Marigolds - an attractive crop for certain diseases and pests

Summary

Lettuce (Lactuca sativa L.) is a widely cultivated salad crop around the world. Its cultivation is accompanied by attacks from a significant number of disease-causing agents and pests, which can compromise crop quality and yield. The accurate and rapid identification and control of these pests is essential for maintaining healthy crops and producing quality produce. This article reviews the main pests (diseases and insects) affecting lettuce, their symptoms, environmental requirements, and management strategies.

High fuel prices have made winter vegetable production in cultivation facilities almost impossible. The open niche was taken over by salads, which transitioned from a seasonal crop to a year-round one. This was also contributed to by their increased consumer demand throughout the year. Lettuce (Lactuca sativa L.) is the most popular of the salad crops. It is highly adaptable and is distributed in almost all climatic zones. It can be grown both in the field and in greenhouses, including hydroponically. Lettuce is an important component of healthy eating, as well as a decorative element for many dietary dishes. It is undemanding and can tolerate lower temperatures. This vegetable has almost no calories, while simultaneously being rich in many beneficial elements. It contains vitamins (vitamin K, vitamin A, B vitamins, and vitamin C) and minerals (manganese, potassium, copper, iron, phosphorus, magnesium, and calcium). It provides the body with fiber and cellulose, improving digestion. Lettuce salad has a specific and interesting taste that was appreciated by the ancient Greeks and Romans. This wealth of nutrients contributes to numerous health benefits.

Lettuce is a delicate and attractive crop for numerous pests (aphids, thrips, cutworms, slugs) and diseases (caused by fungi, bacteria, viruses). Often, pest damage to the leaves worsens the quality and marketable appearance of this crop.

Downy mildew, powdery mildew, gray mold, bottom rot caused by Rhizoctonia solani, and drop caused by Sclerotinia spp. are widespread and economically significant diseases wherever lettuce is grown. Depending on seasonal conditions, some pathogens have greater economic importance in some cases, while others predominate in others. Some additional fungal diseases have greater regional distribution and significance. These include anthracnose, leaf spots caused by Cercospora, Fusarium and Verticillium wilt. Bacterial diseases (Xanthomonas campestris pv. vitians and Pseudomonas viridiflava) are responsible primarily for post-harvest losses in lettuce. They are caused by pectolytic and fluorescent bacteria. Bacterial leaf spots, soft rot, etc., are observed, which sometimes lead to significant losses if appropriate and timely control is not applied. Viral diseases in lettuce are caused by several viruses. The ones with the greatest economic importance are: Lettuce mosaic virus (LMV) causing mosaic in lettuce. It has been identified in all regions where lettuce is grown – from the northernmost to the hottest areas.

The frequent rainfall in January this year and the increased air humidity are prerequisites for the development of some major lettuce diseases:

Gray Mold (Botrytis cinerea Pers.)

The causal fungus is a ubiquitous polyphagous pathogen. It is established in all regions of the world where lettuce is grown, both in open fields and in cultivation facilities. In recent times, the damage has been more severe, which is related to the plant tissues being more fragile and succulent. Furthermore, the substrate has higher moisture. It can develop independently and cause significant damage, especially in cultivation facilities. It is often reported as part of a parasitic complex that attacks the lower leaves of lettuce. It also includes the fungi Sclerotinia sclerotiorum and Sclerotinia minor. Due to its biological characteristics, it mainly attacks the autumn and winter production of these crops.

Gray Mold (Botrytis cinerea Pers.)

Attack by the pathogen is rarely observed in seedling compartments and more often after transplanting. It attacks plants in all phases of their development – from the seedling stage to harvest. In young seedlings, it causes damping-off by infecting the root collar. Infected plants fall over onto the soil surface, and the affected parts become covered with a gray, sporulating mold from the fungus. In mature plants, it most often attacks the lowest, oldest leaves. Large water-soaked spots form on them. They can be located at the base of the leaf and at the tip. Later, the spots turn yellow and become covered with a grayish sporulating mold. Sometimes the pathogen engulfs the entire head and the plant dies. Later, large, black sclerotia form on the dead tissues, with which the fungus persists in the soil for a long period. There is a key moment in the development cycle of lettuce. As the plants develop, the leaf rosette grows, closes up, and control of the fungus becomes increasingly difficult. Well-developed plants completely cover the soil surface. The old leaves remain covered by younger ones, receive less light, become chlorotic, and are pressed against the moist soil. The pathogen more easily colonizes such tissues. Damage can also be observed after harvest, during storage and transportation.

The fungus persists in the soil and plant debris as conidia, mycelium, and sclerotia. The latter survive in the soil for several years. Conidia are spread by wind and air currents, less frequently by rain and water droplets. They penetrate through the cuticle or through wounds. It easily enters through necrotic or dead tissues resulting from damage by low temperatures, water stress, sunburn. The pathogen favors a moist environment. Relative humidity around 95% and temperatures of 17-23 °C are very favorable conditions for its attacks. These occur in cultivation facilities, but also in open fields, during rainy periods or after overhead irrigation. The agrotextile used to protect plants from insects favors its development. Attack by B. cinerea is more significant under these covers due to the higher humidity. In covered tunnels, the quality of the covering matters for the development of some pathogens, especially Botrytis cinerea. The attack is strongest under polyethylene.

Control

Control of this pathogen is difficult because: plants are most often grown under polyethylene, where conditions are favorable for its development; chemical PPP treatments must stop early – before the economic maturity of the plants due to the risk of residue accumulation, and during this period they are most vulnerable; the fungus adapts quickly to PPPs and develops resistance; the number of registered PPPs against this disease is small.

Some preventive measures complement chemical control: the effectiveness of crop rotations is disappointing, which is certainly due to the polyphagous nature of Botrytis cinerea and the fact that the inoculum largely comes from the crop's environment; regular ventilation of cultivation facilities to reduce air humidity; balanced fertilization; irrigation is recommended to be carried out in the morning and early afternoon, never in the evening, so that the plant dries as quickly as possible; it is advisable to heat the facilities in the early morning hours to reduce humidity and remove dew formation on the leaves; removal of plant debris from diseased plants, on which Botrytis sporulates abundantly and sometimes forms sclerotia.

Registered PPPs: Avalon (Erune/Laitane/Pyramid/Pretil) 200 ml/deca; Botribel 0.4-1.5 l/deca; Geox VG 50 g/deca; Kiplant meta-care 0.4-1.5 l/deca; Serenade ASO SC 400-800 ml/deca; Serifel 50 g/deca; Signum 60-75 g/deca; Switch 62.5 WG 60 g/deca; Fontelis SC 150 ml/deca; Fungisei 300 ml/deca.

Downy Mildew (Bremia lactucae Regel)

Downy mildew is one of the oldest and most damaging known diseases affecting lettuce, grown both in the field and indoors. It is particularly widespread in production regions characterized by prolonged periods of moisture and cool weather, especially in Europe. It can cause significant losses and destroy entire crops within a few days if climatic conditions are favorable. It often represents a constant threat and requires preventive fungicide treatments.

Bremia lactucae belongs to the oomycetes. It occurs on about 230 plant species from the family Asteraceae. The species has many specialized forms, parasitizing a wide range of hosts. It attacks lettuce throughout its growth cycle. Seedlings are particularly susceptible to downy mildew. The fungus develops very rapidly on the cotyledons, which turn yellow and become covered with a fluffy white mold. Slowed plant growth and death are observed. In older plants, it first attacks the lower leaves. Large pale green to yellow angular spots appear on them, limited by the veins. Later, the spots become necrotic and turn light brown. Their lower surface becomes covered with a fluffy mold from the sporulation of the pathogen. Subsequently, spots also appear on the inner leaves. In severe attacks, the spots merge, become necrotic, and the leaves die. Systemic infections are also possible. In these, browning of the internal tissues of the stem and leaf base is observed. The damaged tissues provide a nutritional base for the development of secondary bacterial (Pseudomonas spp., Pectobacterium carotovorum subsp. Carotovorum spp.) or fungal (Botrytis cinerea) invasions, which under moist conditions cause wet and soft rots in the field, and sometimes even during storage.

Downy Mildew (Bremia lactucae Regel)

It occurs in cultivation facilities and in the open field. This obligate parasitic fungus is highly influenced by climatic conditions. It favors prolonged periods of cool, wet weather (with relative humidity close to 100%) and cloudiness. Long periods of leaf wetness in the morning are particularly favorable for infections. Overhead irrigation stimulates the development of downy mildew more than other irrigation methods. The temperature range for sporangia germination is between 10 and 15°C. Infections can develop in 2 to 3 hours within a temperature range of 2 to 20°C. Its sporulation is intensive at night temperatures around 5-10°C and daytime temperatures between 12 and 20°C. Conversely, as soon as the weather warms up again, the temperature rises above 20°C and humidity decreases, sporulation sharply declines. The temperature range for pathogen development is 1-15⁰C. If climatic conditions are favorable, yellow spots appear 4 to 7 days after the first infections. Under high air humidity, when plants are covered with dew droplets for a prolonged time, the disease develops massively and causes great losses. Lettuce that has undergone stress during its growth, such as very low temperatures, low light, temporary water shortage, is more sensitive to this fungus. Bremia lactucae achieves a full cycle in less than 5 days if climatic conditions are very favorable. It persists on plant debris in the soil as oospores and mycelium. Seeds sometimes carry the fungus superficially, but it is not proven that they are a source of primary infections.

Control

It should start with preventive measures: regular ventilation of seedling compartments and cultivation facilities; watering during the day when plants will have time to dry by evening; if necessary and possible, turning on heating during the early hours of the day, which will limit the possibility of dew formation on plants; at the appearance of the first spots, diseased leaves are removed and destroyed outside the greenhouse; maximum removal of plant debris at the end of the growing season; deep plowing of those remaining in the soil; Treatment with PPPs – preventive and upon appearance.

Registered PPPs: Bordeaux Mix 20WP 375-500 g/deca; Vitene Triplo P 400–450 g/deca;  Golbex VG (Golbex WP/Keifol VG/Keifol WP/Kilate VG/Kilate WP)  250 g/deca; Enervin Pro 320 ml/deca; Enervin SC 120 ml/deca; Eruan SC 250 ml/deca; Infinito SC 140-160 ml/deca; Iodus 200 ml/deca; Copper Key (Copper Key Flow/Copper Lainco/Codimur 50 WP/Codimur SC/Cupra) 150-240 g/deca; Codimur 38 Flo 200-320 ml/deca; Corseit 60 VG 20-40 g/deca; Limocid 200 ml/deca; Melody Compact 49 VG 185 g/deca; Mikonos Evo 45-250 ml/deca; Oxitec 25% High Bio 300 g/deca; Orondis Ultra 40 ml/deca; Proplant 722 SL 150 ml/deca; Revus 250 SC 60 ml/deca; Ridomil Gold R VG 500 g/deca; Ridomil Gold SL 20 ml/deca; Taegro 18.5-37.0 g/deca; Favia 50 ml/deca; Fungisei 300 ml/deca; Cyclo P Liquido 300-400 ml/deca.

Anthracnose (Marssonina pannattoniana (Berlese) Magnus).

The fungus attacks young plants, which are particularly susceptible. Under high air humidity and severe infection, seedlings become chlorotic and growth is slowed. The pathogen is responsible for spots on leaves located close to the soil. Initially, they are small and water-soaked. Subsequently, they expand and become rounded. Their limitation by the veins gives them an angular appearance. The damaged tissue changes from orange to brown in color. Later, it dries up, breaks off, and falls out. Leaves appear perforated. Spots along the veins are more elongated and often merge. They acquire a dark orange hue. On the periphery of the spots, separate whitish to pink spore masses from acervuli are formed, which ensure asexual reproduction.

In late infections, lesions appear only during storage and transport, which worsens the quality of lettuce ready for sale. It is most often observed in earlier planting dates in autumn, but can appear throughout the crop's growing season. On older leaves, small, water-soaked spots appear. On veins and petioles, the spots are sunken, pale yellow-brown. In wet weather, they become covered with a pale pink mold of mycelium and sporulation of the fungus. The pathogen persists in the soil and is also transmitted via seeds. Spores are dispersed by water droplets. For these reasons, attacks are more frequently observed in crops whose seedlings were grown in the open.