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Symptoms and Control Methods of Potato Powdery Scab

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[Pathogen scientific name] Spongospora subterranea (Wallr) Lagerh. [Host crops] Potato, black nightshade and Nicotiana rustica. [Disease diagnosis] It mainly affects underground tubers and roots, and above-ground stems can also be infected. When tubers are infected, pinhead-sized brown spots first appear on the epidermis, surrounded by a translucent halo, which then gradually expand and rise, forming rash-like "blister spots" 3 to 5 mm in diameter. In the early stage, the epidermis of the lesions does not rupture, which is the "closed blister" stage; as the disease progresses, a suberized halo forms at the boundary between diseased and healthy tissue, and then the epidermis ruptures and curls back, the tissue beneath turns orange-red, and a large amount of dark brown powdery material is released, the blister spots collapse to reveal cavities, forming crater-shaped powdery scabs, which is the "open blister" stage. On tubers that have already formed powdery scab, the ruptured epidermis can cause water loss during storage, or infection by other pathogens can lead to dry rot and shriveling. When roots are affected, tumors the size of a bean, single or clustered, grow on one side of the root; in severe cases, young plants wilt and die. When above-ground stems are infected, the lesion characteristics resemble those of affected tubers. The causal pathogen is Spongospora subterranea. The brown powdery material released from the blister spots of the diseased parts consists of the pathogen's resting spore balls. The spore balls vary in size and are formed by the aggregation of many nearly spherical yellow-brown resting spores, with hollow cavities inside, appearing like spongy spheres, and pore-like structures can be seen under the microscope. Individual resting spores are spherical to polygonal, with thin, smooth walls, and germinate to produce zoospores. The zoospores are nearly spherical, without a cell wall, bearing two flagella of unequal length at the apex, and later become motionless and form amoebae that invade the host root hairs.

[Pathogen scientific name] Spongospora subterranea (Wallr) Lagerh.

[Host crops] Potato, black nightshade and Nicotiana rustica.

[Disease diagnosis] It mainly affects underground tubers and roots, and above-ground stems can also be infected. When tubers are infected, pinhead-sized brown spots first appear on the epidermis, surrounded by a translucent halo, which then gradually expand and rise, forming rash-like "blister spots" 3 to 5 mm in diameter. In the early stage, the epidermis of the lesions does not rupture, which is the "closed blister" stage; as the disease progresses, a suberized halo forms at the boundary between diseased and healthy tissue, and then the epidermis ruptures and curls back, the tissue beneath turns orange-red, and a large amount of dark brown powdery material is released, the blister spots collapse to reveal cavities, forming crater-shaped powdery scabs, which is the "open blister" stage. On tubers that have already formed powdery scab, the ruptured epidermis can cause water loss during storage, or infection by other pathogens can lead to dry rot and shriveling. When roots are affected, tumors the size of a bean, single or clustered, grow on one side of the root; in severe cases, young plants wilt and die. When above-ground stems are infected, the lesion characteristics resemble those of affected tubers. The causal pathogen is Spongospora subterranea. The brown powdery material released from the blister spots of the diseased parts consists of the pathogen's resting spore balls. The spore balls vary in size and are formed by the aggregation of many nearly spherical yellow-brown resting spores, with hollow cavities inside, appearing like spongy spheres, and pore-like structures can be seen under the microscope. Individual resting spores are spherical to polygonal, with thin, smooth walls, and germinate to produce zoospores. The zoospores are nearly spherical, without a cell wall, bearing two flagella of unequal length at the apex, and later become motionless and form amoebae that invade the host root hairs.

[Disease occurrence pattern] The pathogen overwinters as resting spore balls in seed tubers or in the soil with diseased plant residues, becoming the primary infection source for the following year. Long-distance spread of the disease occurs through the transport of seed tubers, while in the field it spreads over short distances through diseased soil, diseased manure and irrigation water. The pathogen invades the host through root hairs, lenticels and wounds. Infected tissues readily disintegrate, allowing the pathogen to fall into the soil to overwinter and spread. When soil moisture is around 90%, temperature is 18 to 20°C, and pH is 4.7 to 5.4 (slightly acidic), it is suitable for pathogen development and conducive to disease occurrence. In years and regions with abundant rainfall and cool summers, the disease occurs severely. The severity of this disease mainly depends on the amount of primary infection; the impact of secondary infection in the field is relatively unimportant.

[Control measures] (1) Strict quarantine system. Block off diseased areas, control the transport quarantine checkpoint, and prohibit the entry and exit of diseased seed tubers. (2) Select disease-free seed tubers. Establish disease-free seed production fields; control the harvest, storage and sowing stages, and remove diseased tubers. (3) Ensure proper disinfection of seed tubers. Soak seed tubers in 2% hydrochloric acid solution for 5 minutes, or soak in 200-fold dilution of 40% formalin for 5 minutes, or soak and seal for 2 hours, then dry and sow. (4) Strengthen cultivation management. Implement crop rotation of more than 5 years in diseased areas; increase the application of base fertilizer and phosphorus-potassium fertilizer; use lime and plant ash to improve soil pH; adopt high-ridge cultivation, avoid flood irrigation, and prevent the spread of the pathogen.

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