Early blight is a common fungal disease primarily targeting plants within the Solanaceae family, including tomatoes, potatoes, and eggplants. Although the name suggests an early-season problem, the disease often appears as plants mature, causing a progressive decline in health and significantly reducing yield. Understanding its symptoms and life cycle allows for timely intervention and protection of susceptible garden plants.
Recognizing the Visual Symptoms
The earliest signs appear on the oldest, lowest leaves, often where humidity is highest. Initial symptoms are small, dark, irregularly shaped spots that expand up to half an inch in diameter. These expanding spots develop characteristic concentric rings of dead tissue, creating a distinctive “bullseye” or target-spot pattern.
As lesions multiply, the surrounding tissue may turn yellow, and the entire leaf eventually withers. This defoliation progresses upward, exposing developing fruit to sunscald and reducing photosynthesis. On young seedlings, the disease can cause dark, sunken lesions near the soil line, known as collar rot, which girdles the stem and leads to plant collapse.
The disease also affects tomato fruit and potato tubers. Tomato fruit develops dark, leathery, and sunken spots, typically near the stem end, which may show the concentric ring pattern. Potato tubers develop dark, dry, and sunken lesions that can become leathery and cause shriveling during storage.
Understanding the Causal Fungus and Conditions
The disease is caused by the fungal pathogen Alternaria solani, which can repeat its infection cycle multiple times within a single growing season. The fungus survives between seasons by overwintering in infected plant debris left in the soil, on the surface of seeds or tubers, and on wild weeds or volunteer Solanaceae plants.
In the spring, the fungus produces spores (conidia) that spread to healthy plants primarily by wind, splashing water, and contaminated tools. The fungus thrives in warm temperatures between 75°F and 85°F. Infection is favored by high humidity or extended periods of leaf wetness, requiring free water on the leaf surface for spores to germinate and penetrate tissue.
Proactive Prevention Measures
Prevention begins with cultural practices that reduce the initial source of infection. Garden sanitation involves removing and destroying all infected plant debris at the end of the season instead of composting it. Implementing crop rotation is also important; susceptible plants should be moved to a new location and only returned to the original site after at least two or three years.
Proper watering techniques limit disease spread. Use drip irrigation or soaker hoses to deliver water directly to the soil, avoiding overhead watering which keeps foliage wet and disperses spores. Watering plants earlier in the day allows moisture to evaporate before nightfall. Applying mulch, such as straw or plastic, creates a physical barrier that prevents soil spores from splashing onto lower leaves during rain or irrigation.
Increasing air circulation within the plant canopy is achieved by ensuring adequate spacing and pruning lower leaves near the soil surface. Staking or trellising plants keeps foliage off the ground and promotes faster drying after dew or rain. Selecting resistant or tolerant cultivars, often indicated by an “EB” designation, can significantly reduce disease severity.
Management of Active Infections
Once symptoms are visible, action is needed to slow disease progression. Gardeners should physically remove and dispose of all infected leaves and stems, especially the lower foliage. Wash and sanitize hands and tools after working with infected material to prevent transferring the fungus to healthy plants.
Fungicides protect healthy foliage from new infections, including copper-based products or synthetic options like chlorothalonil. These treatments are most effective when applied preventively, before the disease is widespread, and often require repeated applications following label instructions. Maintaining plant vigor is supportive, as well-fed and well-watered plants better tolerate the infection and continue to produce fruit.