I. Downy Mildew of Cruciferous Vegetables 
( I ) Symptom Identification 
Downy mildew of cruciferous vegetables can occur throughout the entire fertility period. The disease is severe at the late stage of adult plants, mainly damaging leaves, and then, stems and f lowers, etc. Downy mildew is commonly characterized by leaf damages, which initially produces yellowish-green to brown spots at the topside of leaves; after expansion, the spots shape into polygons and irregular forms due to the limitation of leaf veins; in severe cases, the spots are connected to form pieces, causing part of or the entire leaf to die; injury to pedicels and floral organs often causes hypertrophy and deformity; when humidity is high, white powdery mildew layers can occur on diseased parts.After the onset of the disease, white powdery mildews can appear at the underside of leaves of Chinese cabbages (Brassica rapa pekinensis), while the 
syndromes at the topside of leaves are not obvious. The damaged leaves are dried on one layer after another from outside to inside on the adult plants, in severe cases, leaving only the leafy heads of the plants. Diseases can occur during both the seedling stage and adult stage of common cabbages [Brassica pekinensis (Lour.) Rupr.]. Chlorotic spots with unclear margins initially appearing on the foliage can turn into yellowish-brown polygonal spots after the expansion due to the limitation of leaf veins. Powdery mildews appear at the underside of the spots. The diseased 
apical buds and pedicels of seed strains become swollen and deformed, which are called the “dragon staff” by farmers. The diseased seedpods also deform to varying degrees, resulting in poor seeding. 
After the onset of the disease, spots that are slightly concave, black to purplish_x005fblack, and dotted and irregular in shape can appear at the topside of leaves of wild cabbages (Brassica oleracea) and cauliflowers (Brassica oleracea var. botrytis Linnaeus), while white powdery mildews appear on the spots at the underside of leaves. After the heads of caulif lowers are damaged, the tips turn black and the blackened parts spread to the entire head, which makes the head inedible. Brown to blackish-brown irregular spots and slightly sagged black spots respectively appear on the diseased tuberous roots of turnips (Brassica rapa subsp. rapa) and radishes (Raphanus raphanistrum subsp. sativus). The spots on the stems of bolting plants are the same. 
( II ) Pathogen Identification 
The pathogen causing downy mildew to cruciferous vegetables is parasitic downy mildew [Peronospora parasitica (Pers.) Fr.], which belongs to Peronospora of Oomycota in Chromista. The aseptate mycelium is colorless, entering host cells to absorb nutrients. Asexual propagation produces sporangia, which grow on the sporangiophores. The sporangiophores protrude from the stomata, branching in the shape of binary trees. The sterigma at the top is pointed, bending inward in the shape of pincers. The sporangia are colorless, amerosporous, oblong oval to oval-shaped, producing germ tubes directly at the time of germination. Sexual propagation produces oospores, which can be formed in diseased leaves, stems, flower stalks, and pods, especially in thick tissues such as flower stalks, etc. The oospore is yellow to yellowish-brown, nearly spherical, thick on the walls, smooth or wrinkled on the surface, and produces germ tubes at the time of germination.
( III ) Occurrence Regularity 
Downy mildew occurs mainly in spring and autumn. In northern China, parasitic downy mildew mainly stays in the soil with the oospores and diseased residues or the root tubers of radishes and turnips with spores and dormant hyphae for oversummering and overwintering. Vegetables infected in spring, e.g., bok choy (Brassica rapa subsp. chinensis), rapes, and radishes, produce oospores in the disease tissues at the middle and late stage of the infection, which can germinate in the autumn of that year and infect autumn vegetables, becoming the initial source 
of infection in the northern region. In southern China, pathogens can also stay in host cruciferous vegetables in the field with mycelia for overwintering. Under appropriate conditions, mycelia can form sporangiophores that protrude from the stomata and produce sporangia. Both oospores and sporangia are the initial source of infection for diseases in these areas. In Guangdong, where vegetables are planted all year round, pathogens are transmissible on various host crops throughout the year, so overwintering is not taken into consideration. The sporangia produced on these diseased plants become the initial source of infection for local diseases. The oospores and sporangia can be transmitted to the host through wind, rain, or agricultural tools, germinating and producing germ tubes that enter the host tissues from stodiseasemata or epidermis and develop into mycelia. The mycelia expand in gap junctions, causing lesions in the host tissues. Afterward, the hyphae produce sporangiophores that protrude from the stomata. Sporangiophores produce sporangia. The sporangia are transmitted by wind, rain, or airf low for secondary infection. In a growing season, secondary infection occurs many times, causing the diseases to spread. At the late stage of infection, the hyphae in the diseased tissues produce oogonia and antheridia, and the eggs in the oogonia become oospores after being fertilized, and then, the pathogens overwinter and oversummer with oospores or dormant hyphae. 
Temperature and humidity greatly influence the occurrence and epidemic of downy mildew, among which temperature determines the occurrence time and developmental speed of the diseases, and rainfall determines the severity of disease development. The production and germination of sporangia are most suitable at a lower temperature of 7–13℃. The appropriate temperature for invasion into the host is 16℃. After the invasion, the growth of mycelia in the host requires a higher temperature of 20–24℃. Therefore, an epidemic is most likely to occur in rainy weather lasted for more than f ive consecutive days with an average temperature of 16℃ and relative humidity higher than 70%. High humidity and water droplets are beneficial to the formation, germination, and invasion of sporangia. Therefore, diseases are often severe in rainy conditions. Generally, during the rosette and folding stages of Chinese cabbages, if the weather is hot, the rainfall is heavy, or the f ield temperature is high, the temperature difference between day and night is large, or the weather is sometimes hot and sometimes cold, and it’s dewy and foggy, the diseases are most likely to cause epidemics. Diseases can easily occur on seed strains during anthesis on rainy days. Generally, diseases mostly occur on vegetables planted in late autumn and early spring. Diseases occurring on early sown autumn cabbages, especially under a high temperature and heavy rainfall, are usually severe; situations such as poor ventilation in the field, continuous cropping, insufficiency of bottom fertilizer, late seedling thinning, large density, and the lack of fertilizer at the folding stage can also lead to severe diseases. In addition, there are certain differences in disease resistance between cultivars. The cultivars of straight and slight wrappers are less affected by diseases, while the ones of spherical and central type have severe syndromes; the cultivars with white stems that have a tender and juicy taste also have severe syndromes, while the ones with green stems are less affected by diseases.