Tomato blight is a common plant disease that can swiftly destroy a harvest. It describes the rapid death of plant tissue, often caused by fungal or fungal-like organisms that thrive in garden settings. This issue spreads quickly and leads to significant loss if not managed immediately. Understanding the specific type of blight affecting your plants is the first step toward effective management.
Identification of Blight Types
Proper identification is necessary because the two most common types of blight are caused by different organisms and require varied approaches.
Early Blight
Early Blight is caused by the fungus Alternaria solani, which typically targets older, lower leaves first. The defining characteristic is the appearance of dark spots with concentric rings, often described as a bullseye or target-like pattern. These lesions usually grow to about half an inch and are often surrounded by a distinct yellow halo. Early Blight spreads relatively slowly. The pathogen survives in infected plant debris and soil, making sanitation important for future seasons.
Late Blight
Late Blight is caused by the water mold Phytophthora infestans, an aggressive oomycete known for its rapid spread. Symptoms appear as irregularly shaped, water-soaked lesions that quickly turn dark brown or purplish-black on leaves and stems. Unlike Early Blight, these spots lack concentric rings and often start on younger growth higher up on the plant. A sign of Late Blight is the white, fuzzy growth that forms on the underside of the leaves in humid conditions. This pathogen prefers cooler temperatures (60° to 80°F) combined with high humidity, making immediate containment necessary.
Immediate Removal and Containment
The physical removal of diseased material is the first action upon diagnosing blight. Gardeners should prune off all infected leaves, stems, and severely compromised plants. This work must be done only when the foliage is completely dry to avoid spreading wet spores to healthy plant parts.
Sanitation of tools is important to prevent the transfer of spores during removal. Pruning shears and other tools should be disinfected between cuts, especially when moving between plants. Wiping the blades with a cloth soaked in 70% isopropyl alcohol is a quick and effective method, as it acts rapidly. A 10% bleach solution, made by mixing one part bleach with nine parts water, is also effective. Because bleach is corrosive, tools must be rinsed, dried, and lightly oiled afterward to prevent rust. All removed debris must be destroyed, either by bagging it for landfill disposal or burning it where permitted, as composting will not kill the hardy spores.
Treatment Strategies
After removing infected material, the remaining plants require protective treatment to prevent the spread of spores. Fungicides and biopesticides function primarily as protectants, meaning they are most effective when applied before the disease takes hold. Application must be thorough, ensuring that the top and bottom surfaces of the leaves are completely coated, as new growth remains unprotected.
Organic treatment options center on copper-based products, which provide reliable preventative control against both Early and Late Blight. These treatments must be applied regularly, especially after rain, since they act only on contact with the plant surface. Biopesticides containing beneficial microorganisms, such as those derived from Bacillus subtilis, can also suppress fungal growth.
For conventional management, synthetic fungicides offer broad protection against various pathogens. Products containing active ingredients like chlorothalonil or mancozeb are frequently used. Specialized products are available for the highly aggressive Late Blight water mold. To manage the risk of pathogen resistance, gardeners should alternate between different types of fungicides with varying chemical actions.
Preventing Future Infections
Long-term management involves implementing cultural practices that make the garden environment less hospitable to blight pathogens.
Cultural Practices
One preventative measure is adjusting how plants are watered to minimize leaf wetness. Gardeners should always water the soil at the base of the plant, avoiding overhead irrigation that leaves the foliage damp for extended periods. Improving air circulation helps the leaves dry quickly, inhibiting spore germination. This is achieved by spacing plants appropriately and by pruning the lower leaves and interior suckers to increase airflow. Removing the lower leaves also minimizes contact between the foliage and the soil, which is a major reservoir for Alternaria solani spores.
Rotation and Resistance
Implement a strict crop rotation schedule, which involves not planting tomatoes or related Solanaceae family members (such as potatoes) in the same location for at least three to four years. This practice starves the soil-borne pathogens and breaks the cycle of infection. Choosing tomato varieties specifically bred for disease resistance, such as ‘Mountain Merit’ or ‘Plum Regal,’ provides an extra layer of defense against future infections.