most destructive diseases of the tomato, but its nature is not fully worked out, and a uniformly successful treatment is unknown. It has been thought to be due to bacterial invasion, but complete demonstrations of that fact have not yet been published. The physiological conditions of the plant appear to be important. The disease is worst in dry weather and light soils, where the moisture supply is insufficient, and irrigation is beneficial in such cases. Spraying does not control point-rot so far as present evidence goes.
=Anthracnose--ripe-rot=--(_Colletotrichum phomoides_ (Sacc.) Chest.), is distinguished from the point-rot by the fact that it occurs mainly on ripe or nearly ripe fruits, producing a soft and rapid decay. Widespread losses from this cause are not common, but when a field becomes infected a considerable proportion of the crop within a limited area may be destroyed if humid or rainy weather prevails. Preventive measures only can be employed. These should consist in collecting and destroying diseased fruit and in staking and trimming the vines to admit light and air to dry out the foliage. Bordeaux mixture applied after the development of the disease would be of doubtful efficiency and would be objectionable on account of the sediment left on the ripe fruit.
DISEASES OF THE ROOT OR STEM
=Damping off.=--Young plants in seed-beds often perish suddenly from a rot of the stem at the surface of the ground. This occurs as a rule in dull, cloudy weather among plants kept at too high a temperature, crowded too closely in the beds or not sufficiently ventilated. Several kinds of fungi are capable of causing damping off, under such conditions.
_Preventive measures_ are of the first importance. Since old soil is often full of fungous spores left by previous crops, it is the wisest plan to use sterilized soil for the seed-bed. When the young plants are growing, constant watchfulness is required to avoid conditions that will weaken the seedlings and favor the damping off fungi.
_Watering and ventilation_ are the two points that require especial skill. Watering should be done at midday, to allow the beds to drain before night, and only enough water for the thorough moistening of the soil should be applied. Ventilation should be given every warm day as the temperature and sunshine will permit, but the plants must be protected from rain and cold winds. Work the surface of the soil to permit aeration and do not crowd the plants too closely in the beds. If damping off develops something can be done to check it by scattering a layer of dry, warm sand over the surface, and by spraying the bed thoroughly with weak Bordeaux or by applying dry sulphur and air-slaked lime.
=Bacterial wilt= (_Bacterium solanacearum_ Erw. Sm.).--This disease, which also attacks potatoes and eggplants and some related weeds, is one of the most serious enemies of the tomato. It is known to occur from Connecticut southward to Florida and westward to Colorado, but is most prevalent in the Gulf States, where it has greatly discouraged many growers.
_Its most prominent symptoms_ are the wilting of the foliage and a browning of the wood inside the recently wilted stems. An affected plant wilts first at the top, or a single branch wilts, but later the entire plant yellows, wilts and dies. Young plants wilt more suddenly and dry up. The disease progresses more rapidly in plants that have made a succulent, luxurious growth, while those with hard, woody stems resist it somewhat.
_The disease is due_ to the invasion of bacteria, which enter the leaves through the aid of leaf-eating insects, or through the roots. They plug the water-carrying vessels of the stem, shutting off the water and food supply of the plant. If the stem of a plant freshly wilted from this disease be severed, the bacteria will ooze out in dirty white drops on the cut surface.
=Remedial measures= entirely satisfactory for the control of bacterial wilt have not yet been worked out. The best methods to adopt at present are the following:
(1) _Rotation of crops._--The field evidence is that this disease is in many cases localized in old gardens or in definite spots in the field.
It appears also that the infection left by a diseased crop can remain in the soil for some time. It is therefore advised that tomato growers should always practice a rotation of crops, whether any disease has appeared or not, and that in case bacterial wilt develops they should not plant that land in tomatoes, potatoes, or eggplants for three or four years. The length of rotation necessary to free the soil is not known, but will have to be worked out by the individual grower.
(2) _Destruction of diseased plants._--The bacteria causing wilt not only spread through the soil but are carried by insects from freshly wilted to healthy plants. Diseased plants thus become dangerous sources of infection, and it is evident that all such should be pulled out and burned. This is particularly important at the beginning of the trouble when the eradication of a few wilting plants may save the remainder.
(3) _Control of insects._--To lessen the danger from spread of wilt by insects, the measures advised in the next chapter for the control of leaf-eating insects should be adopted. In this connection it should be mentioned that the use of Bordeaux mixture for leaf blights, as previously recommended, has an additional value in that the coating on the leaves is distasteful to insects and helps to keep them away.