Tomato blight is a common disease complex caused by various fungal or fungal-like pathogens that can severely reduce tomato yields. These pathogens thrive in conditions of high moisture, making prevention significantly easier and more effective than attempting treatment once the disease is established. Proactive planning and adjustments to cultural practices offer the best defense against these infections. This guide outlines the preventative measures necessary to protect your tomato plants.
Identifying the Types of Tomato Blight
The three most frequently encountered forms of blight are Early Blight, Late Blight, and Septoria Leaf Spot, each thriving under slightly different environmental conditions. Early Blight, caused by the fungus Alternaria, favors mild to warm temperatures (68 to 80 degrees Fahrenheit) alongside extended periods of leaf wetness. Its spores can survive over winter on infected plant debris or on seeds and are easily disseminated by wind or splashing rain. Septoria Leaf Spot, caused by Septoria lycopersici, prefers moderate temperatures (68 to 77 degrees Fahrenheit) combined with high humidity and wet conditions. This pathogen is primarily spread by water splash, but it can also be carried by workers, equipment, or insects. Late Blight, caused by the water mold Phytophthora infestans, appears when temperatures are cooler and humidity levels are consistently high. Preventative measures are focused on minimizing the moisture and temperature combinations required for infection.
Optimizing the Growing Environment
Managing moisture is the primary cultural practice for preventing the establishment and spread of blight pathogens. Gardeners should always water the soil directly, utilizing drip irrigation or soaker hoses, rather than employing overhead watering methods. This technique prevents the prolonged leaf wetness necessary for fungal spores to germinate and infect plant tissue. Watering deeply but infrequently encourages a robust root system while keeping the foliage dry.
Adequate airflow around the plants also reduces the risk of disease by promoting faster drying of leaves after rain or dew. Tomatoes should be planted with proper spacing, allowing air to circulate freely between individual plants. Pruning the lower leaves and any suckers removes older foliage that is often the first to be infected and keeps leaves from touching the soil.
Supporting the plants with stakes, cages, or trellises lifts the plant canopy off the ground. This elevation prevents the lower leaves from coming into contact with soil-borne spores, which are often splashed up during rainfall or watering. Maximizing ventilation and minimizing direct contact with the ground disrupt the blight life cycle.
Seasonal Strategy and Variety Selection
Planning the garden layout provides a strong foundational defense against blight diseases. Crop rotation is a strategy where tomatoes and related nightshade plants (peppers, potatoes, and eggplant) are not planted in the same location year after year. This practice is necessary because blight pathogens can overwinter in the soil on residual plant debris. Rotating the planting location for one to two years helps to starve out the pathogens that remain in the soil.
Applying a layer of organic mulch, such as straw or shredded leaves, or using plastic sheeting over the soil surface is another protective measure. Mulch acts as a physical barrier, preventing rain splash from kicking up soil-borne spores onto the vulnerable lower leaves of the plant. This simple addition keeps the immediate growing environment cleaner and less conducive to initial infection.
Gardeners can also choose varieties specifically bred for disease tolerance. F1 hybrid tomatoes like ‘Iron Lady F1’ have demonstrated resistance to multiple pathogens, including Early Blight, Late Blight, and Septoria Leaf Spot. Other blight-resistant hybrid selections offer a greater chance of a successful harvest, even when environmental conditions favor disease development.
Preventative Sprays and Garden Sanitation
For an extra layer of protection, especially during periods of wet or humid weather, supplemental preventative treatments can be applied to the foliage. Copper-based fungicides, such as Bordeaux mixture, are effective because the copper ions prevent fungal spores from germinating on the leaf surface. These sprays must be applied proactively before any symptoms appear, coating the plant tissue to establish a protective barrier.
Bordeaux mixture, a combination of copper sulfate and lime, is known for its persistence, which is beneficial in rainy conditions where other treatments may quickly wash away. Alternatively, biofungicides containing beneficial microbes like Bacillus subtilis can be used as a less toxic option. These biological controls work by colonizing the plant surface, making it difficult for the pathogens to gain a foothold.
End-of-season sanitation is the final step in the prevention cycle, ensuring that the garden is not a source of infection for the following year. All infected plant material must be removed and destroyed immediately, rather than being composted, to eliminate the overwintering source of spores. Tools and supports must be disinfected with a bleach solution to prevent the transfer of disease to new plants.