Detailed Guide to Symptom Identification and Control Methods for Danshen Phytophthora Blight
Danshen (red sage), scientific name Salvia miltiorrhiza Bunge, also known as Chishen. It belongs to the Lamiaceae family as a perennial herb. Its roots are used as medicine and have effects such as removing blood stasis and promoting tissue regeneration, activating blood circulation and regulating menstruation, and nourishing the heart and relieving restlessness. It is produced in Zhejiang, Anhui, Sichuan, Liaoning, Inner Mongolia, Hebei, Henan, Shandong, Jiangsu, Jiangxi, Hubei, Guizhou, Yunnan and other provinces (regions).
Symptoms In the early stage of the disease, the lower leaves of the aboveground plant turn yellow, and the top leaves lose water and wilt at noon under high-temperature weather, but can still recover in the morning and evening. In the later stage, the leaves of the whole plant wilt and do not recover, and rotting begins from the base. When the underground part is dug out, the epidermis of the main root of the plant can be seen to be water-soaked and slightly turns light yellow; when the epidermis is peeled off, the pith can be seen to be completely rotted. The rotted pith does not change color, has a slight foul odor, and is in a wet rot state. This type of disease spreads relatively quickly. Generally, infected plants can die within 1—2 weeks. When the soil is moist, it can occur shortly after planting, spreads very quickly, and white mold-like substances, namely the mycelium or fruiting bodies of the pathogen, can intermittently appear on the roots.
Pathogen Phytophthora cactorum (Leb. et Cohn) Schroet., called Phytophthora cactorum, belongs to the Mastigomycotina fungi. On V8 juice medium, the aerial mycelium is sparse, light-colored, and less branched. Young hyphae are colorless, non-septate, relatively thin, 2—4μm wide. Older hyphae are thicker, slightly light yellow, and sparse septa may appear in the later stage, 5—6μm wide, with branches at right angles; the sporangiophores are indistinguishable from the hyphae, sympodially branched, and the sporangia are terminal or intercalary, relatively numerous, with no layering observed; sporangia are pyriform or nearly spherical, 20-34×18—28 (μm), length-to-width ratio 1-1.2, sporangial papillae obvious or not obvious, single or double, especially intercalary sporangia often having double papillae, papillae 0.5—6μm high and 3.5—6μm wide. Mature sporangia easily detach, with a nearly round base, without a pedicel or with a pedicel, pedicel 0.5—3μm long. Sporangia germinate easily in water and release zoospores. Zoospores are kidney-shaped and nearly spherical when resting. Homothallic. Oogonia are numerous, spherical, light yellow, with a non-smooth surface, 18—26 (21) μm in diameter. Antheridia are club-shaped, mostly paragynous, lateral, relatively small, 6—10×4—7 (μm); oospores are spherical, light yellow, nearly filling the oogonium, 14—20μm in diameter, with a wall 1.5—4μm thick. The optimum growth temperature is 23—28℃, and chlamydospores are not formed. On CMA and PSA, the mycelium is luxuriant and can form sporangia, oogonia, and antheridia, but with great variation.
Transmission Routes and Disease Conditions The pathogen overwinters in the soil as mycelium and oospores with diseased residue tissue. The following year, when the mycelium or oospores encounter water, they produce sporangia and zoospores, which spread to Danshen through irrigation water and rainwater, germinate into germ tubes, produce appressoria and penetration hyphae that penetrate the epidermis and enter the host body. Under high-temperature and high-humidity conditions, lesions appear in 2—3 days, producing large numbers of sporangia on them, which spread and multiply through wind, rain, or irrigation water, causing repeated infections many times. The pathogen produces sporangia in two ways: first, short hypha-like sporangiophores emerge from the stomata, and sporangia form at the tips. Second, hyphae emerge from the stomata, branch, and produce hypha-like sporangiophores on the branches, with sporangia forming at the tips. The sporangia release zoospores that rest on the leaf surface for 2 hours before germinating, or the sporangia germinate directly to produce germ tubes, beginning to invade from the gaps between the guard cells of the stomata. The hyphae spread between and within the leaf cells, and some hyphae extend from the stomata and then invade through the stomata or spread on the leaf surface. After several days of incubation, symptoms appear. The severity of the disease is related to the timing of the rainy season, the temperature, and the amount of rainfall. In years when the disease occurs early and the temperature is high, the disease is severe. Generally, the disease begins when the rainy season starts, and spreads rapidly or causes an epidemic when heavy rainstorms occur. Continuous cropping or flat-bed cultivation in production makes the disease more likely to occur, and long-term flood irrigation, frequent watering, and large water volumes make the disease severe.
Control Methods
1. Implement crop rotation for more than 3 years.
2. Use high-bed cultivation, which can reduce contact with the pathogen. If plastic film mulching can be used, the effect is even better.
3. Strengthen water and fertilizer management, apply compost fermented with effective microorganisms, increase the application of phosphorus and potassium fertilizers, appropriately control nitrogen fertilizer, and use formula fertilization technology where conditions permit. Immediately pull out diseased plants when found and spread quicklime for disinfection.
4. After the appearance of central diseased plants, promptly spray 72% cymoxanil mancozeb (Kekangling) wettable powder 800 times solution, or 72% DuPont Kexiu or 72% Keshuangqing wettable powder 800-1000 times solution, 56% Kaoshan water-dispersible microgranules 800 times solution, 18% metalaxyl·mancozeb wettable powder 600 times solution, 70% fosetyl·mancozeb wettable powder 500 times solution, spraying 50L of the prepared spray solution per 667 m2, once every about 10 days, and applying 2—3 times depending on the disease condition.