There are three types of soil fungi that could be found in a flower bed. Of the three, two are actually beneficial to plants. Beneficial fungi make it easier for plants to absorb nutrients, whether by breaking down organic material or immobilizing the nutrients already in the soil. There are no obvious symptoms to your flowers when beneficial fungi are working. The third group of fungi is harmful to plants and is the type most gardeners are concerned about.
Decomposers are saprophytic fungi. They break down dead organic matter, creating nutrient-rich soil that most plants thrive in. Not only is this decomposition process an important part of the ecological cycle, but the organic acids released during the process help the soil retain its nutrients, which can remain for hundreds of years without dissipating in the right conditions.
Mutualists are mycorrhizal fungi that have a mutually beneficial relationship with the plant they are inhabiting. Mycorrhizal fungi live in the soil and colonize a plant's roots. Unlike harmful fungi, which can destroy the roots of flowers or plants, mutualists obtain carbon from the plant as they work to solubilize phosphorus--break it down and make it easier for the plant to absorb and use. Mycorrhizal fungi also attract other beneficial nutrients to the plant's roots.
Pathogens (or parasites) are fungi that can cause great damage or even death to flowers and other plants. Some fungi only cause minor, cosmetic issues, such as small black spots on the flower stems. Others are known as deadly fungal diseases, such as the Verticillium fungus, which causes Verticillium Wilt in many trees and is not curable. These harmful fungi enter a plant through its roots, quickly colonizing them and moving up the plant as they spread. Some soil fungi can also be spread from flower to flower on the bodies of insects as they move between the flowers during pollination. Fungi pathogens that spread by water or air tend to be less harmful than soil-based pathogens.