Early blight of tomato is caused by

Early blight of tomato is caused by Fungus.
Early blight is a common tomato disease caused by the fungus Alternaria solani. It can affect almost all parts of the tomato plants, including the leaves, stems and fruits. The plants may not die, but they will be weakened and will set fewer tomatoes than normal.
Tomatoes that have early blight require immediate attention before the disease takes over the plants. Thoroughly spray the plant (bottoms of leaves also) with Bonide Liquid Copper Fungicide concentrate or Bonide Tomato & Vegetable.