Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/10/03
Contacts:
Paula Flynn, Plant Pathology, (515) 294-0581, pflynn@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning February 14, 2003
Where to Plant Pathogens Spend the Winter?
By Paula Flynn
Extension Plant Pathology
Iowa State University
Plant pathogens can be an annoyance to the home gardener. They cause spots on leaves, galls on branches, rots on roots and wilts of vines. Many of these problem-causing pathogens thrive in the wet conditions of spring and summer. We might hope that the cold, barren, snowy conditions of winter would eliminate pathogens from our yards and gardens.
Unfortunately, the fungi, bacteria, viruses and nematodes that cause problems on our plants have found ways to survive in the absence of green, actively growing leaves, stems and roots. There are thousands of known pathogens, and many have developed a diversity of ways to "hang around" between growing seasons.
Pathogens can survive in a variety of tissues in perennial plants, such as in buds, roots, vascular vessels, stem lesions or fruit that clings to the plant. Some fungal pathogens form tough survival structures that protect them during adverse conditions of drought or cold temperatures. Some of these structures can even survive for years in the soil without a host plant. These tough structures also have hard names, such as sclerotia, ascocarp and pseudothecia.
Although annual plants die at the end of the season, their pathogens can survive on the dead plant material, also referred to as debris. Fungi are composed of thread-like strands called mycelium. Mycelium nestled in diseased leaves, fruit, roots or other plant parts can often survive winter conditions. Other fungi form resting spores or resistant structures on the plant debris of annual plants. Spores are the microscopic reproductive structures that serve spread pathogens, similar to seeds of a plant.
Some plant pathogens, especially bacteria and viruses overwinter in the bodies of insects, called vectors. Bacterial wilt of cucurbits, such as cucumbers and melons, is a problem that can be carried over from season to season if the insect vector, the cucumber beetle, survives the winter and feeds on new plants in the spring. The disease-causing bacterium is moved into young plants as the beetle vector feeds.
Fortunately, not all of our plant pathogens have developed crafty ways to stay put for the winter. There are groups of fungi that do not survive the cold conditions of winter. These fungi may find their way to our gardens on diseased plant material. Other fungi, such as some of the rust fungi, are blown here on southerly wind currents.
With the knowledge that many plant pathogens can survive the winter in their suit of armor or nestled in plant tissue, it seems spring conditions might bring worry rather than cheerful anticipation of the growing season. Fortunately, our knowledge can help us get an upper hand on these persistent pests.
Be a careful, persistent observer. If a problem is noticed, take action early by removing a plant, pruning out a cankered tree branch, staking stems to improve air circulation or investigating soil fertility levels. Sometimes no action is necessary. For example, no control measures are necessary for a minor leaf spot on a big, established tree. Because certain bacterial and virus diseases survive in insects, scout for insect pests as well.
In some cases, a problem noticed during the season is a cue to take action after the first frost. After cold temperatures kill the foliage of a perennial or the tomato plants in your garden, get busy and remove any diseased plant debris to reduce the chances of the pathogen surviving over the winter. Deep tilling also can speed up the breakdown of plant debris.
As you are planning for the growing season, be sure to rotate the location of plants in the garden from year to year when feasible. Many plant pathogens are specific to a plant or related group of plants. Planting your plants in a new spot each year can put space between any leftover debris from last season and the new plants. When shopping for plants or seeds, look for plants that are listed as resistant or tolerant to diseases. For instance, you can save yourself the trouble of spraying fungicides and strict clean-up chores if you buy a crabapple that is resistant to scab, a troublesome leaf disease.
The last recommendation is an easy task for a home gardener. Spend time tending to plant needs. Plants that are growing vigorously and provided with good growing conditions are best able to resist and endure attack by pathogens. Good cultural practices, such as selecting a suitable location for planting, proper pruning, maintaining correct soil fertility, monitoring water needs, weeding and maintaining adequate spacing, also help keep plants healthy.
Despite forming tough survival structures that have intimidating names, pesky pathogens can be discouraged from your yard and garden by planning ahead, scouting and using diligent sanitation practices.
-30-
ml: isugarden
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.