By Paula Flynn
Extension Plant Pathologist
Iowa State University
An oak leaf fluttered across the parking lot on a recent windy day and settled in with the light cover of snow. A white coating on the underside of the brown leaf caused it to appear camouflaged in the snow. The white appearance on the leaf was not caused by snow, but rather a matted mass of fungal strands.
The powdery mildew fungus commonly infects the leaves of various plants, including oak, causing them to appear as if they have been dusted by powder. A careful observer may also notice pinpoint specks of black scattered throughout the white coating. The small black specks are actually tough fungal structures that help the fungus survive winter conditions.
With temperatures rapidly fluctuating between the 50s and below zero this winter, some gardeners have asked if problematic plant pathogens will have difficulty surviving until springtime. Unfortunately, there is quite a variety of microbes out there that thrive under many different conditions.
Some microbes actually prefer to live in extreme environments, such as very hot or cold, salty or even very acidic. Examples of hostile environments include hot springs, volcanoes, acid bogs, salty lakes and areas covered in glacier ice. The organisms that live in environments that we would classify as highly undesirable are given the name extremophiles.
Although most of the microbes that cause harm to the plants in our yards and gardens prefer much more moderate conditions, many have adaptations that allow them to survive from season to season, even when extremes in temperature occur. The fungal strands, known as mycelium, of some fungi can survive the cold days of winter, especially if imbedded and protected in plant tissue, such as a bud. The tiny black structures produced by powdery mildew fungi, known as cleistothecia, serve to aid in the survival of the fungus. Cleistothecia do not crack open to release the spores of the fungus until favorable temperature and moisture conditions occur in the spring.
The crown rot fungus that causes the stems of hostas and other perennials to collapse in the heat of the summer forms tough structures, called sclerotia, that allow the fungus to survive when temperatures are cooler. Sclerotia look like mustard seeds, and appear yellow when young and brick red when mature. These resistant structures first appear at the base of rotten stems, and then fall to the soil. It can be easy to mistake sclerotia for the small, round, slow-release fertilizer pellets that are commonly used.
Resistant structures of plant pathogens can survive in plant debris left on or in the soil, on garden tools or equipment, in insects, or on plant tissues such as buds, twigs, or branches. With such diverse means of survival, it can be easy to unknowingly spread plant pathogens from one area to another area or from one plant to a neighboring plant.
Good sanitation practices are always helpful at reducing infectious disease problems. When possible, remove diseased plant material to avoid continued spread of the problem and to avoid season-to-season survival of the pathogen. This may involve pulling out entire plants at the end of the season, raking and removing fallen leaves, and pruning out diseased twigs and branches.
Just as microbes thrive under certain environmental conditions, so do plants. If extremes in temperature or moisture have caused stress to trees, shrubs, lawns, or other ornamental plants, they can be more vulnerable to attack by disease-causing microbes. Foil their attack by taking measures to restore plant health and vigor, such as watering and mulching in the spring. The fungi that hang around in nearby parking lots will have difficulty finding a good home when the weather turns warm again.