Celery leaf spot has become a significant disease in winter and spring protected cultivation areas, greatly affecting both yield and quality of the crop. The disease primarily affects the leaves, but can also damage the petioles and stems. Lesions on the leaves appear in two forms: small and large spots. In the early stages, small spots look like oil-like, light brown patches that gradually grow into central brown necrotic spots. These small spots are usually less than 3 mm in diameter, with a yellow-brown border and a yellow-white to gray-white center. Some may have a yellow halo around them. Large spots can expand up to 7–10 mm, with dark brown edges and a brown center. Both types of lesions show scattered black spots, which are conidia. Petiole and stem lesions often appear as diamond-shaped or oblong spots, slightly sunken, with a brown outer layer and a lighter interior, also containing small black spots. The pathogen responsible is *Septoria apiicola* Speg., also known as celery culm, along with *S. apii* and *S. apii-graveolentis*, all belonging to the fungal subphylum. Conidiospores are embedded in the epidermis, measuring 87–155 × 4–25–56 microns. They emerge through spore orifices and spread via rain splash. Spores are colorless, transparent, and elongated, with a blunt tip and 0–7 septa, most commonly 3. Their size ranges from 35–55 × 2–3 microns. When germinating, conidia may split into sections, each producing germ tubes. Mycelium and conidia are killed at 48–49°C after 30 minutes. Germination occurs between 9–28°C, with optimal growth at 20–27°C. Growth slows above 27°C, and the pathogen only infects celery. The disease survives in the soil through mycelia and remnants of infected plant material, or remains latent in seeds. Endogenous conidiospores serve as primary and secondary inoculums, spreading via water splashes. Infection occurs through stomata or the epidermis, under high humidity and temperatures around 27°C. Cold, wet, and rainy weather promotes disease development. In protected environments, poor ventilation and high humidity increase the risk of infection. Plants in low-fertility soils or weak growth conditions are more susceptible. Resistant varieties include Italian winter and summer celery, Florida 683 (susceptible to black heart disease under heat and drought), Utah 10-B, Emerson, June Bell, Early Green, Jinnan Ciqin, Daqin, and Beijing 8401, which can be selected based on local conditions. To manage the disease, choose resistant varieties such as Jinnan, Winter Celery, Xiaqin, Jinqin, Tianma, Shanghai Daqin, Ventura, American Glass Crisp, Celery 3, and Chunfeng. Use disease-free seeds or treat infected seeds. Store seedlings for two years from disease-free sources. Improve field management by applying proper fertilizers, ensuring strong seedling growth, and enhancing plant resistance. In open fields, apply 75% chlorothalonil WP at 600 times, 40% Fuxing (fluorosiliconazole) at 8000 times, 45% Tecto suspension at 1000 times, or 50% acetamiprid wettable powder at 1000 times. For ground protection, use 5% chlorothalonil or 6.5% moldoyl dust, or 45% chlorothalonil aerosol, applying the treatment 2–3 times every 7–10 days.

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