Fungi are the number one cause of plant diseases. Thousands of species of fungi cause disease. Fungi reproduce by producing spores in huge numbers. Fungal spores spread by wind and water, and are unintentionally transported from one plant to another by creatures, including humans. When conditions are right – cool and damp – spores germinate on the surface of plants. As a fungus develops it penetrates the surface of the plant to grow through and/or between plant cells. It then lives off the plant, stealing its nourishment from the plant as it grows. Fungi cause disease in various ways. Some produce toxins that kill cells. Others block the plant’s internal transport of food or water, causing wilts. Examples of fungal diseases include powdery mildew on lilacs, black spot on laurel, black spot on rose, tomato blight, and fusarium wilt.
Plan for health in advance. Start with resistant plants when possible, and plan your planting site to be inhospitable to fungi. Remember that there is no such thing as a disease free garden. The goal is to minimize disease, and therefore the need for fungicides.
1. Since most fungi germinate in high-moisture conditions, good soil drainage is crucial.
2. Good air circulation is also important, because it helps reduce moisture on foliage, stems, flowers and fruits.
3. Space plants so they have ample access to light and enough space so air can circulate freely.
4. Avoid working in the garden when foliage is wet to keep from spreading fungal spores from plant to plant.
5. Try not to wet foliage when watering, and water early in the day so leaves can dry before nightfall.
6. Try not to damage plants by digging into crowns or brushing into or tearing foliage, since damaged foliage invites fungal infection. Deadhead blooms to improve air circulation and drying of plants after rains, but do it carefully.
7. At the first sign of fungal infection, remove all the plants or plant parts that have symptoms and destroy them safely.
8. If the fungi are ones that live or spread through the soil, dig and dispose of the infected soil.
9. Practice superb cultural practices to encourage plant vigor and health as a first line of defense against infection. This includes soil fertility, avoiding plant stress by watering during dry periods, mulching etc.
10. If fungal disease does occur, step back and ask yourself some “big picture” questions. Was the season that produced the outbreak unusually ideal for fungal growth? I.e. was it unusually cool and wet – in which case these problems are unlikely to reoccur in future more normal seasons? Or is fungal disease a problem year after year? If this is a recurrent problem then it is worthwhile to attempt to change the microclimate your plant is living in by improving air circulation etc. Some pruning of surrounding plants or overstory trees may be called for to increase air circulation to the plant. Some thinning of the interior of the crown of shrubs like laurel may also be called for to improve the airflow through the plant. If improving airflow and drainage fail, it may even be worth considering moving the plant to a more suitable site.
