Early and late blight are two devastating tomato diseases that attack the leaves and fruit of a tomato plant. Early blight will commonly cause small yields of growth and foliage damage to the plant, while late blight will also destroy fully grown plants. Early blight is caused by the fungus Alternaria solani, while late blight is caused by Phytophthora infestans. Both diseases require destruction of the infected plant and rotation of the crop to prevent further infection.
Examine the leaves of the plant for spots of yellowing, withering or for leaves that are dropping from the plant for early and late blight. Look for the existence of insects on the plant before considering late blight, as some insect infections can cause similar symptoms.
Look for spots on the leaves that appear to be water-soaked, beginning at the leaf tips. This indicates the presence of late blight.
Inspect green tomatoes for the appearance of brown, leathery lesions on the skin of the fruit, or wet rot, indicating late blight.
Look for tomatoes that have turned black, or have black concentric circles running around the stem connecting the fruit to the plant. This indicates early blight.
Sniff around the plant to see if you recognize any foul smells, indicating rotting fruit. This is a symptom of both diseases.