Potato tuber necrotic spot disease primarily affects tubers and roots, although stems can also become infected. In the early stages of the disease, small brown spots appear on the epidermis, surrounded by a translucent halo. These spots gradually swell and expand into "freckles" with diameters ranging from 3 to 5 mm. The epidermis remains intact and powdery during this phase, referred to as the "closed" stage. As the disease progresses, the epidermis ruptures, folds back, and reveals orange-red subcutaneous tissue filled with numerous dark brown powders—sporangia. The freckle becomes sunken, resembling a crater, with cork-like structures around it. The halo ring is known as the "open pod" stage for whiteflies. Infected roots may develop nodules, similar in size to soybeans or clustered together.
The pathogen responsible for this disease is *Spongospora subterranea* (Wallr.) Lagerheim, commonly known as the "powder white fungus." It belongs to the phylum Mastigomycota. The brown powder released from the "freckles" consists of dormant sporangia, which are spherical to ovoid, yellow to yellow-green in color, and measure 19–33 µm in diameter. These structures have internal cavities. Dormant spore capsules are round to polygonal, measuring 3.5–4.5 µm, with thin, smooth walls. When germinating, they produce zoospores that are nearly spherical, wall-less, and lack long flagella. These zoospores can swim in water and transform into resting bodies when stationary. They then invade the host through root hairs or lenticels. Zoospores and their resting forms serve as the primary infection sources.
The disease spreads through infected seed potatoes, diseased soil, and plant debris. Seed potatoes are the main vector for long-distance transmission, while short-distance spread occurs via contaminated soil, fertilizers, and irrigation water. Resting sporangia can survive in the soil for up to 4–5 years. Under favorable conditions, zoospores germinate, form resting bodies, and penetrate the host through root hairs, lenticels, or wounds. Inside the host, protoplasts divide into multinuclear cells, eventually forming mononuclear dormant sporangia. These accumulate as sponge-like structures within host cells. When infected tissues decay, the sporangia fall back into the soil, surviving through winter or summer. Optimal conditions for pathogen development include soil moisture of about 90%, temperatures between 18–20°C, and a pH of 4.7–5.4. Heavy rainfall and cooler summers often lead to higher disease incidence. The severity of the disease depends mainly on the initial infection pressure rather than secondary infections in the field.
To manage the disease, strict quarantine measures should be enforced to prevent the movement of infected seed potatoes. Crop rotation for more than five years is recommended. Use only certified disease-free seed potatoes, and remove any infected tubers. Soaking seed potatoes in a 2% hydrochloric acid solution or 40% formalin diluted 200 times for 5 minutes, followed by covering with plastic for 2 hours and drying, can help reduce infection. Applying phosphate and potassium fertilizers, adding lime or wood ash to adjust soil pH, and improving field management practices such as avoiding overwatering and promoting proper drainage can further control the disease.
Anti-fungal
According to the site of fungal infection,it can be divided into superficial fungal infection and deep fungal infection, superficial fungal infection is caused by ringworm infection of the skin, hair, fingers, toe fingers. The incidence rate is high with less harmful. Deep fungal infection is caused by Candida and cryptococcal invasion of internal organs and deep tissue, the incidence is low with big harmful. Classified based on mechanism action classification can be divided into (1) Antifungal agents for sterol synthesis in fungal cell membranes (2) Antifungal agents for fungal cell wall synthesis (3) Antifungal agents acting on nucleic acid synthesis, The Antifungal drugs easily affect the white blood cells and liver function, if in long-term use,it may caused a transient increase in GPT or leukopenia, withdrawal is normal.
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