Cultivation technology and plant protection for floral transplants in a greenhouse

Growing technology and microclimate in the greenhouse

Greenhouses are used to grow a wide range of transplants for flowering crops. In the UK, the most common ones are alyssum, antirrhinum, begonia, dahlia, pelargonium, lobelia, marigold, nemesia, petunia, and salvia. All these species, except for pelargonium (which is propagated vegetatively as well), are grown from seed. Although each crop requires specific agricultural practices, the general principles are similar — drastic deviations from the technology make greenhouse growing impossible.

Peat compost is used for seed germination and pricking out, while ordinary soil is rarely used. The nutrient content of the sowing mixture (peat and sand in a 50:50 ratio) is very low. The compost for pricking out (peat and sand in a 75:25 ratio) already contains sufficient elements for plant rooting. Subsequently, the transplants are regularly top dressed with liquid fertilizers, adjusting the application according to the growth rate: slow-growing species (begonia, antirrhinum, salvia, lobelia) require nutrients for a longer period than fast-growing ones (alyssum, marigolds, asters).

Seeds are sown in wooden or plastic seed trays. Once pricked out, the seedlings are kept in a warm place for the first two weeks, after which the temperature is reduced to the lowest possible safe level. To obtain high-quality marketable products, the transplants are distributed into pricking-out boxes or plastic trays combined into modules.

• Size of seed trays — 36×22×5 cm
• Seed germination temperature — 13–21 °С (usually 18–21 °С)
• Post-pricking out regime — 2 weeks at 10–13 °С
• Planting density in a box — 48–54 pcs. (sometimes up to 70 pcs.)
• Number of containers in a module — 12 pcs.
• Good yield per 1 ha — 200–250 thousand boxes

Transplants are grown for spring sales, so sowing of slow-growing species begins as early as January. Some cold-hardy crops, such as violet, pansy, and aubrieta, are sown in the autumn. They are left to overwinter in open soil or under minimal cover.

Plant protection for transplants against pathogens and prevention of damping-off

The success of transplant production largely depends on protecting plants from pathogens in the greenhouse. Infections can affect emergence, roots, and stems of young plants at various stages of growth. Fungal and bacterial diseases pose the main threat.

• Emergence (damping-off, seedling collapse, and root rots): pathogens — species of Pythium, Phytophthora, Alternaria, Pseudomonas, Rhizoctonia solani.
• Roots (root rots): species of Pythium, Phytophthora, Fusarium, Rhizoctonia solani, Thielaviopsis basicola.
• Stems (stem rots): Botrytis cinerea, Sclerotinia sclerotiorum, Alternaria spp..

Damping-off (seedling collapse and root rots of emergence, caused by Pythium, Phytophthora, Alternaria, Pseudomonas, Rhizoctonia solani) is the most important disease of transplant crops, affecting almost all plant species. Its most common pathogens are Pythium spp.. Pre-emergence damping-off kills sprouts while still in the soil, destroying rootlets and stems before the cotyledons unfold. If infection occurs later, a constriction forms on the hypocotyl at the soil surface, causing the stem to break and the seedling to collapse.

Foci of collapse in seed trays spread very quickly, especially if the sowing density is too high. After pricking out, the disease manifests as root rot and growth stagnation. As they age, seedlings become more resistant to Pythium species, but under high humidity, they may lose their root system even before planting. Identifying a specific disease pathogen is difficult.

Damping-off caused by Phytophthora spp. is much rarer. However, some crops, such as petunia, show increased susceptibility to them. To combat these pathogens, it is critically important to follow the rules of substrate preparation.

The effectiveness of soil disinfection is clearly confirmed by the example of growing aster (Callistephus chinensis). When sown in untreated compost, emergence dies en masse from black leg caused by Pythium ultimum. Using the same substrate, but pre-steamed, allows for obtaining absolutely healthy and strong transplants.

Rhizoctonia solani most often causes black leg in stock, aubrieta, and salvia. In some cases, damage by this pathogen can be identified by the drying or shriveling of the affected area of the stem, which gradually acquires a light brown color. This type of lesion is often called "wire stem". The brown hyphae of the fungus sometimes inhabit the affected stem tissues, and a mycelial coating covers its basal zone and spreads to the surface of the compost.

Disease caused by Alternaria species is often accompanied by leaf and stem rot (see p. 344, lobelia alternaria blight). Spotting develops on infected seedlings.

Bacterial pathogens (Pseudomonas species) affect leaves and stems, and not just their basal part.

Pythium spp., Phytophthora spp. and R. solani are common soil-borne fungi, with the first two groups often contaminating pond water. Compost, which serves as a source of infection, is also often contaminated with these pathogens. Both the conditions favorable for the development of the disease and the measures for its control are determined by the pathogen species. Therefore, to carry out effective control, it is primarily necessary to identify the cause of the disease.

A moist environment is favorable for Pythium and Phytophthora species; moreover, their harmfulness increases with slow seed germination and seedling development. Under unfavorable conditions, both pathogens survive in the form of oospores, which produce motile zoospores upon germination. Zoospores infect seedlings, and sporangiophores and numerous sporangia form on decaying plant tissues in moist soil. R. solani is most dangerous in moist, warmed soil, which is ideal for seedling growth. As a rule, this pathogen does not form spores, but small sclerotia appear on pieces of mycelium — the form in which the pathogen survives under unfavorable conditions. Greenhouse soil is a reservoir for all damping-off pathogens, and it develops very rapidly in seed trays placed on contaminated shelves, when splashed with water during irrigation, or when uncleaned trays are reused. Water, in particular pond water rather than tap water, also serves as a possible source of infection.

Control. Hygiene is the most important factor in the fight against this disease. The disease will not appear if pathogen-free compost and tap water are used, and seed trays and the greenhouse itself are thoroughly cleaned. Some highly susceptible plants, for example, antirrhinum, lobelia or salvia.

Fig. 16.2. Checking compost or soil for damping-off pathogens using "trap" plants, in this case cucumber and watercress: a and b — steamed soil, c and d — the same, but unsteamed soil. Such tests allow detecting damping-off pathogens in moist and warm (12–15 °C) soil. Test plants must be very susceptible to pathogens. The test lasts about two weeks and allows clearly distinguishing between heavily contaminated soil and healthy soil, but it is not always reliable at low inoculum levels. Conditions may require special protection measures, in particular regular fungicide treatments. As a rule, standard compost is not contaminated with damping-off and root rot pathogens, although this cannot be guaranteed. To be absolutely sure of the purity of the compost, it must be steamed. Such disinfection is very effective, but is sometimes accompanied by the accumulation of ammonium or manganese ions in toxic concentrations (see

Factors leading to the development of damping-off and seedling lodging include high sowing density, conditions that delay seed germination and the growth rate of seedlings (low temperatures, excessive humidity or excessive soil dryness followed by flood irrigation), and root damage due to a high content of soluble salts in the compost or the presence of phytotoxic chemicals.

Compost treatment with fungicides can be carried out before sowing or after seed germination by watering the seedlings. Pricking out outwardly healthy plants from seedbeds where disease has been noted is often pointless, since there is a danger of introducing pathogens into the compost of the pricking-out trays. In addition, it is very difficult to determine infection in the early stages of disease development.

To choose the right fungicide, it is necessary to accurately know the pathogens. Etridiazole and propamocarb hydrochloride are most effective against Pythium and Phytophthora species when incorporated into compost before sowing or applied after sowing. Copper-containing preparations, in particular Cheshunt*, have moderate effectiveness, but they are sometimes phytotoxic, retarding the growth of seedlings and young plants. R. solani is suppressed by quintozene, tolclofos-methyl and iprodione, but not by etridiazole. Both quintozene and tolclofos-methyl are best incorporated into the compost before sowing seeds, and iprodione should be applied by spraying the soil. When working with fungicides, it is very important to observe the recommended dosages to prevent the occurrence of phytotoxicity, especially with quintozene, which evaporates rapidly when compost is warmed and can inhibit plant growth.

To combat the disease caused by Pythium and Phytophthora species, as well as R. solani, several fungicide mixtures are produced, for example, etridiazole and chlorothalonil. Protection measures against Alternaria and Pseudomonas are described on pages 344—345.

Root rots (Pythium, Phytophthora, Fusarium, Rhizoctonia solani, Thielaviopsis basicola, Nectria radicicola, syn. Cylindrocarpon destructans)

Symptoms of root rots usually appear after pricking out, when the transplants are almost ready for planting in open soil. Earlier infection is usually associated with the development of damping-off. In affected plants, the root color changes from cream or light brown to dark brown or black. The first indicator of root rot is general stunting of the affected plants, often accompanied by uneven development of the transplants in the box. It is not easy to determine the cause of root rot, but the task is simplified if one can establish how the seedling development proceeded and whether they were affected by damping-off. Blackening of roots often indicates the presence of Thielaviopsis basicola, which forms black chlamydospores on the affected root tissues. This pathogen thrives in low temperatures. At the beginning of the disease development, it can be detected on the roots of plants, sweet peas, and delphiniums.

* Contains 2 parts copper sulfate and 11 parts ammonium carbonate. When dissolved in water, it forms copper ammonium sulfate, which converts into basic sulfate after application. — Ed. note.

Nectria radicicola is a weaker pathogen, usually affecting plants with retarded development.

Pathogens of damping-off and seedling collapse can cause progressive root rot, which leads to wilting and death of the transplants.

All mentioned fungi are soil-borne and persist in the soil or compost for a long time in the form of resting spores. Most transplant crops are susceptible to one or more root rot pathogens.

Control. The same control methods described above are effective against fungi that cause root rots, i.e., hygiene, soil disinfection, and careful irrigation with clean tap water. Treatment with specific fungicides (see page 342) is accompanied by the suppression of Pythium and Phytophthora species and R. solani. Successful suppression of T. basicola and N. radicicola is achieved by watering the compost or transplants with one of the benzimidazoles, zineb, or captan.

Stem rots (Botrytis cinerea, Sclerotinia sclerotiorum, Alternaria species)

Stem rot may appear at the seedling stage, causing seedling collapse (see page 339), or on more mature plants, which then partially wilt. The most common stem rot pathogen is Botrytis cinerea. The fungus infects wounds or aging plant tissues, which at first turn slightly brown and then quickly become covered with gray-brown mycelium and fungal sporulation, especially under constant humidity. Under such conditions, spores germinate within two hours.

Sclerotinia sclerotiorum causes signs of white rot on fleshy plant stems, for example, dahlias, antirrhinums, and marigolds, although many other species are also susceptible to this pathogen. A diagnostic feature of the disease is abundant white cottony mycelium developing on the stems. Soon, sclerotia form in the mycelial layer, which are resistant to adverse environmental conditions. Upon germination, they produce apothecia and ascospores; the second cycle of the disease begins with these forms. The spread of infection in a transplant box due to growing mycelium occurs very rapidly.

Different Alternaria species, most often A. alternata, cause stem lesions that differ from those described above by the presence of black mold — the pathogen's conidia — on the affected surface.

Control. Control measures for stem rots are determined by the type of pathogen. The best way to suppress B. cinerea is to maintain a regime that avoids long periods of high relative air humidity and leaf wetness, premature tissue aging, and mechanical damage.

If environmental conditions cannot be controlled, spraying with fungicides is carried out. The most effective among them are iprodione, vinclozolin, thiram, or benzimidazoles, applied via high-volume spraying every 10—14 days.

To prevent the development of white rot, it is primarily necessary to destroy S. sclerotiorum sclerotia. To do this, soil steaming or chemical disinfection is performed. If the soil is covered with polyethylene film or another protective material, such as a layer of peat or gravel, germinating sclerotia do not form apothecia. The spread of an already developed disease is contained using high-volume spraying with vinclozolin or iprodione.

Methods of control for Alternaria spp. are provided below. Alternariosis (Alternaria alternata and other Alternaria species)

Alternaria leaf spot most often affects lobelia, zinnia, alyssum, antirrhinum, aster, carnation, nemesia, pelargonium, marigolds, and violet. The disease is caused by various species of Alternaria. As a rule, all these pathogens cause brown spotting on the affected leaves. As the disease progresses, the spots turn black during the pathogen's sporulation. These same fungi cause symptoms of damping-off, lodging, and stem rot. Many of them are seed-borne. Alternaria develops on seedlings from infected seed and spreads to neighboring plants via water droplets. Increased air humidity and leaf moisture are favorable conditions for Alternaria pathogens.

Alternaria and leaf spots: how to disinfect seed and protect seedlings

Alternaria leaf spot, as well as infections of snapdragon and Phlox drummondii, are primarily seed-borne. If the infection is not suppressed at the start, the diseases will quickly spread throughout the greenhouse via water splashes during irrigation. The main focus in seedling protection should be on pre-sowing preparation of seed material.

• Heating with a steam-air mixture — 60 °C
• Thiram concentration by active ingredient — 0.2%
• Temperature of thiram solution — 30 °C
• Soaking time for lobelia and phlox — 12 hours
• Dry heating of snapdragon seed — 49 °C

To combat Alternaria, a short-term seed treatment with a steam-air mixture is used. Another reliable method is soaking in a 0.2% thiram solution for 24 hours. This procedure not only destroys the pathogen but also stimulates germination. Dusting seed with iprodione is also effective, and at the first symptoms on plants during the growing season, spraying with the same product is recommended.

Leaf spots on snapdragons are caused by two pathogens: the fungus Phyllosticta antirrhini and the bacterium Pseudomonas syringae pv. antirrhini. Both pathogens produce round brown spots up to 0.5 cm in diameter, which move from the lower leaves upwards. Distinguishing them visually is difficult, as a characteristic purple border can appear in both cases, especially on older leaves. In the case of a bacterial infection, a water-soaked zone or chlorotic halo is more often noticeable around the spots, while in the case of a fungal one, black dots (pycnidia) are visible.

The following measures are used to combat these pathogens on snapdragons:

• at the first symptoms of bacterial leaf spot, spray with copper-based fungicides;
• against fungal leaf spot, seed is treated with iprodione, and seedlings are sprayed with zineb, mancozeb, or benzimidazole-group preparations;
• irrigation is adjusted to avoid water splashing, ensuring plant leaves dry quickly.

Leaf spot on Phlox drummondii appears on transplanted seedlings that are a few weeks old. Gray spots up to 0.5 cm in diameter with a red or purple edge and black pycnidia in the center form on the leaves. The disease is seed-borne, so before sowing, seed is treated with thiram just like lobelia seed. In the greenhouse, it is important to promptly discard trays with diseased seedlings and irrigate the transplants strictly without splashing.

Downy and powdery mildew: humidity control and fungicidal treatments

Downy mildew (peronosporosis) poses a threat to cruciferous crops — stock, alyssum, and wallflower. A sporulation coating appears on the underside of the leaves, while chlorotic spots appear on the upper side. The disease can spread to stems, deforming the plants. Spores are easily carried by wind and water, but their germination requires liquid moisture or very damp air.

To protect seedlings from downy mildew, microclimate parameters are controlled to prevent stagnant moist air and over-wetting of the leaf surface. If symptoms are detected, regular high-volume spraying with dithiocarbamates (mancozeb, zineb) or their mixtures with acylalanines is carried out.

Powdery mildew rarely affects seedlings but is dangerous for sensitive species — salvia, begonia, and Myosotis. Unlike downy mildew, the pathogen does not require liquid moisture on the leaves: spores germinate perfectly at high temperatures and high air humidity. The disease manifests as a white or grayish powdery coating on the upper side of the leaf blade.

At the first signs of powdery mildew, transplants should be immediately treated with specialized fungicides using the high-volume spraying method.

In some species, particularly nemesia and petunia, leaf deformation develops. The leaves of seedlings thicken and curl, sometimes fusing to form pitcher-like structures. Occasionally, the infestation covers the entire tray, even if neighboring plantings from the same seed source remain healthy.

Various factors may be the pathogens of this type of deformation. One of them is considered to be the bacterium Corynebacterium fascians. It can be introduced with seed, but does not survive in plant debris. When young plants are affected, gall-like outgrowths appear on the leaves, especially the lower ones. A correlation has been established between deformations and the application of certain fungicides, particularly thiram, although in most cases, symptoms of infestation do not appear upon their application. Apparently, for deformation to manifest, fungicide application during the critical phase of seedling development must coincide with a period of low temperatures. Somewhat similar seedling deformations in transplants appear during infestation by stem and leaf nematodes. Control. There is little that can be done when symptoms of deformation appear. All diseased plants should be separated from healthy ones, but if they are not severely damaged, continue growing them, as they sometimes recover. Flower diseases Flower infestation in transplants is rarely observed, mainly because only a few species reach the flowering phase before planting into open soil. On flowering transplants, such as marigolds, flower rot sometimes appears, caused mainly by B. cinerea. Since pollen stimulates the germination of fungal spores, the disease intensifies in its presence. Gray mold on flowers is easily controlled using the methods described on page 343. FURTHER READING Forsberg, J. L. (1975) Diseases of Ornamental Plants. University of Illinois Press, Urbana, Ill. Shurtleff, M. C. (1966) How to Control Plant Diseases in Home and Garden. Iowa State University Press, Ames, Iowa. CHAPTER 17. POT CROPS AGROTECHNICS