Septoria Leaf Spot
Septoria leaf spot/blight (Celery leaf spot) Septoria apiicola is a common and serious disease of celery (closely related septoria species attack parsley, cilantro, tomato, cucurbits and other crops). Like many fungus diseases septoria leaf spot is most severe in warm wet weather.
Leaf spot first appears as sunken light brown spots with yellow haloes on the outer leaves. These eventually turn tan or gray in the center and may have visible black spore producing bodies in the center. Spots then appear on stalks and other leaves and in severe cases the leaves may eventually die and drop off, giving a stand of plants a scorched appearance.
Celery leaf spot is often transmitted through seed, but spores can also survive on crop debris in the soil for several years. The spores are often transmitted when soil is splashed on to leaves, so it is important to avoid getting the leaves wet (use drip irrigation or water in the morning so leaves can dry rapidly). It is also important to give the plants good air circulation and avoid planting in the same place for 3 years. Seedlings should be grown in clean containers and given good air circulation, Discard any plants with spots on the leaves.
You can reduce the incidence of seed borne disease by soaking the seed in hot (118˚ F) water for 30 minutes before planting (this has to be done fairly accurately).
Image: William M. Brown Jr., Bugwood.org