Uh-oh, you've survived the tomato insects, only to be confronted with tomato plant diseases. There is hope, and most can be treated organically.
Let's take a look at the diseases that can wilt, blight or kill your garden-grown tomato plants.
Tomato Plant Diseases
Some of the diseases reduce the number of fruits, but leave them edible; some don't. The most common diseases are as follows.
Bacterial Canker: Wilting and upward rolling of margins of leaves; generally one side of plant is affected before the other. Leaves brown, wither and die, but remain attached to the stem. Stems may have open cankers and yellowish decay of inner tissues. Spotting of the fruit; spots 1/8" to 1/4" in diameter; small cavities in central portion of the fruits. Some protection is afforded if soil is not allowed to be splashed onto the stems/leaves (use mulch soon after planting). Slight additional protection if mulched with compost.
Bacterial Wilt: Gradual drooping and eventual dying of the leaves over the entire plant without marked yellowing of the foliage. Stem decayed at ground line and covered with a whitish fungus growth, with small, light-brown circular spots. Some protection is afforded if soil is not allowed to be splashed onto the stems/leaves (use mulch soon after planting). Slight additional protection if mulched with compost.
Blight, Early: Rather large spots, up to 1/2" in diameter. The spots are brown with concentric rings giving a bulls-eye appearance. Plants partially or completely defoliated. Seedlings may be girdled at the ground line. Fruits developing large, dark, leathery spots near stem ends, with dark, dry decay of the flesh underneath. Development favored by warm, humid conditions, so this is prevalent in the Southeast.
Blight, Late: Grayish, water-soaked patches on leaves, increasing in size rapidly. Fungus growth also evident on underside of the leaves. Water-soaked spots on fruits; spots enlarging rapidly and sometimes cover over half of the surface. Spots become brown with a firm corrugated surface. Disease spread in the wind from overwintered debris for great distances. Development favored by cool, moist conditions, so this is more of a problem in the Pacific Northwest.
Fusarium: Yellowing, wilting, and death of the leaves from the base upward, followed by the gradual death of the plant. Dark-brown discoloration of woody tissues just below the green outer cortex of stem. No soft decay of stem. Symptoms may be apparent on only one side of the plant, or may appear on both sides. Does not effect the fruits (except in their quantity). A problem largely in the Southern USA (controlled by temperature in the north). Tomatoes resistant to Fusarium are marked with a capital "F" following the variety name. Note: There are at three strains of Fusarium, so it it's especially a problem for you, look for a tomato resistant to at least two, if not all three, strains.
Nematodes: Plants are dwarfed, sickly, and will sometimes wilt readily in dry weather. Roots showing swelling or galls. This is actually caused by a tiny worm, and if you get rid of the nematodes, you will remove the problem (see Insect section). Tomatoes resistant to Nematodes are marked with a capital "N" following the variety name. Nematodes are especially troublesome in sandy soil.
Septoria: Older leaves show numerous small, roughly circular spots 1/16" to 1/8" in diameter with dark margins and gray centers dotted with tiny dark specks. Plants often seriously defoliated and fruits exposed to sun, thus making the fruits suseptible to sunscald. Fruit remains edible.
Southern Blight: Slow wilting and eventual death of the plant; soft tissues of the stem decayed, with cavities that contain a grayish-white fungus growth and large, black spots. Fruits sometimes show a watery soft rot.
Stem Rot: Emergence failure or collapse and wilting of seedlings, caused by a decay of the stem at the ground line or roots. Use a sterile seed starting mixture to help prevent this problem.
Tobacco Mosaic Virus: Green or yellow mottling of leaves; some curling, malformation, and dwarfing of leaflets. Plants slightly stunted. Fruits mottled by yellow strains of the virus. Can be passed from plant to plant by handling wet plants that are infected, then handling healthy plants (for example, when pinching back shoots on a damp morning.) This disease is widespread throughout the USA. Tomatoes resistant to Tobacco Mosaic are marked with a capital "T" following the variety name.
Verticillium: Yellowing, wilting, and loss of foliage, beginning at the base of the plant. Plants wilt during the day, but often appear to recover in the evening. Plant growth is stunted, but the plant usually survive the season. Discoloration of the woody tissues of the stem, generally confined to the lower part of the plant. Does not effect the fruits (except in their quantity). Tomatoes resistant to Verticillium are normally marked with a capital "V" following the variety name (in seed catalogs, at any rate).