Potato blight is a common and serious plant disease targeting members of the nightshade family, most notably potatoes and tomatoes. The term “blight” refers to two distinct conditions: Late Blight and Early Blight, caused by different pathogens. These diseases present with unique visual markers and require different approaches for effective management. Understanding these differences is the first step toward protecting a crop.
Visual Markers of Late Blight
Late Blight, caused by the water mold Phytophthora infestans, is destructive and spreads rapidly in cool, damp conditions. Initial symptoms on foliage are small, dark green, water-soaked spots that quickly enlarge into irregular lesions. These lesions are often found near leaf tips or edges where moisture accumulates.
The lesions quickly turn dark brown or black, often appearing greasy, and expand across the entire leaflet, unrestricted by leaf veins. A pale green or yellowish border, known as a chlorotic halo, frequently surrounds the expanding dark lesion. On stems, the disease appears as dark, firm, sunken areas that can cause the stem to collapse.
The most telling sign is the presence of white, fuzzy growth, which is the pathogen’s spore-producing structure (sporangia). This characteristic “fuzz” usually appears on the underside of the leaf, particularly around the lesion edge, during high humidity or early morning dew.
When the pathogen infects potato tubers, external symptoms appear as irregularly shaped, slightly depressed patches on the skin. These discolored areas range from brown to purplish. Spores wash down from the foliage and infect the tubers primarily through the eyes.
Cutting into the infected tuber reveals a tan to reddish-brown, dry, granular rot directly beneath the discolored skin. This rot is firm and distinct, typically extending less than half an inch into the flesh. The margin between diseased and healthy tissue is often indistinct, sometimes showing brown, finger-like extensions.
Visual Markers of Early Blight
Early Blight is caused by the fungal pathogen Alternaria solani and progresses slower than its late-season counterpart. Symptoms usually appear on the oldest, lower leaves, often after the plant has begun to set fruit. Initial signs are small, dark brown to black spots on the foliage.
As the lesions enlarge, they develop the characteristic “target spot” or “bullseye” pattern of concentric dark rings. These spots can grow up to a half-inch (1.3 cm) in diameter, and the surrounding leaf tissue may turn yellow. The affected tissue inside the spot is dry and papery, lacking the soft, water-soaked appearance of Late Blight.
Lesions on the stems are dark, slightly sunken, and may exhibit the same concentric ring pattern as the leaf spots. This stem infection can lead to basal girdling, sometimes called collar rot, especially in young plants. While Early Blight primarily affects foliage, it can also cause dry, dark, and slightly pressed-in lesions on potato tubers. The underlying tuber flesh turns dry, leathery, and brown, unlike the granular rot of Late Blight.
Differentiating Between Early and Late Blight
Distinguishing between these two blights is important because they are caused by different pathogens—a water mold versus a fungus—and thrive in different conditions. A primary difference is the speed of progression and the initial location of symptoms on the plant canopy. Late Blight spreads rapidly, often attacking young, tender growth first.
Early Blight is a slower disease that typically begins on the older, lower leaves during periods of warm weather. Late Blight, favoring cooler temperatures, will spread to all parts of the plant regardless of the plant’s age. The location of the first spots—low and old versus high and new—is a strong initial clue.
The texture of the leaf lesions offers another clear distinction. Late Blight lesions are water-soaked and soft, frequently accompanied by the white, fuzzy growth on the leaf underside. Conversely, Early Blight spots are dry, brittle, and papery in texture, and they never exhibit this white, fuzzy spore structure.
The shape of the lesions is also unique, with Early Blight displaying the distinct concentric “bullseye” rings that Late Blight lacks. Regarding tubers, Late Blight causes a destructive, granular, reddish-brown rot that tunnels into the flesh. Early Blight’s impact on tubers is less severe, resulting in dry, superficial, pressed-in dark spots near the surface.