Take an infected dogwood leaf to your County Agriculture Extension department for indentification. Meanwhile, Rain is making the fungus grow. Fungi love moisture. Soils, leaves and stems need to dry out. Fungus will not thrive in dryness. Allow air to circulate between plants. Are your plants too close together? Tomatoes need at least 5 hours of sun. When the sun does come out, do your plants have sufficient sun to dry out the soil and plants? Do you water in the morning? Plants then will have an opportunity to dry. Fungus exists in the air. We have to learn to live with it.