Fungus can be one of the most destructive organisms to attack plants. Root rot, which often occurs when a plant is overwatered, is one of the most common killers of plants and is difficult to cure. Root rot is especially a problem for plants that are grown in containers, where moisture can accumulate and overwhelm the plant.
Plants that are suffering from root rot will develop leaves that lose much of their color. As the plant loses its ability to take up water as the fungus makes the roots more brittle, the plant will start to droop. Since root rot occurs only if there is sufficient moisture, a plant that is drooping under dry conditions likely does not have root rot and probably needs watering.
Fungicide can often be very expensive, so you should avoid using fungicide unless absolutely necessary and should instead use management techniques that reduce the risk of root rot, such as watering the plant in a way that is appropriate to the species, according to the University of Alabama.
The fungicides used on root rot need not harm the plant or contaminate the soil in the process of killing the fungus. One fungicide called Phosacid kills the fungus and then breaks down into phosphorus, which is one of the top three nutrients that plants need the most. The fungicide is sprayed on the plant's leaves, but the fungicide is taken into the plant and travels to the roots, where it fights the root rot in a targeted way. Since it is phosphorus-based, it does not harm the plant.
Fungicide has been successful in some cases at killing root rot, though the fungicide is not always strong enough and sometimes will harm the plant, causing the plant to become more susceptible to root rot. However, plants that receive fungicide while in the shade are more likely to recover from root rot.
Pots that have been contaminated with the root rot fungus should be soaked in bleach. Otherwise, plants later placed in the pot will also develop root rot, since the spores from the fungus are still in the pot.
The University of California tested a product known as Subdue on plants that were grown in soil infected with root rot fungus. The researchers found that plants given Subdue grew taller than the plants not given any fungicide, suggesting that the fungicide does not harm the plants and helps fight the fungus. Gardeners who wish to grow plants in soil infected with root rot can use fungicides such as Subdue to fight the fungal pathogens within the soil.