Technology of greenhouse lettuce cultivation and methods of disease control

Cultivation technology and microclimate parameters

The soil for lettuce is prepared immediately before sowing by treating it with methyl bromide. Using this technology, one or two lettuce crops are grown in the greenhouse, after which tomatoes are planted. Peat compost is used for the nursery. To prevent the appearance of damping-off, fungicides are incorporated into it if necessary.

• Seedling block area — about 4 cm²
• Planting density — 23 plants/m²
• Plant nutrition area — 201×210 mm
• CO₂ concentration in the air — 0.1 g/l
• Winter yield — 120 g/plant
• Yield in March — 250 g/plant

Temperature regime during seedling cultivation depends on lighting conditions. Under standard conditions, the temperature is maintained at 18 °C during the day and 4 °C at night, with a minimum threshold of 7 °C. Under constant additional lighting, these values are increased to 23 °C during the day and 20 °C at night. During further lettuce cultivation, the temperature in the greenhouse is maintained at 4 °C at night and 10 °C during the day, while maintaining the concentration of carbon dioxide.

Lettuce sowing dates are not limited by the season. However, the duration of the growing season from sowing to harvest varies significantly depending on the time of year. During the summer period, the crop matures much faster than with winter planting.

Protecting lettuce from damping-off and grey mold

Damping-off (seedling rot) affects lettuce seedlings relatively rarely. The disease is caused by pathogens of the genus Pythium, as well as the pathogen of root rot Rhizoctonia solani. The infection manifests as a characteristic constriction of the stem at the soil surface. Dense sowing and excessive soil moisture contribute to the development of the disease.

Grey mold (Botrytis cinerea) is a common disease that affects lettuce at all stages of development, especially at the beginning of growth and before harvesting. The pathogen enters through injuries and actively colonizes aging tissues, as well as areas affected by downy mildew (Bremia lactucae) or rhizoctonia (Rhizoctonia solani). Soft brown rot with profuse sporulation develops on leaves and stems. Diseased adult plants wilt, acquire a grey-green tint, and die. Black sclerotia up to 3 mm long can form on rotting parts, which are significantly smaller than the sclerotia of Sclerotinia sclerotiorum.

The infection is most active at high air humidity and wet leaves, which is characteristic of the autumn-winter rotation. Temperature conditions are less critical for the pathogen — it maintains high activity in a wide range up to 25 °C. Fungal spores are easily carried by wind and water splashes during irrigation. Lettuce protection is based on compliance with agricultural practices and careful handling of plants.

1. High-quality soil preparation. Perform shallow loosening of the surface layer to avoid injuring tender lower leaves with clods of earth.
2. Timely transplanting. Plant transplants into the soil before the plants become overgrown and easily damaged during transplanting.
3. Microclimate control. Regularly reduce the relative air humidity in the greenhouse using preventive heating or intensive ventilation.

When carrying out chemical control against grey mold, it is very important to start treatments in the initial phases of crop development. In this case, the lower leaves of the lettuce will be covered with fungicide before they are covered by subsequent leaves. High-volume spraying with iprodione or benzimidazoles is most effective, although resistance to benzimidazoles often develops in B. cinerea populations (see page 131 and table 6.1). Root rot, seedling rot, or rhizoctonia (Rhizoctonia solani)

This fungus is the most common pathogen of lettuce seedling damping-off; in addition, it causes root rot in adult plants. Typical symptoms of seedling damping-off develop on emergence: a dry, light or dark chocolate-colored constriction of the stems at the soil surface or slightly above. Plants droop, and grey mold and bacterial rot may develop in the affected areas.

In plantings of adult plants, the pathogen affects stems and leaf stalks. Dry red-brown rot develops on the petioles of the lower leaves and on the stems, gradually moving to the heart. Affected tissues are covered with fungal mycelium. Diseased plants do not always wilt, but they are quickly colonized by secondary pathogens (grey mold and soft rot agents). Fig. 9.2. Lettuce affected by Rhizoctonia rot;

a — healthy and severely wilted leaves of the same plant (comp. Fig. 9.1). In this sowing, common chickweed (1), shepherd's purse (2), and groundsel (3) serve as hosts for viruses of lettuce (see p. 229); b — damage to the stem and leaf petioles at the soil surface; 8 — reddish-brown staining of the root collar and spotting of petioles, typical for root rot.

bacterial rots), and as a result, the plant wilts and dies off.

The source of infection is peat and mixed composts. The pathogen can also be introduced into disinfected soil if seedling flats are contaminated with it, in which case rhizoctonia spreads very rapidly. In general, however, traditional composts used for raising transplants and preparing soil blocks are free from A. 50/14.

The fungus can persist in the soil for a long time. It grows rapidly in soil and is often most harmful after partial soil disinfection, which kills many competitors of K. 50am.

Control. Seedling rot develops rapidly when raising transplants in poorly disinfected substrate. Compost used in nurseries must be steam-treated or treated with chemicals.

To control rhizoctonia, the soil is treated with quintozene or tolclofos-methyl before planting transplants. Coarse-droplet spraying with iprodione combined with pre-planting soil treatment with tolclofos-methyl (Rizolex) is very effective, but spraying should be carried out at early stages of crop development to protect the lower part of the stems and lower leaves from infection.

This disease develops at high temperatures (22 °C and above) and is therefore most common in summer greenhouse crops, severely affecting plants at the beginning of the ripening phase. Soft stem rot, accompanied by complete wilting of the leaves, can serve as the first indicator of white rot infection, and these symptoms are easily confused with those caused by the gray mold pathogen. However, dense fluffy mycelium on the affected tissue, and in the case of plant decay — large black fungal sclerotia embedded in the white mycelium and in the affected plant tissues, help to establish the true cause of the disease. Sclerotia of S. sclerotiorum reach 1 cm in length, survive in the soil for a long time, and germinate by forming mycelium or apothecia with ascospores, which are released into the air and infect any suitable host plant. The host range of this pathogen is very wide; it includes tomato, cucumber, chrysanthemum, and many other greenhouse crops, as well as herbaceous plants.

Control. All affected tissues must be removed, taking care that the fungal sclerotia do not fall onto the soil. Immediately after identifying the symptoms of the disease, spraying with iprodione or benzimidazoles is carried out. It is difficult to control the disease once it has already developed. Fungal sclerotia usually die during soil steaming or chemical treatment.

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The disease appears on lettuce quite often, especially on crops ripening in the first months of the year. Sometimes affected plants can only be discovered during harvesting by the black-green dry rot of the lower stem part of the head. Rot spreads along the veins of the lower leaves, and sometimes to the root tissue. However, the affected plant wilts and dies only after secondary rots appear on it. Bacterial rot develops especially strongly on plants grown under insufficient light and excessive leaf moisture. Most cultivars are susceptible to the pathogen, although on neighboring sowings of different cultivars sown at different times, the degree of infestation may differ sharply.

Mainly the bacterium P. marginalis is isolated from plants with signs of bacterial rot, although it has not yet been definitively established that it is the primary cause of the disease. Apparently, for bacterial rot to become truly harmful, a complex of conditions is required, i.e., the presence of the pathogen, a susceptible host, and certain environmental conditions.

Downy mildew: humidity control and treatment schedules

Downy mildew (pathogen — Bremia lactucae) can completely destroy a crop of greenhouse lettuce during prolonged high humidity and moisture on the leaves. It is difficult to recognize the disease in the early stages, especially in seedlings. The first symptoms are barely noticeable: young leaves turn slightly yellow and curl, but sporulation is already occurring on both sides of the leaf blade. Later, bright yellow angular spots appear on the upper side of the leaf, bounded by veins, and a white powdery coating appears on the lower side. In case of severe infestation, the leaves die off completely.

• Optimum for spore germination — 15 °C
• Time for infection start — 2–3 hours
• Disease development arrest — at 25 °C
• Conidia viability — 6 days

For infection, the pathogen requires only moisture on the leaves and a temperature range from 2 to 20 °C. If the temperature rises above 20 °C, sporulation slows down sharply. Dry air promotes the release and rapid spread of spores by wind to neighboring crops. The pathogen is not seed-borne but is easily transmitted through the air. Downy mildew is most dangerous when growing lettuce in monoculture or when alternating summer open-field cultivation with winter greenhouse production.

The genetic resistance of cultivars (11 resistance genes are known) is quickly overcome by new multivirulent races of the fungus. For this reason, relying solely on breeding in industrial production is not justified. An agronomist must combine genetic protection with a complex of preventive measures:

• Do not place new lettuce crops near old, infected areas.
• Completely remove plant residues immediately after harvesting.
• Control the microclimate, avoiding stagnant humid air and condensation on leaves.

In cases of mild disease development, chemical treatment yields satisfactory results. Contact and systemic preparations are used to suppress the infection. It is important to strictly adhere to the application regulations to prevent the accumulation of active ingredients in the produce.

Bacterial rot and anthracnose: prevention in the greenhouse

Bacterial rot of lettuce becomes active in the first months of the year. An outbreak is provoked by excessive air and soil humidity combined with the loss of turgor in plants. Another critical risk factor is an excess of nitrogen in the soil coupled with a deficiency of potassium. This often happens if soil disinfection was carried out immediately before planting transplants.

Anthracnose, or ring spot (pathogen — Marssonina panattoniana), manifests on mature leaves as brown rounded spots 4–5 mm in diameter. Brown sunken ulcers, resembling damage by slugs, form on the veins. Over time, the centers of the spots fall out, and in a humid environment, a pink sporulation coating appears at the edges of the lesions.

The infection persists in the soil on plant residues, is transmitted via seed, and is carried by water droplets. Anthracnose is rare in greenhouses, occurring mainly due to drips under gutters. The weed prickly sowthistle (Sonchus asper) often serves as a reservoir for the infection. Fungicides are usually not required to protect greenhouse lettuce from anthracnose, but at the first signs of the disease in field conditions, high-volume spraying with thiram is effective.

Lesser-known lettuce diseases: symptoms and control measures

Some pathogens occur locally on lettuce, but when favorable conditions arise, they quickly spoil the marketability of the leaves. It is important for an agronomist to notice the first signs of infection in time so as not to confuse them with other diseases and to implement protective measures promptly.

• Diameter of rust pustules — up to 1.5 cm
• Diameter of initial Septoria spots — about 1 cm
• Length of olive-colored Septoria areas — up to 1.5 cm

Lettuce rust has been recorded in the Netherlands, although it has not yet been detected in the UK. The disease manifests as clusters of small cup-shaped aecia on the leaves, which coalesce into large convex cushions. Among the fungicides permitted for use on the crop, dithiocarbamates (zineb, mancozeb) are the most effective against rust.

Septoria leaf spot is widespread in Europe, although there is little data on its development in the UK. Romaine lettuce (summer endive) is particularly often affected, with uneven light-yellow spots appearing on the leaves. Gradually, these develop into olive-colored rounded or irregular areas with a light-yellow halo, on which pycnidia develop. The tissue in the center of the spots falls out, leaving holes with jagged edges. Benzimidazoles should be effective against Septoria, but chemical control trials have not yet been conducted.

Powdery mildew is occasionally found in European countries; in Great Britain, only a few reports of its occurrence have been recorded. The white powdery fungal coating develops primarily on the upper side of the leaf blades. Among the products approved for use on lettuce against powdery mildew, only fungicides of the following groups are effective:

• benzimidazoles;
• thiram.

Also, small light-brown round spots may appear on the outer leaves of lettuce, which merge and cover large areas of tissue. Sometimes the center of the spots falls out, leaving a brown border around the holes. This disease is extremely rare, so there is no need for protective treatments against it.