Terra cotta pots do not cause fungus in plants. Terra cotta (clay) pots are naturally beautiful and work well in most gardens. The pots are naturally porous, which means water--and other chemicals--travel easily through the clay pot.
The porousness of the pots is a plus for gardeners because it's hard to over-water a plant. In addition, porous clay pots allow for breathability--good for plants in the summer months--but can cause frost damage to plant roots if left outside in colder months.
Because of the porous nature of the pots, it's likely gardeners will see a white film on the outside of the pot during the growing season. This is not fungus. The white debris is often salt or another run-off ingredient from watering, fertilizers and pesticides that have been used on the plant.
Green algae also might grow on the outside of the pot. This won't hurt a plant. Just clean that off after the growing season.
Though terra cotta pots can crack or break, they are durable and can be used for several years.
But any pot can harbor fungus and bacteria. The best way to avoid fungus in plants is to remove the old soil and thoroughly clean the terra cotta pots before reusing.
Gardeners can clean and sterilize terra cotta pots in several ways. Smaller to medium pots can be washed and placed in an oven--250 degrees or less--for an hour. Do not move the pots when they are hot. Turn off the oven and let them cool before moving. The oven heat kills bacteria, fungus and other diseases.
Another method is to wash the pot and then fill with a bleach water (1:10 ratio) and let it sit for 30 minutes to an hour. The clay pots absorb the bleach, so it's best to rinse with clean water and let air dry for a day or so before using the pot again. Some gardeners also wash clay pots in the dishwasher with a small amount of vinegar.
To remove the white build-up on the outside of the pot, scrub using a soft brush to and baking soda paste, then rinse off. This also works well for any algae growth on the outside of the pot.
Terra cotta pots should not be stored with old soil when not in use. Removing soil and cleaning pots is the best way to avoid any fungus or other disease on plants.
During the growing season, monitor the plants' growth. Keep moisture low by allowing the soil to slightly dry between watering, keeping the plant in sunlight also will prevent mildew.
For year-round plants, repot at least once a year (usually in the spring) to avoid fungus. Clean the pots and reuse. Watering and root growth cause degeneration of soil in potted plants. Repotting gives the plant an added boost of nourishment.