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Fungus Amongus

In the past few weeks I have been inundated with samples of sick and dying plants, and for the most part, these plants have been suffering from a variety of fungal diseases. Fungi cause more plant diseases than any other group of plant pests in Minnesota gardens. They attack trees, shrubs, fruits, vegetables and ornamentals throughout the summer, and account for the bulk of information requests during the growing season. Some common fungi you may find in your garden are:

1. Leaf Spots: One of the most common fungal symptoms is leaf spotting. These can be caused by a variety of different fungal organisms. Usually, leaf spots have a distinct dark brown or red margin. The inner spot is dead, usually brown or black. Outside the margin you will find healthy green leaf tissue. Some common leaf spot diseases include: Anthracnose, Rust, Septoria, and Early Blight.

2. Wilt: This type of fungi can remain in your garden soil for years, and is often spread by improper crop rotation. Wilting symptoms occur in midsummer when air and soil temperatures are high. Leaves infected with Fusarium wilt turn yellow, wilt and die. Wilting progresses upward and may first appear only on one side of the plant. Verticillium wilt has the same characteristics as Fusarium wilt, but yellowing symptoms are not restricted to one side of the plant.

3. Mildew: Typical symptoms include a white or grayish powdery growth on leaves. (Like someone shook talcum powder on damp leaves.) It grows in warm climates when nighttime humidity is high, and is often seen in areas with limited air circulation.

The good news is that most fungal infections are more cosmetic in nature, and do not permanently damage affected plants. Fungal spores are spread by wind, water, soil, animals, tools and other infected plant material. Infections can be prevented by implementing good cultural practices. Avoid crowding plants (adequate air circulation will aid in disease prevention); purchase disease-free and resistant seeds and plants; water at the base of the plant (use drip irrigation, or water early in the day to allow leaves to dry); remove affected plant material from the garden; and use a thick layer of organic mulch to keep plant material off the ground.

If you have a plant infected by a fungal infection, you can prevent the spread of the disease by applying a fungicide. Fungicides must be applied early in the season prior to the onset of disease. It does not cure infected plants; it protects unaffected plants from getting the disease in the first place.

For more information about pathogens in the garden, visit the University of Minnesota Extension website at Until next time, happy gardening!