Tomatoes are subject to various wilts and blotches. Some are transferred in seed. Others survive in the soil. If you had problems with tomatoes the year before, cover the soil with clear plastic for three weeks before you plant the next crop.
Fruit fly (maggots in fruit)
If you pick your tomatoes every few days you won't get any nasty surprises. Don't let fallen fruit lie on the ground. As a last resort, pick green tomatoes and let them ripen indoors or cover them with a fruit-fly-proof net. (And fruit-fly-stung tomatoes are squishy anyway, so you don't have to worry that your nearest and dearest will find the classic half a grub in their salad).
Mites (mottled, dehydrated leaves; small insects under leaves)
Water under the leaves as well as on top — especially bad in dry weather or in glasshouses.
Uneven watering — too dry then very wet. Mulch, mulch and mulch.
Slugs and snails
These are less of a problem for tomatoes than other veges, though they will eat your nice ripe tomatoes. Put a sharp collar made from the top of a can or plastic soft drink bottle around the stem or put pellets inside an old ice cream container with a little door cut out for the snails et al to enter. This way the pellets won't kill worms and other soil life (or kids, dogs, cats and lizards).
Pick fruit often; use bird nets.
Plants appear suddenly cut off at the base. Try collars of old tin cans, old plastic soft drink bottles or stiff cardboard or two toothpicks as stakes on either side of seedlings. A well mulched soil will deter cutworms.
Damping off (seedlings suddenly die; a brownish band may appear at ground level.)
Dip seedlings in chamomile tea before planting, pour the rest around each one on planting. Make sure all organic matter in the soil is decomposed and no mulch is actually touching the seedlings.
Red spider mites (webbed or dehydrated leaves)
A good solid overhead watering every day may be all this problem needs.
Hard yellow patches on ripe or near ripe tomatoes are usually caused by phosphorus or potash deficiency. Use a seaweed-based organic fertiliser.
Root knot nematodes (unthrifty plants that fail to thrive)
Mulch well to encourage earthworms — these are the best possible destroyers of nematodes, as well as the fungal webs of decomposing organic matter. The nematodes get caught and die. Grow marigolds close around the plants — marigolds deter root knot nematode (but not some other problem species) but if they are grown too far away the nematodes will be deterred away from the marigolds and towards the tomatoes.
With fusarium wilt the whole plant wilts, rotted at the base of the stem and stems may be cracked. In mild cases the plant may just grow slowly or be pale and stunted.
Fusarium wilt is a soil-borne fungus. Try covering the soil before you plant out tomatoes with clear plastic sheeting for three weeks to kill off spores in the soil. Look for tomato seedlings that say they are resistant to fusarium wilt. Some of the old fashioned varieties are very susceptible to it.
Small holes in fruit
This is caused by tomato fruit worm. Don't worry about them — the damage usually isn't severe enough to worry about unless you are growing them commercially. As a last resort use DIPEL, a form of bacterial warfare on caterpillars, but harmless to other species.
Poor fruit set
This may be caused by too much nitrogen, particularly if foliage is thick and green. Use mulch and compost instead of artificial fertilisers. Some tomato varieties don't set well in cool weather, cold climate varieties may not set in very hot weather.
Poor pollination, possibly caused by very hot or very cold weather. Possibly also a virus.
'Rolled over' older leaves
This can occur when you have hot days and cold nights. Don't worry about it. It's not a virus.
Yellow older leaves means nitrogen deficiency. Feed more.