Your garden is starting to grow and your tomatoes seem to be doing well; they look healthy, green, and are beginning to flower. Then you notice the dark spots on the lower leaves; these spots quickly spread to the whole plant, including the fruit, and your tomato harvest is over before it's begun. Early blight is a disease caused by the fungus Alternaria solani. It is one of the most common leaf diseases of tomatoes, and also affects potato plants.
Early Blight Development
Infection by early blight is most common in plants that aren't receiving enough nutrients or are otherwise stressed. The spores of the Alternaria fungus can germinate within two hours on your tomato plant. Depending on the temperature, it may take another three to 12 hours to penetrate and infect the plant. If the temperature is above 60 degrees and enough moisture is present, the fungus can cause lesions on the leaves within 2 to 3 days.
Survival and Spread
Early blight fungus can overwinter and survive up to several years in plant debris or soil. The spores can be transported by water, wind, insects and animals. Once the fungus has infected a plant, it produces new spores and rapidly spreads to other plants.
The tomato plant can be affected by early blight at all stages of its growth. The fungus may cause damping-off (stem rotting off at soil surface), collar rot (rotting, spongy areas of stem), leaf blight and fruit rot. The most recognizable symptom occurs on the leaves; circular lesions up to a half inch wide form with dark, concentric circles visible. These lesions usually form first on lower leaves, then move up the plant.
You can take several steps to minimize the spread of early blight. Plant blight-resistant varieties of tomatoes. Removing and destroying plants at the end of the season eliminates plant residue the fungus needs to overwinter. Rotate crops on a three-year cycle with non-susceptible plants to reduce the chance of re-infection each season. Don't work in the garden unless it's dry to avoid spreading spores along with water.
Apply the preventative fungicide chlorothalonil every seven to ten days to help prevent early blight. Fungicides that can help treat already-infected plants include azoxystrobin and pyraclostrobin, applied every seven to 14 days; bacillus subtilis, applied every five to seven days; and potassium bicarbonate, applied every five to 14 days. For safety, remember to thoroughly read and follow label directions on any fungicide.