There are many species of Alternaria fungus. A. alternata is the most common followed by A. solani. They cause disease in both plants and people. Mold prevention tips.
These fungi mar the leaves of many ornamental plants, the fruit of many food crops and cause skin and respiratory infections in humans.
Alternaria alternata is the most commonly occurring and the most broadly studied. It causes brown spot in certain susceptible citrus trees and asthma in sensitive humans.
This is the organism responsible for
early blight in tomatoes.
There is no cure for an infected plant. The best solution is to plant blight resistant tomatoes and keep their foliage as dry as possible.
Vulnerable plants should be spaced far enough apart that air can circulate freely between them. They should also be so situated that air is constantly but gently moving around them.
Avoid watering or feeding them too much as this will only encourage weak growth which is more disease prone. And fungi is fond of water.
Splashing water is the primary means of transportation for fungal spores in the garden.
Drip irrigation or soaker hoses are a better option than overhead sprinklers.
Copper based fungicides also work against this and many other fungal diseases.
Similar advice applies to the home environment. Keep the air on the dry side, replace any water damaged wood and prevent excess moisture from building up on interior surfaces.