How to prevent and control brown spot disease in field-grown lettuce?

Lettuce is the common name for leaf lettuce, belonging to the genus Lactuca in the Asteraceae family. It is an annual or biennial herbaceous crop, and is also a popular vegetable in European and American countries, widely loved by people. Lettuce originated along the Mediterranean coast of Europe, domesticated from wild species. The ancient Greeks and Romans were the first to eat it. Lettuce has a long history of introduction into our country, with more cultivation in the southeastern coastal areas, especially in the suburbs of large cities and the Guangdong and Guangxi regions, and it is especially widely grown in Taiwan, China. In recent years, the cultivation area has expanded rapidly, and lettuce has also moved from hotels and restaurants onto the dining tables of ordinary people.

Growth habits of lettuce

Lettuce prefers a cool environment; it is neither cold-tolerant nor heat-tolerant. The suitable growing temperature is 15°C-20°C, and the growth period is 90-100 days. The seeds are relatively tolerant of low temperatures and can germinate at 4°C. The optimum germination temperature is 18-22°C, and above 30°C they almost do not germinate. During the plant's growth period, it prefers a cool climate, with 15-20°C being the most suitable for growth, giving high yield and excellent quality; when continuously above 25°C, growth is poorer, the leaf quality is coarse and old, and there is a slight bitterness. However, its cold tolerance is also quite strong, and 0°C or even short-term subzero low temperatures do not greatly hinder growth. Lettuce has a well-developed root system and a waxy leaf surface, and its drought tolerance is quite strong, but when cultivated in fertile, moist soil, the yield is high and the quality is good. The suitable soil pH is 5.8-6.6.

Main types of lettuce

According to leaf growth form, lettuce can be divided into head lettuce, curled-leaf lettuce and upright lettuce.

Head lettuce: The main characteristic of head lettuce is that its terminal leaves form a head. The leaves are entire, serrated or deeply lobed, the leaf surface is smooth or wrinkled, and the terminal leaves form a head that is round or nearly round. The main varieties include Dahu 659, Qianwei 75, Salinas, Beishan No. 3, Mamike, Bilu and so on.

Curled-leaf lettuce: The main characteristic of curled-leaf lettuce is that it does not form a head. The basal leaves are long ovate, the petioles are relatively long, the leaf margins are wavy with notches or deep lobes, the leaf surface is wrinkled, and the clustered leaf rosette resembles a large flower. The main varieties include “Hongfan” purple-leaf lettuce, red-leaf lettuce, Lubo, Dongshan lettuce and so on.

Upright lettuce: The main characteristics of upright lettuce are narrow, long leaves that grow upright, entire or serrated leaf margins, thick leaves, relatively coarse texture and poorer flavor. This variety has fewer cultivars, is not widely distributed, and is rarely grown in China.

Prevention and control of brown spot disease in field-grown lettuce

Introduction to lettuce brown spot disease

Lettuce brown spot disease is a fungal disease caused by infection by Cercospora lactucae or Cercospora musae of the Deuteromycotina, and it mainly damages the leaves. The lesions on the leaves show two kinds of symptoms: one is water-soaked at the early stage of onset, then gradually expands into round to irregular shapes, with brown to dark gray lesions, 2-10 mm in diameter; the other is dark brown lesions with irregular edges and a water-soaked halo on the outer margin. When environmental humidity is high, dark gray mold-like substances grow on the lesions, and in severe cases the lesions merge with each other, causing the leaves to turn brown and dry up. The first kind of brown spot symptom is caused by Cercospora lactucae, while Cercospora musae causes the second kind of brown spot symptom. The pathogen overwinters on diseased residues as mycelium and conidial clusters, and conidia carry out primary infection and re-infection, spreading and expanding by airflow and rain splash. During continuous rainy and snowy weather, when plants grow poorly, or when excessive nitrogen fertilizer is applied causing overly vigorous growth, the disease will occur and worsen.

Prevention and control measures for brown spot disease in field-grown lettuce:

1. Clean the field, and promptly carry diseased residues out of the field and burn them.

2. Fertilize rationally. Adopt formula fertilization techniques, apply more organic fertilizer and phosphorus and potassium fertilizers, avoid excessive nitrogen fertilizer, so that the plants grow robustly and disease resistance is enhanced.

3. Chemical control. At the early stage of onset, spray 40% carbendazim-sulfur suspension concentrate at 500 times dilution, or 75% chlorothalonil wettable powder at 1000 times dilution plus 70% thiophanate-methyl wettable powder at 1000 times dilution, or 50% iprodione wettable powder at 1200-1500 times dilution, or 60% fosetyl-aluminum wettable powder at 500 times dilution, etc., spraying once about every 10 days, for 2-3 consecutive treatments. Stop using the chemicals 5-7 days before harvest.