Tomato blight is a common, aggressive plant disease that threatens tomato crops worldwide. It is a general term for fast-spreading infections that cause significant foliage and fruit damage. Because these pathogens can quickly destroy a plant, accurate identification and immediate action are necessary to save the harvest. Understanding the specific type of blight is the first step toward effective treatment.
Distinguishing Between Common Tomato Blights
Identifying the correct type of blight is crucial. The two most frequent and damaging types are Early Blight, caused by the fungus Alternaria solani, and Late Blight, caused by the water mold Phytophthora infestans. Early Blight typically appears first on the older, lower leaves. The lesions are distinct, dark brown spots that often display concentric rings, giving them a characteristic “bullseye” appearance. This fungus thrives in warmer, humid weather.
Late Blight, in contrast, is a destructive pathogen that spreads rapidly, often starting on the upper leaves and stems. Its lesions are irregular, greasy-looking spots that quickly turn dark brown or purplish-black. During high humidity, a white, fuzzy mold may be visible on the underside of infected leaves. This pathogen prefers cooler, moist conditions and requires immediate, aggressive treatment.
Immediate Physical and Cultural Treatment
Upon recognizing symptoms, the first line of defense is removing all infected plant material. Carefully prune any leaves, stems, or fruit showing lesions to reduce infectious spores. Handle the diseased material delicately to avoid shaking spores onto healthy parts of the plant or the soil below.
The removed debris must be immediately discarded in the trash or burned, and never added to a home compost pile. Blight spores can survive composting and re-infect the garden in future seasons. Adjusting cultural practices is also necessary to slow the spread of the disease.
Switch from overhead sprinklers to drip irrigation or soaker hoses, as wet foliage provides ideal conditions for spores to germinate. Water early in the day so moisture on the leaves has time to evaporate quickly. Improving air circulation by staking plants and removing suckers or lower branches also helps keep the foliage dry.
Selecting and Applying Effective Fungicides
Fungicides are protective treatments that must be applied to healthy tissue before spores land and begin an infection. Using a combination of products, often applied in an alternating schedule, provides the best defense against developing resistance. Organic options include copper-based fungicides and biological controls such as products containing Bacillus subtilis.
Copper products are contact fungicides that disrupt fungal spore cell function on the leaf surface. They must be reapplied frequently, especially after rain, to maintain a protective barrier. Bacillus subtilis is a biofungicide that colonizes plant surfaces and produces compounds inhibiting pathogen growth. This organic option is safe to use up to the day of harvest and requires reapplication about every seven days.
For a conventional, broad-spectrum approach, synthetic fungicides containing Chlorothalonil are effective against both early and late blight. It must be applied before the disease is established. Application involves thoroughly covering all plant surfaces, including the undersides of leaves, until the spray begins to drip.
Synthetic products require protective gear, such as gloves, long sleeves, and safety glasses, during mixing and application. Chlorothalonil application frequency is typically every 7 to 14 days, using the shorter interval during high disease pressure. Always check the product label for the specific re-entry interval (REI) and the days to harvest (PHI).
Preventing Blight Recurrence
Long-term management requires shifting gardening practices to reduce overwintering spores. Crop rotation is an effective preventative measure, as blight pathogens can survive in the soil or on plant debris for several years. Tomatoes and other Solanaceae family plants, like potatoes and peppers, should not be planted in the same location for at least a three-year cycle.
Selecting varieties that possess genetic resistance to blight is another proactive strategy for future seasons. Varieties labeled with resistance codes like VFN indicate tolerance to common diseases, but specific blight resistance is also available. Cultivars such as ‘Mountain Merit’ and ‘Plum Regal’ are known to carry resistance to Late Blight, while ‘Tropic VFN’ has resistance to Early Blight.
A thorough end-of-season clean-up eliminates sources of infection for the next year. This involves removing and destroying all spent tomato plant material, including roots and fallen leaves, from the garden area. Tools, stakes, and cages used with infected plants should be sanitized with a diluted bleach solution to prevent the mechanical transfer of spores.