How to Achieve Good Control of Downy Mildew in Rapeseed
Rapeseed, also called oil cabbage or bitter cabbage, Latin name: Brassica campestris L., a plant of the Brassicaceae family and Brassica genus, native to our country, its stem is dark green, and its petiole is like that of Chinese cabbage; it is a variety of Chinese cabbage in the Brassicaceae family, and its flowers are yellow. In agronomy, the various species whose seeds contain oil are collectively called rapeseed.
Morphological characteristics of rapeseed
Rapeseed is an annual herb of the Brassicaceae family and Brassica genus, with a taproot system, an erect stem, relatively few branches, and a plant height of 30-90 cm. The leaves are alternate and divided into basal leaves and stem leaves. The basal leaves are not well developed, grow prostrate, are elliptical, 10-20 cm long, have petioles, are lyrate-pinnatifid, with the terminal lobe round or ovate, and 5 pairs of lateral lyrate lobes, densely covered with bristles and with a waxy bloom. The stem leaves and branch leaves have no petioles; the lower stem leaves are pinnatifid, with an expanded base that clasps the stem, and both surfaces have stiff hairs and marginal hairs; the upper stem leaves are fiddle-shaped or lanceolate, with a cordate base, clasping the stem, with auricles on both sides, and entire margins or branched fine teeth. The raceme is an indeterminate inflorescence, borne at the top of the main stem or branches. The flowers are yellow, with 4 petals, typically cruciform. There are 6 stamens, tetradynamous. The silique is linear, 3-8 cm long and 2-3 mm wide, with a beak 9-24 mm long at the tip, and a pedicel 3-15 mm long. The seeds are spherical and purplish brown. Chromosome number: 2n=20. The fruit is a silique, composed of two valves, with a septum in the middle, and about 10 seeds on each side; the seed color is dark red, black, or yellow, varying among different varieties.
How to control downy mildew in rapeseed?
Rapeseed downy mildew is an important disease in all rapeseed regions of China, and damage is severe in the Yangtze River basin and the southeast coast.
Symptoms of rapeseed downy mildew:
In spring rapeseed regions, the disease occurs less and is mild. The disease mainly damages leaves, stems, and siliques, causing the affected parts to turn yellow and bear white mold-like growth. When pedicels are infected, the top swells and bends, forming a “dragon-head crook” shape, the petals become thick and green, do not set seed, and white downy mildew-like growth appears on them. When leaves are infected, small light green spots first appear, then expand into polygonal yellow patches, and white mold grows on the underside of the leaves.
Conditions for the occurrence of rapeseed downy mildew:
In winter rapeseed regions, the pathogen oversummers as oospores with diseased residues in the soil, in manure, and inside seeds; after germination in autumn, it infects seedlings, and sporangia are produced on the lesions for secondary infection. In winter, the disease spreads slowly and overwinters as mycelium in diseased leaves; the following spring, as the temperature rises, sporangia are again produced and spread by wind and rain to reinfect leaves, stems, and siliques. When rapeseed enters maturity, the diseased parts again produce oospores, and multiple secondary infections can occur. Long-distance spread mainly relies on oospores mixed in seeds. As for short-distance spread, in addition to oospores mixed in seeds and manure being directly carried to diseased fields, it mainly relies on airflow and irrigation water or rainwater; sporangia, because the sporangiophores shrink and twist, are discharged from the tips of the small stalks into the air and carried by airflow to healthy plants, with a spread distance of 8-9 m, and oospores on residues in the soil flow with water, and the sporangia produced after germination splash with rainwater onto healthy seedlings. The optimum temperature for sporangium formation is 8-21degC, the optimum temperature for infection is 8-14degC, and the relative humidity is 90%-95%. It has been reported that below 15degC germination occurs after 4-6 hours, and appressoria form after 12 hours.
When the light period is less than 16 hours, infection can occur at the cotyledon stage of seedlings, and the degree of infection is positively correlated with the number of sporangia. When sporangia land on a susceptible host, under suitable temperature they first produce germ tubes, form appressoria, and then grow infection hyphae, penetrating directly through the cuticle, and sometimes also entering through stomata. After the hyphae invade, they extend 7-8 μm and grow in the middle lamella region between the anticlinal walls of epidermal cells, then branch in all directions between cells, and within host cells they produce haustoria. Observation under an electron microscope found that the cell wall initially in contact with the hyphae swells locally, showing a microfibrillar structure; the branches of the haustorium invade through pores 1-2 μm in size, and a callose-like ring structure forms around the base of the haustorium; when the haustorium swells, it produces sheath formation by the host plasma membrane. After the nutrients in the cell are consumed, the cell begins to die, and the tissue turns yellow or dies. The occurrence of this disease is closely related to climate, variety, and cultivation conditions. A temperature of 8-16degC, relative humidity above 90%, and weak light favor infection by this fungus. In production, low temperature and frequent rain, high humidity, and few sunshine hours favor disease occurrence. In the rapeseed regions of the Yangtze River basin, winter temperatures are low and there is little rain, so the disease is mild; in spring, temperatures rise, there is more rain, and field humidity is high, making the disease likely to occur or causing an epidemic during the bolting and flowering stages. Continuous cropping land, early sowing, excessive or over-application of nitrogen fertilizer or plots lacking potassium, as well as plots with high density and retained field moisture, are prone to the disease. Low-lying land, poor drainage, and planting of cabbage-type or mustard-type rapeseed result in severe disease.
Control methods for rapeseed downy mildew:
1. Chemical seed dressing: Use 25% metalaxyl, 50% thiram, or 75% chlorothalonil for seed dressing.
2. Agricultural control: Fertilize and water reasonably, promptly remove severely diseased leaves, and maintain good field sanitation after harvest.
3. Chemical control: 40% phosethyl-Al, 25% metalaxyl, or 75% chlorothalonil can be used for spraying, focusing on spraying the underside of leaves.