Blight is a common and destructive term used by gardeners to describe the rapid death and browning of plant tissues, which often affects tomato crops. This condition is caused by specific plant pathogens that can quickly spread under favorable environmental conditions. For home gardeners, the immediate identification of blight is important because it determines the necessary response to save the plant and prevent the infection from destroying the entire harvest.
Differentiating Types of Tomato Blight
The term “blight” generally refers to two distinct diseases in tomatoes. Early Blight is caused by the fungus Alternaria solani. This disease usually appears earlier in the growing season, often when plants are stressed or beginning to set fruit, and is generally less aggressive than the other form of blight.
Late Blight, conversely, is caused by Phytophthora infestans, an aggressive microorganism classified as an oomycete, or water mold. This pathogen can result in total crop loss if left untreated and typically strikes later in the season.
The primary difference lies in the nature of the pathogen: one is a true fungus (Alternaria) and the other is a water mold (Phytophthora), which affects their preferred environmental conditions and speed of destruction. Alternaria solani can survive over winter in infected plant debris, soil, and on seeds. Phytophthora infestans is often introduced each season through infected potato tubers, wind-blown spores from distant regions, or contaminated transplants.
Visual Identification of Symptoms
Identifying the specific type of blight requires a close look at the lesions on the leaves, stems, and fruit. Early Blight lesions are characterized by small, dark, circular spots that first appear on older, lower foliage. As these spots enlarge, they develop distinct, dark concentric rings, giving them a classic “bullseye” or target-like appearance. The tissue surrounding these spots may also turn yellow, and severe infection leads to significant leaf drop.
Late Blight symptoms are more rapidly destructive and can appear on any above-ground part of the plant. Leaf spots are typically large, irregularly shaped, and may look dark, water-soaked, or greasy. Under cool, moist conditions, a telltale sign is the appearance of a thin, white, velvety or fuzzy mold growth on the underside of the leaves or on the lesions themselves. Infected fruit develops firm, dark brown or golden, sunken areas, and the entire plant can quickly collapse within days.
Environmental Conditions Leading to Blight
Both types of blight thrive when specific weather patterns create an ideal environment for pathogen growth and spread. A primary factor is extended periods of leaf wetness, often caused by rain, heavy dew, or overhead irrigation. High relative humidity, especially above 90%, is necessary for the spores of both pathogens to germinate and infect plant tissue.
Temperature ranges vary between the two diseases, influencing which one is more likely to appear. Early Blight favors warm conditions, with optimal development occurring between 72°F and 86°F. Late Blight, however, prefers cooler temperatures, with spore production ideal around 65°F to 70°F, making it a threat during mild, damp weather.
Spores are commonly spread through rain splash, moving from the soil or infected debris onto the lower leaves of the plant. Late Blight is particularly problematic because its airborne spores can be carried long distances by the wind, sometimes traveling up to 40 miles to establish new infections.
Strategies for Managing and Preventing Blight
Preventing blight begins with cultural practices that reduce the presence of the pathogen and limit the conditions it needs to flourish. Implementing a crop rotation of at least two to three years, avoiding planting tomatoes or related plants like potatoes in the same spot, helps to reduce the pathogen load in the soil. Staking or caging plants and pruning lower leaves that touch the soil increases air circulation, which helps foliage dry quickly and minimizes the splash-up of spores.
Proper watering techniques are also important; using soaker hoses or drip irrigation at the base of the plant keeps the leaves dry, which is less favorable for spore germination. Promptly removing and disposing of infected plant material, and never composting it, helps to eliminate sources of the pathogen. Choosing resistant tomato varieties, when available, provides a primary layer of defense against infection.
If blight is detected, especially the fast-moving Late Blight, immediate action is necessary to halt the spread. Fungicide application is a crucial part of the management strategy, with protectant fungicides often applied on a seven to ten-day schedule. Organic options like copper-hydroxide solutions can be used, while synthetic options such as chlorothalonil are effective at killing fungal spores and suppressing disease progression. A combination of these cultural practices and timely, targeted treatment is the most effective approach to protect a tomato harvest.