The Difference Between Early Blight, Late Blight, and Septoria Blight

Early blight is a common disease affecting plants, particularly those in the nightshade family such as potatoes and tomatoes. It's caused by a soil borne fungus called Alternaria solani. The name "early" does not imply that the disease occurs only early in the season, but does most commonly occur in the early season.

Symptoms of early blight usually start appearing on the lower, older leaves as small brown spots. These spots enlarge, often forming a pattern of concentric rings giving a "bull's-eye" appearance. The surrounding leaf area may turn yellow. If left uncontrolled, it can cause significant defoliation, usually starting at the base of the plant and moving upwards.

Early blight can also affect the fruits of the plant. On tomatoes, dark, sunken, leathery spots can form, usually at the point where the fruit is attached to the plant

Late blight is a serious and devastating plant disease caused by the pathogenic water mold Phytophthora infestans. It's most infamous for causing the Irish Potato Famine in the 1840s, but it also affects other plants in the nightshade family, such as tomatoes, peppers, and eggplants.

This pathogen prefers cool, damp, and humid conditions and can spread rapidly, especially in these environments. It is one of the few plant pathogens that can produce spores both asexually and sexually, leading to a great genetic diversity and adaptability.

Symptoms of late blight typically appear later in the growing season. Initial symptoms include irregularly shaped dark spots on leaves, often surrounded by a pale "halo". As the disease progresses, it may cause the entire plant to wilt and die. It can also cause dark, firm lesions on tubers and fruits.

Septoria leaf spot, also known as Septoria blight, is a common fungal disease that primarily affects tomatoes but can also occur on other plants. It is caused by the fungus Septoria lycopersici.

Septoria leaf spot usually appears on the lower leaves after the first fruit sets. The disease begins as small, water-soaked spots. These spots darken and enlarge to form circular or angular grey or tan areas that may be up to 1/4 inch in diameter. The center of these spots later turn white and tiny black specks, which are the fruiting bodies of the fungus, can often be seen in the white centers.

This disease does not typically affect the fruit directly, but by defoliating the plant, it can lead to sunscald on the fruit. Also, heavily defoliated plants will produce fewer, smaller fruit.

Prevention of blight