Control Methods and Technical Key Points for Litchi Downy Blight

Litchi downy blight can harm litchi from young fruit to ripe fruit, fruit stalks, and the small bearing branches; when humidity is high, leaves also develop the disease.

Symptoms: local chlorosis on leaves, with white mold on the underside, the fruiting bodies of the pathogen, but usually at the peak period of this disease, the leaves have already aged, so leaf damage in the field is rarely seen. When ripe fruit is affected, the lesions are irregular with no obvious margins, and white mold grows when moist; the lesions spread rapidly, often turning the whole fruit brown, the flesh becomes sour, and later rots into a pulp, exuding brown liquid. After young fruit is affected, it quickly drops off, and white mold grows on the diseased area. Etiology and principle: Peronophythora litchii Chen ex Ko et al., called litchi downy blight fungus, belongs to the Mastigomycotina fungi. The sporangiophores are 5-7 PyA wide and 92-1332μm long, but on corn meal agar medium, the sporangiophores are extremely long, even exceeding l000μm, colorless, septate, with the apex branching singly 2-3 times, then dichotomously branching at the branch tips, the terminal branchlets slightly pointed at the tip, on which 1 sporangium forms at the same time. On corn meal medium, new sporangiophores mostly arise at a distance from the apex of the previous sporangiophore, or continue to form in the axils of branches; sometimes a terminal branchlet does not produce a sporangium but forms a new sporangiophore, or pushes the sporangium to one side while extending the sporangiophore, and its apex branches, thus forming sporangia at the same time; each sporangiophore bears more than ten sporangia. Sporangia are lemon-shaped, colorless or slightly pale yellow, with a papilla at the apex, size 17-40×15-24(μm); after detachment they carry a small stalk at the base, and upon germination generally form zoospores that escape from the apical papilla. On corn meal agar medium, oogonia are spherical, colorless, with thin and smooth walls, 23-33×17-27(μm); antheridia are mostly amphigynous, a few lateral, colorless; oospores are spherical, colorless to pale yellow, with smooth walls, and can also form oospores on host branchlets in the later stage, but this is relatively rare. Disease conditions: the pathogen may overwinter as mycelium on diseased fruit and diseased leaves. In late spring and early summer of the following year, it produces sporangia that spread with wind and rain, and the sporangia produce zoospores that invade host leaves or fruit and cause damage. Humidity is the key factor for the occurrence and epidemic of this disease. During the plum rain season, low-lying litchi orchards, poor drainage, dense branches and leaves, and poor ventilation can all aggravate the occurrence of this disease. In addition, nearly mature fruit is susceptible, and early and mid-season varieties (such as “Dingfeng”, “Heiye”, etc.) are susceptible. In Guangzhou, from the end of April to early June each year, the average temperature in April and May is 21-24°C, in early June 25°C, and the minimum temperature is 11-21°C; the pathogen harms litchi by means of zoospores, the infection process is short, and reinfection is frequent, so it often becomes epidemic in production. Control methods: (1) Eliminate the primary infection source. In late autumn and early winter, in conjunction with clearing the orchard, thoroughly remove residual leaves, residual fruit, fallen leaves, and fallen fruit in the orchard, and burn them in a centralized manner. (2) Strengthen orchard management. Pay attention to drainage in low-lying waterlogged areas, and appropriately prune excessive vegetative branches to enhance air permeability. (3) Chemical control: at the flower bud stage, young fruit stage, and before fruit ripening, spray 72% Kelkangling wettable powder 800-fold solution, 70% ethylicin-mancozeb wettable powder 500-fold solution, or 58% metalaxyl-mancozeb wettable powder 800-fold solution, 75% chlorothalonil wettable powder 600-fold solution, 90% fosetyl-aluminum wettable powder 500-fold solution, 64% sandofan wettable powder 400-fold solution, 60% Baijuntong (fosethyl-aluminum) wettable powder 500-fold solution. Apply once every about 10 days, for 3-4 treatments.