Diseases Of Greenhouse Crops
ASPARAGUS (Asparagus Officinalis)
Cultural Considerations. Asparagus plants lend themselves admirably to forcing. It is now grown for commercial purposes on a fairly large scale indoors. Light is not essential for this crop. The beds may be in total darkness, although a diffused light is preferred. Any variety which produces large shoots is desirable for forcing. However, the variety Reading Giant has been developed for its resistance to rust, and therefore should be given preference. Forced asparagus may be grown in any soil, even sand or coal ashes, provided it contains plenty of organic matter. At the beginning of the forcing process, the temperature should not run higher than 45 to 50 degrees F. for at least one week. As soon as strong shoots are made, a temperature of 65 to 70 degrees is desired. In order to obtain high yields, profuse watering is necessary.
DISEASES OF ASPARAGUS
Greenhouse asparagus seem to be subject to but few diseases.
DAMPING OFF, see Rhizoctonia, p. 20.
Rust (Puccinia aspargi D.C.) seems to be of no importance as a disease of forced asparagus.
BEAN (Phaseolus Vulgaris)
Cultural Considerations. Beans are not to be grown as a fall or winter crop. They are produced more easily and more profitably as a spring crop. In this case they follow well a winter crop of lettuce. The night temperature should not go below 6o degrees F. The best soil for bean culture is a rich light sandy loam. The soil should never be al-lowed to become damp and cold for any length of time. It must not be allowed to become packed and soggy through overwatering. As a safe guide to success, the crop should never receive a check in its growth in any period of its development. Of the varieties adapted to forcing may be mentioned, Black Valentine, Long Yellow Six Weeks, Kentucky Wonder.
DISEASES OF THE BEAN
Forced beans may be attacked by several important diseases.
Caused by Pseudomonas phaseoli Ew. Sm.
Symptoms. If the soil is too wet during planting time, the seed may rot in the ground and never germinate. At other times the roots of the young seedlings may decay and the result will be a very poor and uneven stand. Under drier conditions a better germination is obtained. The disease also works on the older plants, forming irregular spots. When their root system is attacked, affected plants become yellowed and wilted by daytime, but slowly revive at night. Should the air become muggy by over-watering and high temperatures, infected plants appear as though they have been drenched with hot grease, the leaves having a burned appearance. The injured plants then seem to make a desperate attempt to produce new foliage, which in turn becomes affected; the pods cease filling, and ripening is very uneven.
In carefully examining diseased seed, it is found to be yellowed and shriveled; or, in light cases of attack, covered with irregular yellow blotches. On the leaves, the trouble appears as watersoaked spots which later become amber colored. The cankers on the stems somewhat resemble the canker produced by Colletotrichum lindemuthianum.
The Organism. Pseudomonas phaseoli Ew. Sm. is a short rod, rounded at both ends, and motile by means of polar flagella (fig. 15, d and e.). It liquefies gelatin slowly, coagulates milk, and produces no gas. For methods of control, see Anthracnose, p. 112.
Caused by Sclerotinia libertiana Fckl.
Sclerotinia rot is a disease which attacks snap beans. During a period of several hot humid days the disease may suddenly break out in great se-verity. Usually withering and decaying of stems and pods where the plants are thickest is the first symptom that attracts attention. On closely examining infected stems and pods, we find that they are watersoaked, and overrun by a white mycelial. growth on which appear numerous hard, black sclerotia. In the field, the Black Valentine snap bean seems to be more resistant to rot. For a description of the causal fungus and methods of control, see lettuce drop, p. 150.
Caused by Erysiphe polygoni D. C.
Symptoms. Powdery mildew is a serious bean disease. It is characterized by white, mealy patches on the surface of the leaves and stems. The foliage soon turns yellow and dry. Powdery mildew may be controlled by dusting the plants with flowers of sulphur. Care in the proper amount of watering and ventilation will also help to keep it in check.
Symptoms. Anthracnose is so characteristic that it cannot be mistaken for any other disease, except perhaps the blight. In light attacks, the seeds are covered with sunken brown to black specks. These are especially evident on the white seeded varieties. In severe attacks, the seeds are covered with deep sunken black spots (fig. 15, c.) which are rifted in the center. On the leaves the disease attacks the veins, which become blackened and somewhat shrunken. Frequently it attacks the petioles, especially at the point of leaf attachment. In this case, the foliage drops off, leaving the bare petioles or stems. Anthracnose on the pods begins as small, circular, pin-point, dark red spots which enlarge, and later elongate into maroon colored pits, cracks, or cankers (fig. 15, a.). On young seedlings the stem rots off a short distance above ground.
The Organism. Spores are formed on the spots or cankers of all parts affected (fig. 15, b.). These are imbedded in a gelatinous substance and may be-come loosened only by water splashing upon it. It is at this stage that the disease becomes serious, for it is then spread from plant to plant. When the spores are lodged on a new bean plant or on a new part of the same plant, infection takes place through the penetration of the tube of the germinated spores.
Control. Spraying has not given satisfactory results. The best control is to plant clean seed selected from clean pods. The latter, before shelling, may be dipped for ten minutes in a solution of one part of corrosive sublimate to a thousand of water. The treated pods are then dried in the sun, shelled, and the seed put away in dry Mason jars until planting time. Should weevils threaten these seeds, they may be fumigated with carbon bisulphide. Under no circumstances should infected plants be syringed. When this is done the spores of the fungus are scattered broadcast. Recently Burkholder * has succeeded in developing a resistant bean by crossing the Well's Kindney Bean with the White Marrow variety.
Root Rot, see Rhizoctonia, p. 20.
Root KNOT, see Nematode, p. 28.
BEET (Beta vulgaris)
Cultural Considerations. Beets are not grown very extensively in the greenhouse. They are, however, raised on a small scale for greens or for the roots. It is often used as a companion crop with tomatoes. The Egyptian or any other early variety is preferable. The cultural requirements of the beet are the same as those of the lettuce, see p. 145. However, beets will grow more rapidly under higher temperatures than lettuce.
DISEASES OF THE BEET
Indoor beets are subject to less diseases than those grown out of doors. The following are the more important ones :
Caused by Pseudomonas tumefaciens Sm. and Town.
Crown gall is a very important disease because of its cosmopolitan nature, for it is widely prevalent and attacks a large number of hosts.
Symptoms. The disease does not usually manifest itself until the roots are nearly half grown. Abnormal outgrowths or galls (fig. 16, b.) appear which vary in size from that of a garden pea to nearly two inches in diameter, according to the se-verity of the attack. The galls are usually attached to the beet by a narrow string. In light cases of infection there may be but one gall on the root; in severe cases, however, the roots may be covered with numerous galls.
The Organism. The cause of crown gall is a bacterial organism, Pseudomonas tumefaciens Sm. and Town. It is a short rod, multiplying by fission, and moves about by means of polar flagella. On agar or gelatin it forms small round white colonies. Under unfavorable conditions it readily develops in-volution forms; the organism is short lived in pure culture. P. tumefaciens lives over in the soil from year to year.
Control. The disease may be introduced with infected soil. Sterilizing the soil with steam or formaldehyde (see pp. 32-43) is recommended.
Caused by Actinomyces chromogenus Gasp. Scab on beets is the same as the scab of the Irish potato, the radish, and the carrot.
Symptoms. The symptoms of the disease on beets do not differ from those of the Irish potato. Occasionally, the scabs which arise before the beet is full grown disappear entirely, leaving merely a small scar. This is somewhat sunken and has a definite outline. In normal cases of infection, the scabby areas on the beet are rough; while the corky layer of the spots decidedly bulge out. Immediately below them, the tissue is a discolored reddish brown.
The Organism. The cause of beet scab is the same as that of the scab of the white potato. The parasite is a soil organism, and thrives best under alkaline conditions.
Control. The disease is introduced with infected soil, or with the compost. Care should be taken that no infected potato peelings find their way to the manure pile. Soil sterilization with steam, or formaldehyde (see pp. 32-43) is recommended.
DAMPING OFF AND ROOT ROT
Caused by Pythium de Baryanum Hess.
Symptoms. Damping off very commonly occurs just as the seedlings emerge from the ground. These topple over and die in the characteristic way so familiar to truckers. The greatest damage follows from overwaterings, when a hard crust is formed on the surface, a condition which prevents the seedlings from emerging normally. On old and mature roots, Pythium de Baryanum may cause a rot. A peculiarity of this disease is that it seldom starts at the top of the crown. The latter appears to be perfectly healthy, although the leaves turn yellow, indicating a diseased condition further down. Rotted roots are found to be overrun by a varied flora, although Pythium is the original cause of the trouble. For a further description of the organism.
Control. The methods of controlling this disease are the same as those for lettuce drop, see p. 151.
Caused by Peronospora schachtii Fckl.
This disease is of little economic importance in the United States. The trouble, however, is prevalent in Europe. The mildew attacks the young seedlings in grayish patches on the under side of the foliage. On older plants, the mycelium of the causative fungus works downwards into the root, causing it to rot.
Caused by Sclerotinia libertiana Fckl.
Drop, which attacks young seedlings of the beet, but not the older plants, is not very different from a similar trouble on lettuce. The high temperature of the soil soon after making the hot bed, is important in favoring the disease. Sterilizing the soil with formaldehyde, careful regulation of temperature, and watering are methods to be observed in the control of the trouble.
Caused by Cercospora beticola Sacc.
There is perhaps no beet disease that is of greater economic importance than leaf spot. The trouble is well known to truckers and it seems to be found wherever beets thrive.
Symptoms. The disease first makes its appearance on the leaves as tiny circular whitish spots. These gradually increase in size and assume a brownish color. The spots soon multiply and involve the entire leaf area (fig. 16, c), which becomes dry and brittle. Leaf spot attacks the outer and older leaves first. As the inner foliage advances in age, they too become infected in turn. Serious though the disease may appear, it never kills the plant. The result, however, is noticeable on the roots, which are undersized and elongated instead of round. Leaf spot generally appears in overwatered and poorly ventilated houses. The disease increases in severity as the- plants are weakened by heat.
The Organism. The fungus, Cercospora beticola, Sacc., like most fungi, is composed of a vegetative part of mycelium and of spores. The latter are microscopic in size, somewhat needle-shaped, and divided by means of a cross wall into cells numbering from two to seven (fig. 16, d.). Each of these cells may germinate by sending out a thread-like tube, which penetrates the leaves through the stomata. The spores are borne on a cluster of stalks or conidiophores, at the base of which is formed a small stroma. The temperature and relative humidity of the air influence the production and infection of conidia. Conidia are generally formed on the lower surface of the leaves, no doubt because these are subject to a higher humidity.
Control. Infected material should be destroyed by fire. Spraying with Bordeaux mixture 4-4-50 is also recommended.
Caused by Rhizoctonia solani Kuhn.
Symptoms. This disease produces a damping off of the young seedlings, and on older plants a rotting of the crown. Upon pulling out an infected plant, we find that the outer leaves are dead and dry, while the inner ones are somewhat curled. The roots of such plants invariably are rotted at the crown, the rot generally working inwards to a considerable ex-tent. The peculiarity of this disease is that the lower half of the root is generally sound. Frequently, the rotted crowns are also found to be cracked at various places. Beets thus affected are worthless for the market. For a description of the fungus seep.
Control. There are no methods of control known. The factors which favor the trouble are poor drainage, an excess of soil moisture, and lack of sufficient ventilation. Every step taken to overcome these will in a degree help to control the rot. Soil sterilization is also recommended.
ROOT KNOT, see NEMATODE, p. 28 (fig. 16, a.).
The organisms Pseudomonas teutlium Met., Ps. beticola Ew. Sm., Urophlyctis leproides (P. Mag.) Trab., Cystopus bliti (Biv.) Lev., Uromyces bette. Kuhn, and Phoma beta Fr. seem to attack beets out of doors only.
CARROT (Daucus carota)
Cultural Considerations. Carrots are forced in about the same way as the radish. The soil, how-ever, should be more sandy. The variety best adapted for forcing is the early small topped Short Horn type.
DISEASES OF CARROTS
Carrots are very hardy and subject to but few diseases of consequence.
SOFT ROT, see Cauliflower, p. 126. RooT RoT, see Rhizoctonia, p. 20.
CAULIFLOWER (Brassica oleracea var. botrytis)'
Cultural Considerations. Cauliflower is not so extensively forced because large amounts of it are shipped from California during the forcing season. Nevertheless, indoor cauliflower is far superior in quality, and with the proper advertisement the forced product should gain greater recognition from the consumer. There are few varieties which lend themselves well to forcing. The Snowball and the Erfurt are preferred by most growers. The soil should not be too heavy, although the plants are heavy feeders. The compost should contain a fair amount of well rotted manure. In addition, a well balanced fertilizer may be added at the rate of 1,000 pounds per acre. Lime should also be added to the beds at least once every two years. This will sweeten the soil in case it has a tendency to sour. Cauliflower requires an abundance of water. Lack of sufficient water may check the growth, an effect that will result in small or no heads. On the other hand, overwatering may produce an excess of foliage at the expense of head development. In warm weather, the plants and the walks should be syringed in order to keep the atmosphere moist. The best night temperature is about 50 to 55 degrees F. and the day temperature from 65 to 70 degrees. Plenty of ventilation should be provided; but drafts should be avoided.