Fungal diseases have changed the world. Fungal infections are the most common plant diseases and the most common cause of famine throughout history. Between 1845 and 1847, the Irish potato famine, which was caused by a fungus, killed 1-million people and was responsible for one of the largest human migrations in history. Fungus afflicts plants by forcing their way into the plant's tissue, often draining their nutrients until they die.
Most people are familiar with fungi as mushrooms, but the anatomical structure responsible for infecting plants is called the hypha, according to naturist Jim Conrad. Mushrooms sprout as reproductive structures from hyphae located underground. Hyphae appear as white cobwebby strands, and gardeners can observe hyphae growing along the leaves of infected plants. The hypha is responsible for growth of the fungus and the absorption of nutrients, of particular importance with respect to fungal diseases of plants.
Fungal infections damage plants because they don't just remain on the surface but invade the cells themselves. At the tip of the hypha, a swollen structure called the appressorium grants the fungus access to the inside of the plant. The appressorium sometimes dissolves the plant's cells using enzymes, and others enter through existing wounds. The appressoria of some pathogenic plant fungi invade the cells by swelling with water until the pressure is so great that it forces its way into the plant.
Not all fungal pathogens are deadly, according to Ohio State University. While saprophytes can survive on living or dead tissue, parasitic fungi require a living host to survive. This forces them to achieve a delicate balance, extracting enough nutrients to ensure their own survival but not so much that they kill the plant. Common plant fungi such as downy mildew, powdery mildew, rust and smut are parasites and require a living plant to sustain them.
Bacteria can only enter a plant through a wound, making them less important as pathogens. Because fungi can force their way into a plant, they are much more dangerous to the plant, and according to the Ohio State University Extension, more than 8,000 fungal plant pathogens are known, causing immense agricultural damage annually. Once the hypha forces its way into the plant, it is free to grow amid the cells, leaching them of nutrients.
The University of Ohio Extension recommends cultural practices as the best defense against plant fungal diseases. Whenever possible, select cultivars resistant to fungal diseases found in your area. Healthy plants are less likely to get sick, so ensure that your plants have access to healthy, fertile soil, adequate water and the correct amount of sunlight. Destroy or turn under dead plant material at the end of the growing season, and handle plants carefully to prevent wounds. If infection occurs despite your precautions, use a fungicide approved for that particular pathogen, always following the instructions on the label.