Greenhouse - The Sweet Pea
( Originally Published 1920 )
Cultural Considerations. The best position for a sweet pea house (fig. 69, a) is east and west with a full southern exposure. The sides of the house should be considerably higher than for other crops. Sweet peas require an abundance of water during the growing season. Frequent syringing of the foliage is also necessary to keep the red spider in check. In ventilating care should be taken not to admit cold drafts; nor should the plants ever be chilled. Exposure to drafts or sudden falls in temperature will predispose the plants to bud drop and to powdery mildew.
DISEASES OF THE SWEET PEA
Indoor sweet peas are subject to numerous diseases. These, if allowed to spread, will deprive the grower of his year's labor and profits.
These troubles are usually induced by improper conditions of the soil. Root burn, for instance, may be induced by the excessive use of wood ashes applied with the manure. It is not uncommon for growers to use wood ashes at the rate of 1,500 lbs. to 4,500 square feet of bed space. This would be equivalent to nearly seven and one-half tons per acre. Under such conditions the roots actually bum up because of the strong alkalinity of the soil. More-over, hard-wood ashes contain about 30% caustic lime and from 5 to 12% potash. Both of these elements in excess in soil render it too alkaline for plant growth. To remedy this trouble, acid phosphate is used, followed by a good drenching with water. This will help to neutralize the alkalinity and to restore the balanced ration.
Symptoms. The symptoms of malnutrition of sweet peas are identical with those of greenhouse cucumbers (see p. 134).
Cause. Overfertilization may be mentioned as one of the many causes of malnutrition. An analysis of an overfed soil will readily show that the soluble salts present are in excess of what the plant requires and is able to withstand. Table 17 a by Haskins clearly shows that with the exception of nitrogen, the soluble salts were in excess of what the plant could stand. It further indicates that a large amount of horse manure was used in this particular case.
Control. It seems that the average greenhouse beds are faulty in construction. One of the main requisites is to provide good drainage. This is far more important in the greenhouse than in the open. Crops need plant food. A slight excess is desirable to force quick growth. An overdose of horse manure or chemical fertilizers will produce more harm than good. No guesswork should be permitted to take the place of accurate calculation in applying fertilizers. The surface area of the greenhouse bed may be easily expressed in terms of acreage. Sup-pose that 500 pounds of sulphate of potash is required per acre (4,800 square yards), then in the greenhouse 1.6 Ounces would be required per square yard. In this way, all fertilizers may be applied accurately. In relying on guesswork the general results are likely to be the application of an overdose of fertilizers or manure.
In greenhouses when the ill effects of overfertilization become apparent, the soil should be leached out with hot water as soon as the crop is removed. This may also be done with lukewarm water, while the crop is still growing. In either case good drain-age should always be provided in order to carry off the salts in solution. Where conditions for leaching are not favorable as may be the case with a cucumber crop, about 3 inches of fresh loam should be applied to the surface of the bed and thoroughly worked in.
As the name implies, the young flower buds at a very early age turn yellow and drop off. This drop should not be confused with the drop produced by the anthracnose disease. In the latter case, the flower develops into a normal spike but it is at-tacked soon by the fungus Glomerella rufomaculans which girdles it at the point of attachment between the flower and the peduncle. Here the flower often drops off, leaving behind the beheaded peduncle. In the former case, however, the minute young flower bud never develops, instead it turns yellow and drops off. There seems no doubt that the drop is a physiological disease and is induced by an unbalanced condition of the food elements in the soil. This may occur in a soil that has been excessively fed or in a soil that is lacking in plant food.
Bud drop may be readily remedied by the application to the soil of small quantities of muriate of potash and acid phosphate.
MOSAIC, see p. 102.
Caused by Bacillus lathyri Manns and Taub.
Streak is a very serious disease of outdoor sweet peas. Fortunately it is not known to attack greenhouse sweet peas.
Symptoms. Although not occurring indoors, the symptoms of streak are here given to enable the grower to know the disease should it ever appear in the greenhouse. Like the Bacteriosis of beans, streak makes its appearance in wet houses. On the sweet pea the disease usually appears just as the plants begin to blossom. It is manifested by light reddish-brown to dark brown spots and streaks (the older almost purple) along the stems, having their origin usually near the ground, indicating the fact that the distribution of the disease is by the spattering of water droplets and soil particles, and that infection takes place through the stomata. The disease be-comes quickly distributed over the more mature stems until the cambium and deeper tissues are destroyed in continuous areas, whereupon the plant dies. Occasionally petioles and leaves show infection; the latter exhibiting the usual water-soaked spots and resembling the bacterial leaf blight of beans.
The disease is not a vascular infection; it confines its attacks to the mesophyll, the cambium and deeper parenchymatous tissues. The lesions of the stems gradually enlarge and deepen until they come together.
Caused by Peronospora trifoliorum De By.
Symptoms. This trouble usually makes its appearance when the plants are a few inches high or it may attack older plants. Affected leaflets become sickly yellow, finally white, shrivel and die, ,lowing but very little of the fruitings of the causal organism on the surfaces of the affected areas. Un-der very moist conditions, however, the spots become covered with a delicate grayish lilac colored mold growth. The latter consists of the summer spores of the fungus. The winter or resting spores are found imbedded in the dead tissue of the host. Massee claims that downy mildew is a serious disease of outdoor sweet peas in England. Although the fungus Peronospora trifoliorum is very common on other legumes in the United States, it has not yet made its appearance on the sweet pea. Mr. Massee says that Peronospora vicier Berk. also attacks sweet peas in England.
STEM OR COLLAR ROT
Caused by Sclerotinia libertiana Fckl.
Symptoms. This is usually a seedling disease, although it may attack plants of all ages. Like the Rhizoctonia rot, it attacks many different kinds of seedlings. The trouble is most severe in poorly ventilated houses in beds overwatered or lacking proper drainage, and in damp places out of doors. The disease spreads very quickly and is soon fatal. Affected plants first show a wilting of the tip and a flagging of the leaves, and then the seedlings fall over and collapse (fig. 69, b.). The causal fun-gus does not seem to attack the roots, but penetrates the collar of the stem and completely invades the vessels, thus clogging the upward flow of the water from the roots to the stem. Freshly collapsed plants usually have a water-soaked appearance, and are later overrun by a white weft, which is merely the mycelium of the fungus; this is followed 'by sclerotia (fig. 6g, d) (resting bodies), which are found scattered here and there on or within the affected stems. The fungus is a soil organism which occasionally causes trouble in clover fields. It is introduced with animal manure. For a description of the causal organism and methods of control, see p. 151.
THIELAVIA ROOT ROT
Caused by Thielavia basicola Zopf.
Symptoms. Plants severely infected with Thielavia develop practically little or no root system, since the new roots are destroyed as soon as they are formed. Generally all that is left is a stub, which is charred in appearance (fig. 70, b.). The dis-ease often works up from the affected roots to the stems, some 2 or 3 inches above ground. Affected plants neither die nor wilt, but remain dwarfed, stunted, and sickly pale in color, and produce few or no blossoms. For a description of the causal organism and methods of control, see p. 355.
Caused by Microsphera alni (Waller) Salim Symptoms. Powdery mildew is a very common trouble of greenhouse sweet peas. The causal fungus grows on the surface of the leaves in powdery white patches. Affected leaves become pale and drop off prematurely. The ascus stage of Microsphera almi is rarely found on the freshly affected foliage. It is, however, fairly abundant on the dead and fallen leaves on the ground. The mildew may be controlled in the same manner as for the rose (see p. 323).
Caused by Mycosphśrella pinodes (Berk. and Blox.) Niesel.
Symptoms. Although it is a dangerous enemy of the garden pea, this disease has not attacked sweet peas very often, especially where they are grown under greenhouse conditions. The pycnidial stage of the fungus is found on foliage of the garden pea and of the sweet pea. The winter stage may be found on dead leaves and vines of the sweet pea and of the garden pea.
The Organism. The pycnidia are brown, erumpent, globose, with thin walls. The spores are hyaline, cylindrical, one septate and rounded at both ends ; they are guttulate only when young. The perithecial stage was discovered by Stone. The brown perithecia are found under the epidermis or deeply sunken in the tissue of the spot. Their mouths are elongated and beaklike. The asci are cylindrical, while the ascospores are elliptical to ovate. Both are two-celled hyaline bodies. Spraying with a standard fungicide may keep the disease in check.
Caused by Glomerella. rufomaculans (B.) V. Sch. and Sp.
Symptoms. Like streak, anthracnose has not yet made its appearance on greenhouse sweet peas. Whether this is merely accidental, or whether indoor conditions are unfavorable to the disease remains to be discovered. It is necessary, however, that the grower be familiar with the disease in order to pre-vent its spread indoors. The symptoms of Anthracnose are manifested in a wilting or dying of the tips, which become whitish and brittle and readily break off. At other times the wilt works downwards and involves the entire branch. Frequently, also, leaves thus infected become brittle and soon drop (fig. 70, a.). Examination of an infected leaf with a hand lens shows that it is peppered with minute salmon-colored pustules. At the time of blossoming the fungus makes its attack on the peduncle, or the fungus attacks both the flower bud and the peduncle, in which case both dry up, although they do not fall off. The most distinguishable symptoms of this disease are on the seed pods. Infected pods lose their green color, become shriveled, and are soon covered with salmon-colored patches, which cannot fail to attract attention.
Organism. The cause of the anthracnose is the fungus Glomerella rufomaculans. This fungus causes also the bitter rot of apple and the ripe rot of grapes. Cross inoculations have definitely proven that the fungus can go back . and forth from the apple to the sweet pea and vice versa. Anthracnose begins its destructive work early, even in the seed-ling stage.
CHAETOMIUM ROOT ROT
Caused by Chetomium spirochete Pratt.
This disease is of minor importance. It is found on plants weakened by poor cultural conditions, such as overwatering. The symptoms of this trouble greatly resemble the injury from Thielavia root rot. The only way, however, to tell them apart is microscopically.
Caused by Fusarium lath yri Taub.
Symptoms. This disease is of much greater importance to greenhouse men than root rot. The writer has known of instances where the disease has ruined the entire crop of indoor sweet peas. After several resowings, the owners gave up in despair any further attempt to grow them.. Florists should do everything to prevent the introduction of the dis-ease into the house. In places where this disease has made its appearance the growing of greenhouse sweet peas had to be abandoned within less than two years. The disease produces a sudden flagging of the leaves which is accompanied by a wilting and col-lapse of the seedlings (fig. 70, c.). Usually, upon sowing the seed, a fair percentage germinate and reach a height of about eight to ten inches, when they are attacked by the fungus. If the collapsed seedlings are allowed to remain on the ground, the dead stems will soon be covered with the sickle-shaped spores of the Fu- sarium fungus. Eventually the dead tissue rots, attracting small fruit flies, which begin to distribute the spores to different places in the same house. The trouble usually appears in widely separated spots on the bench. These spots, however, quickly spread, involving the entire bed, the plants of which may suddenly assume a wilted appearance, Here and there, however, and in the same bench, a few plants remain alive and keep on growing in spite of the disease.
The Organism. The mycelium of the fungus is hyaline, septate, and branched. At an early age the hyphe begin to form chlamydospores. These are round hyaline bodies filled with oil globules and are, formed in the center of the hypha, whereupon the contents of the cell collects into the chlamydospores. Usually, the chlamydospores are also borne at the tip end of the hyphe in chains of twos, threes and even fours. Old cultures are practically one mass of chlamydospores. There are also two spore forms present, which appear as early as the third day in the pure culture. These comprise microconidia which are fairly abundant and macroconidia, varying from two, three to four celled. The aver-age form of macronidia is the three celled. Both micro- and macroconidia are hyaline and smooth. In old cultures the macroconidia shrink so that the septa become slightly pronounced. These old macroconidia soon lose their protoplasm or let it break up until it presents a granular appearance. In young cultures, the outer wall of the chlamydospore is smooth, but in old cultures it becomes slightly warty or covered with minute points. No perfect stage has been found to accompany this fungus either in pure culture or on the host. This disease may be con-trolled by the steam method of sterilization.
Caused by Rhizoctonia solani Kuhn.
Symptoms. This trouble is of considerable importance to greenhouse men. The disease may destroy the entire stand, or cause it to be uneven, thus necessitating several resowings. Severely infected plants have practically no root system (fig. 71, b.). In less infected plants only one or two rootlets may be destroyed. The fungus produces a browning effect of the root before total destruction sets in. In very early stages of infection the seedlings are seen to have a wilted appearance; as the disease progresses the infected seedlings fall over and collapse. The fungus is not confined to the roots alone. It is often seen to work its way up the stem and produce a constricted area marking it off from the healthy part. As the fungus is a soil organism, it is usually introduced with manure, infection taking place at any part of the roots or stems. In the latter case reddish sunken spots are observed at the base. Root rot is primarily a seedling disease, although older plants, too, may be affected. Such plants linger for some time but are valueless. Corticum vagum is a soil fungus which attacks a number of other green-house as well as outdoor plants. In this case, the organism (fig. 71, c) either produces a damping off among young seedlings, or deep cambium lesions on the stem. With sweet peas the injury is the same. Root rot is introduced in the greenhouse with infected soil or manure. Overwatering and a sour condition of the soil favor the disease. For a description of the causal organism and method of control.
ROOT KNOT OR NEMATODE
Caused by Heterodera radicicola (Greef) Muller.
Symptoms. The disease is characterized by swellings on the roots (fig. 70, d.). These are either small swellings formed singly, in pairs, or in strings, thus giving the affected root a beaded appearance. Again, the swellings may be very large so as to be mistaken for root nodules. However, these galls cannot be mistaken for the normal root nodules, be-cause the latter are lobed and are attached at one end, whereas the root gall produces a swelling of the entire surface of the part affected. Infected plants usually linger for a long time, but they can be distinguished by a thin growth and yellow, sickly looking leaves and stems. The disease is introduced with infected manure or with compost. For a description of the causal organism and methods of control.