Early blight is a fungal disease caused by Alternaria solani. If untreated it can significantly reduce yields and even kill plants.
How it Spreads
The fungal spores of Alternaria solani can be spread by water droplets (splashing water), insects (particularly sap sucking insects like aphids), wind and human contact. As with any plant disease, the early blight fungus can also spread if healthy plants are pruned using secateurs that were used to prune infected plants.
Early blight affects plants from the Solanaceae family - particularly tomatoes and potatoes.
Symptoms of early blight include a characteristic bulls eye patterned spot on leaves as well as loss of leaves. The lowest leaves tend to be affected first. Any above ground part of the plant may be affected so the stems may also exhibit damage as can the fruit.
The fungus can also cause 'damping off' - ie. new sprouts suddenly die.
If all else fails and you notice an infection, fungicides are available to help control it. If you're an organic gardener, try an infusion of garlic, chamomile or chives but prevention is definitely the best method of control.
To prevent early blight infections, water early in the day and aim to only water the soil as wet foliage will increase the likelihood of infection. Watering methonds that prevent water from splashing onto plants (such as dripper systems or watering by hand with a watering can at the base of individual plants) also help prevent the spread of early blight.
The fungus can survive over winter in the soil for a number of years so always practice crop rotation, ensuring that no member of the Solanaceae family is planted in the same spot for at least 3 years - longer if possible. Mustard plants produce chemicals that kill or inhibit the growth of some fungi and bacteria so following Solanaceae crops with mustard will help to prevent early blight.
Make sure plants are spaced appropriately so that there is good air circulation and try to plant more than one garden bed with any given crop to reduce the likelihood that you will lose your entire crop (plants in one bed might become infected but good garden hygiene can prevent the spread of the infection to other beds).
Companion plants that can provide nutrients or prevent fungal infections are useful as they can boost plant immune systems or protect plants from fungal spores that come into contact with the plant. As the early blight fungus can be spread by pest insects, companion plants that attract beneficial insects are also useful because the beneficial insects can reduce the numbers of pests that might spread the disease.
If you choose to prune your tomato plants (a practice that I don't recommend partly because it increases the chance of spreading plant diseases), make sure you thoroughly disinfect your secateurs in between plants. If early blight or other fungal infections are a common problem in your area, you might also consider washing your hands in between handling different plants to prevent you from transferring diseases on your skin. If you wear gardening gloves when handling nightshade plants (a good idea because they can irritate the skin - tomato plants in particular) it isn't practical to wash your gloves in between plants but if you wear rubber gloves instead, you could wash them as you would your hands.