What are the control methods for powdery mildew on Schisandra chinensis?

1. Disease symptoms: Powdery mildew damages the leaves, fruits and new shoots of Schisandra chinensis, with young leaves and young fruits suffering the most severe damage. It often causes leaves to dry up, new shoots to die back and fruits to drop. In the early stage of leaf infection, needle-like spots appear on the underside of the leaves, gradually covered with a layer of white powdery substance, and in severe cases it spreads to the entire leaf. Diseased leaves turn from green to yellow, curl upward, wither and drop. Young fruit infection starts near the rachis, and in severe cases gradually spreads outward to the entire fruit cluster; diseased fruits become wilted and drop, and dark brown spots appear on the fruit stalk and new shoots. In the late stage of the disease, large numbers of small black dots are produced on the midrib, veins, petioles and new shoots on the underside of the leaves; these are the cleistothecia of the pathogen.

2. Disease occurrence pattern: It has been identified that the disease is caused by a fungus of the genus Trichocladia in the subphylum Ascomycotina; the sexual state of the pathogen is Trichocladia schisandrae, an ectoparasitic fungus, and the white powdery substance on the diseased parts is the mycelium, conidia and conidiophores of the pathogen. The mycelium grows on both sides of the leaves and also on the petioles; conidia are solitary, colorless, elliptical, oval or nearly cylindrical; cleistothecia are scattered to clustered, oblate-spherical, dark brown, with mostly 10-14 appendages, which are 0.8-1.5 times the diameter of the cleistothecium, 8.0-14.4 μm thick at the base, straight or slightly curved, occasionally geniculate. The outer wall is rough at the base and gradually smooth upward, without septa or with 1 septum below the middle in a few cases, colorless or light brown at the base and below the septum, with the apex mostly 5-6 times dichotomously branched; asci number 4-8, elliptical, oval, broadly oval; ascospores number 5-7, colorless, elliptical, oval.

High temperature and drought conditions favor the onset of the disease. In Northeast China, the initial onset occurs from late May to early June, and the peak period is reached in late June (if there is no drought and high temperature weather, onset mostly occurs in early to mid-July). Judging from the disease occurrence in plants, environmental conditions such as overly dense branches and vines, excessive vegetative growth, excessive nitrogen fertilizer application and poor ventilation favor the occurrence of the disease. The pathogen overwinters as mycelium, ascospores and conidia in diseased plant residues in the field. From mid-May to early June of the following year, when the average temperature rises back to 15-20°C, the conidia overwintering on the field residues begin to activate, and with the help of rainfall and dew they begin to germinate, infecting the plants, and the disease begins in the field. Mid-July is the peak period for conidial dispersal, the diseased leaf rate and diseased stem rate rise sharply, and fruits become heavily infected. In mid-October the temperature drops noticeably, Schisandra chinensis leaves senesce and fall, the diseased residues scatter in the field, and the pathogen carried on the residues enters the overwintering dormant period. Under natural conditions, the overwintering pathogen produces conidia that spread through airflow and continuously cause re-infection, allowing the disease to develop; infected seedlings and fruits are transported by vehicles, ships and other means of transport, causing long-distance spread of powdery mildew, which is the most important route of transmission of the disease.

3. Control techniques. (1) Strengthen cultivation management, pay attention to the rational distribution of branches and vines, and improve ventilation and light conditions on the trellis surface through pruning; appropriately increase the proportion of phosphorus and potassium fertilizer to improve the disease resistance of the plants and enhance tree vigor. (2) Eliminate sources of inoculum, clear diseased branches and leaves before sprouting, promptly prune diseased clusters in the early stage of the disease, pick up fallen diseased fruits, and burn or bury them deeply in a centralized manner to reduce sources of pathogen infection. (3) Chemical control: spray 1:1:100 equal-strength Bordeaux mixture in late May for prevention; if no disease occurs, it can be sprayed once every 7-10 days; choose 0.3-0.5 degree Baumé lime sulfur mixture, or 25% triadimefon wettable powder 800-1000 times dilution, or thiophanate-methyl wettable powder 800-1000 times dilution, spraying once every 7-10 days for 2-3 consecutive sprays, with very good control effect; choose 40% sulfur suspension concentrate 400-500 times dilution, or 15% triadimefon emulsifiable concentrate 1500-2000 times dilution for spraying, or 25% azoxystrobin aqueous suspension concentrate 1500 times dilution, or 50% kresoxim-methyl dry suspension concentrate 3000-4000 times dilution for spraying, once every 7-10 days for 2 consecutive sprays; fungicides such as Xiansheng, myclobutanil and Cuibei can also be used for control.