More than 30 viruses can malform tomato plants and fruits. Septoria Leaf Spot Figure 1. These spores are spread from this source to growing plants by wind or by water droplets from rain or sprinklers splashing them onto the lower leaves. From these early infection sites, the fungus spreads throughout the tomato plant during warm, wet weather. Rain, dew or sprinkler irrigation provides the necessary moisture on leaves to keep the disease active during the growing season.
Symptoms usually appear after fruit set.
Key Symptoms Small, circular spots with dark margins and gray centers on leaves. Leaf spots may be peppered with tiny black dots. Lesions on stems and petioles are dark elongated spots. Infected leaves and petioles yellow and die. Disease Management Remove plant debris from last season or till it into the soil.