How to control downy mildew of rape

Rape, also known as rape, sowthistle, Latin name: Brassica campestris L. Cruciferae, Brassica plants, native to China, its stem color dark green, such as cabbage, is a variety of cruciferous cabbage, flowers are yellow. In agronomy, a number of species with oily seeds in plants are collectively referred to as rape.

Morphological characteristics of Brassica napus

Rape belongs to Cruciferae, Brassica, annual herb, erect root system, erect stem, few branches and plant height 30~90cm. Leaves alternate, divided into basal leaves and cauline leaves. Basal leaves underdeveloped, creeping, elliptic, long 10~20cm, petiolate, pinnately divided, terminal lobe orbicular or ovate, lateral piano-shaped lobes 5 pairs, densely prickly, waxy. Cauline leaves and branched leaves without petiole, lower cauline leaves pinnatifid, base extended and clasping, with bristles and ciliate on both sides; upper cauline when violin-shaped or lanceolate, base cordate, clasping, with drooping ears on both sides, entire or branched denticulate. Inflorescences racemose, inserted on apices of main stem or branches. Yellow flowers, petals 4, is a typical cross type. There are 6 stamens and 4 strong stamens. Siliques strip-shaped, long 3~8cm, wide 2~3mm, apex with long 9~24mm beak, fruiting pedicel long 3~15mm. Seeds globose, purple-brown. Cell chromosome: 2n=20. The fruit is a long pod, which is composed of two pods, with a diaphragm in the middle and about 10 seeds on each side. The color of the seeds is crimson or black or yellow, and the colors of different varieties are different.

How to prevent and cure rapeseed downy mildew?

Downy mildew of rapeseed is an important disease in all rape areas in China, which is seriously damaged in the Yangtze River Basin and the southeast coast.

Symptoms of rape downy mildew:

The incidence of spring rape is less and mild. The disease is mainly harmful to leaves, stems and pods, causing the injured parts to turn yellow and grow white mildew. The top of the pedicel is swollen and curved, showing a "bibcock" shape, petals thickening and turning green, not strong, white downy mildew on the top. The leaves infected with the disease appeared light green spots at first, then expanded into polygonal yellow patches, and white mold grew on the back of the leaves.

The incidence conditions of rape downy mildew:

In winter rape area, the pathogen spent summer in soil, manure and seeds with oospores. After germination in autumn, the seedlings were infected, and sporangia were produced on the disease spot for re-infection. The disease did not spread rapidly in winter, and the hyphae overwintered in the diseased leaves, and the temperature increased in the next spring, and sporangia spread through wind and rain to infect leaves, stems and pods again. Rape entered the mature stage, and oospores were produced in the diseased part, which could be re-infected many times. Long-distance transmission mainly depends on oospores mixed in seeds. As for close transmission, except the oospores mixed in seeds and manure were transmitted directly to the diseased field, they were mainly transmitted by air flow and irrigation water or Rain Water. The sporangia were radiated from the top of the peduncle to the air and spread to the healthy plants with a distance of 89.9 m. The oospores on the remnants of the soil flowed through the current, and the sporangia produced after germination were sputtered to the healthy seedlings with Rain Water. The optimum temperature for sporangia formation was 8: 21 ℃, the optimum temperature for infection was 8: 14 ℃, and the relative humidity was 90%-95%. It was reported that sporangia germinated in 4-6 hours and attached spores formed in 12 hours below 15 ℃.

When the light time is less than 16 hours, the seedling can be infected at the cotyledon stage, and the degree of infection is positively correlated with the number of sporangium. The sporangium falls on the susceptible host, and the temperature is suitable to produce germ tubes to form appressorium and then grow invasive filaments, which can invade directly through the stratum corneum. Sometimes it can also invade through stomata, expand 7 ~ 8 μ m after hyphae invasion, grow in the middle glue layer between the anticlinal walls of epidermal cells, and then branch in all directions between the cells. Haustorium grows again in the host cell. Under electron microscope, it was found that the cell wall initially in contact with the hyphae expanded locally and the microfiber structure appeared, and the branches of the haustorium invaded through holes with a size of 1 ~ 2 μ m, forming a bacterioid ring structure around the base of the haustorium. Sheath formation of the host protoplast membrane. When the nutrients in the cells are consumed, the cells begin to die, showing that the tissue turns yellow or withered. The occurrence of the disease is closely related to climate, varieties and cultivation conditions. The temperature is 8-16 ℃, the relative humidity is higher than 90%, and the weak light is favorable for the infection of the bacteria. In production, low temperature and rainy, high humidity and less sunshine are conducive to the occurrence of diseases. In the rape area of the Yangtze River basin, the temperature is low in winter, the incidence of Rain Water is light, the temperature rises in spring, Rain Water is more, and the high humidity in the field is easy to cause the disease or lead to the epidemic of the disease at the bolting stage. Continuous cropping land, early sowing, partial application of nitrogen fertilizer or potassium deficiency, high density and moisture retention in the field are prone to disease. Low-lying land, poor drainage, planting Chinese cabbage type or mustard type rape is serious.

Control methods of rape downy mildew:

1. Seed dressing: choose 25% metalaxyl, 50% thiram or 75% chlorothalonil.

2. Agricultural prevention and control: reasonable fertilization and watering, timely removal of seriously diseased leaves, and good pastoral hygiene after harvest.

3, chemical control: you can choose 40% carbendazim, 25% metalaxyl, 75% chlorothalonil spray, focus on spraying leaf back.