Prevention and treatment of downy mildew of dry lettuce with yellowing white mildew layer on the back of leaves

Lettuce is one of the early vegetables on the market in early spring. due to low investment, simple cultivation and continuous expansion of planting area, with the increase of planting years, diseases and insect pests are aggravated. Downy mildew is one of the main diseases of lettuce, which seriously damages the leaves, affects the growth and quality of lettuce and affects the income.

Lettuce downy mildew

1 harmful symptoms:

Downy mildew is mainly harmful to lettuce leaves. At the initial stage of the disease, light yellow near round or polygonal disease spots were produced in the lower leaves of lettuce. The white mildew layer on the back of the leaf gradually spread upward, and the disease spot became yellow and brown in the later stage. When the disease was serious, most of the disease spots were connected with one piece, the leaves were yellow and dry, and finally the lettuce surface turned brown and black, and even the whole plant rotted.

(2) the occurrence rule of disease:

The pathogen of lettuce downy mildew is Cladosporium lettuce. The pathogen used oospores, disease remains in soil or mycelia in seeds, or on autumn-sown lettuce as the source of infection at the beginning of next season, and sporangia were produced in the next spring. By means of wind and rain, insect transmission, contact with the host, invade from the host epidermis or stomata, can be repeatedly infected. The occurrence of the disease is closely related to the climate, especially the temperature and humidity. The optimum temperature for sporangium germination is 6: 10 ℃, the optimum temperature is 15: 17 ℃, and the pathogen infection needs more than 85% relative humidity and a certain condensation time, so low temperature and high humidity are conducive to the occurrence of the disease. In the land where the planting is too dense, the population is too large, the nitrogen fertilizer is used too much, and the ventilation and light transmission is poor, the disease occurs seriously.

(3) Prevention and control methods:

3.1 crop rotation: rotation with non-Compositae vegetables for more than 2-3 years in seriously diseased plots and other non-Compositae vegetables for more than 2-3 years. After harvest, the diseased remains were cleared out of the field or shed to reduce bacterial sources.

3.2 strengthen cultivation management: planting density should be reasonable, avoid excessive density, depression in the plant, which is not conducive to ventilation and dampness, and scientific management in the greenhouse, less watering or under-film irrigation. High ridge cultivation, full plastic film mulching, combined use of nitrogen, phosphorus and potassium fertilizer and more organic fertilizer can improve the disease resistance of lettuce.

3.3 Chemical control: before sowing, 0.3%-0.4% thiram or 0.2%-0.3% metalaxyl can be used to kill the germs on the seed surface and reduce the source of infection. In the early stage of the disease, chemical control should be carried out in time to control the spread of germs. Chemical control, you can use 75% Dakening 500 times or 72% frost urea manganese zinc 600 times, or 64% alum 500 times or 69% acylmorpholine 700 times for spray control, 5 times every 7 days, 2 times in a row. The acres of lettuce planted in the shed were smoked with 45% chlorothalonil 350 g fumigant.