4 common fungal diseases and how to manage them
We’ve certainly had our share of rain this spring, and as a result, I’ve been hearing from gardeners about various fungal issues appearing in their landscapes. Fungal diseases tend to be exacerbated in wet weather due to the spores easily germinating on wet plant foliage.
While there are dozens of common fungal pathogens that can strike our gardens, some are more common than others. And, some are certainly more damaging than others, too.
Today, I’d like to introduce you to four of the most common fungal diseases found in gardens.
Botrytis or gray mold (Botrytis spp.)
Thousands of different plant species can be affected by botrytis, but geraniums, strawberries, grapes, chrysanthemums, roses, dahlias and peonies are among the most susceptible.
Because it affects a broad range of plants, botrytis is very common in greenhouses. As a result, this pathogen is most often introduced to gardens when infected plants are brought home and planted. It’s fast growing and fast spreading, and often enters via an injury site or pruning cut. Appearing first as white or gray splotches with fuzzy spores clearly visible, botrytis can affect leaves, stems, flowers, fruits and buds. The fungal spores easily spread by water, wind or physical contact. Eventually the infected plant tissue turns slimy and rots away.
The best way to prevent the spread of botrytis is to remove and dispose of any infected leaves and stems. Sanitize all pruning equipment after working on a plant with botrytis, and sanitize all containers that housed an infected plant before planting something else in them. Spores of this fungus overwinter in the soil and on stem and leaf tissue, but it can also overwinter on infected fruit left clinging to branches; remove and discard all infected berries and fruits (called “mummies”) at the end of the growing season.
Effective controls include copper-based fungicides, Bacillus subtilis (brand name: Serenade) and bicarbonate products, though good sanitation is definitely key to controlling this disease.
Cercospora leaf spot (Cercospora spp.)
Since there are many different species of this fungal pathogen, it also affects many different plants. Common hosts include figs, hydrangeas, eggplants, okra, carrots, roses, peppers, beans and beets.
This fungal disease starts as small, circular yellow lesions on the lower foliage of a plant. These lesions eventually develop a soft grey fuzz at the center and a dark brown ring around the exterior. Sometimes concentric rings appear, leading this pathogen to be called “frog eyes.” In severe infestations, defoliation may occur. Fruit size and production may be reduced as well.
Cercospora leaf spot survives the winter in plant debris, and when spring arrives the spores are spread by wind, rain, people and animals. Sanitation is key to controlling cercospora leaf spot. Clean up all diseased plant debris at the end of the season and sanitize pruning equipment before every use.
Cercospora leaf spot is largely an aesthetic issue for home gardeners and fungicide use isn’t typically necessary.
Powdery mildew (many species)
Many common plants are prone to powdery mildew infections, including lilacs, phlox, bee balm, cucumbers, grapes, squash and melons.
Powdery mildew infections are caused by several different species of fungal organisms. Since these fungi live on the leaf surface, powdery mildew is largely an aesthetic issue. Signs of a powdery mildew infection appear as powdery white spots on the leaves and stems, most often on the lower leaves first, that make the plant look as if it were dusted with talcum powder.
Aside from choosing resistant varieties, keep the leaves as dry as possible when watering plants and space plants properly to allow for adequate air circulation. Powdery mildew is mostly an aesthetic issue, so there’s no need to go to extreme measures to save an infected plant.
Organic fungicides based on Bacillus subtilis (brand name: Serenade), neem oil and bicarbonates help keep powdery mildew in check.
Anthracnose (many species)
Many different deciduous shrubs and trees, especially ash, oak, sycamore, walnut, tulip tree, maple and dogwoods, and some evergreens, can be affected
Also called leaf blight, anthracnose is a general term used to identify a number of related fungal diseases, all of which result in marred or distorted foliage, defoliation or twig dieback. Most of the fungi responsible for anthracnose have specific host plants they infect. Early signs can include irregularly shaped brown spots on foliage, distorted leaves, leaf blisters, branch cankers and shoot death. The disease is often worse on lower and interior branches.
The fungi responsible for anthracnose may overwinter in leaf buds, on twigs and fruit, and even on fallen leaves. Prevention tactics include choosing resistant plant varieties, making sure susceptible plants are adequately irrigated during drought and raking up and destroying any leaves that fall from an infected plant before winter’s arrival. Proper pruning to improve air circulation also helps prevent this pathogen. Anthracnose fungi need water to infect plant tissue, so keep foliage dry when irrigating.
Anthracnose will seldom kill a plant unless it’s present for several years in a row and causes repeated defoliation. Fungicides are not effective once the symptoms develop, though copper- and sulfur-based products, neem oil and those made from Bacillus subtilis are effective when used as preventatives.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.