All plants from vegetables and flowers to shrubs and trees are susceptible to root rot that is most often caused by fungal diseases. Waterlogged soil hosts fungi that infect roots, sometimes spreading to the crown of the plant and beyond. Because fungicides to treat root fungi living in moisture under the soil are limited, gardeners and growers are best advised to use good cultivation practices to prevent root rot from developing.
Root rot usually causes dull color of foliage; the leaves may turn yellow and wilt. Trees and shrubs may decline for years before they die.
If root systems are healthy, you should see white feeder rootlets; rotted roots look brown and mushy. If a root has rot, you can pull off the outer tissue or slide off the inner core of the root. The remaining core will look like a tiny piece of brown or white thread.
On soft-stemmed plants, you'll see symptoms on the surface of the stem just up from the soil. On woody plants, you'll be able to peel back the bark where the plant meets the roots. You may see brown or brick-red streaks on the wood under the bark.
Buy plants only from reputable sources; make sure they are free of root rot before you buy them.
When you plant potted plants, fill the hole with water; it needs to drain within 24 to 28 hours. If it doesn't, you need to figure a way to make it drain or choose a plant that can tolerate wet soil.
The hole should be at least twice the width of the root ball. Break up the soil that you removed before returning it. The roots should not be crowded, and the sides of the hole should be jagged and rough.
To encourage root growth, break some of the roots on the root ball; straighten twisted roots.
After settling, the plants should be slightly higher than they were originally grown. If the site doesn't drain well, plant on raised beds. You can surround raised beds with old railroad ties, rocks and other materials.
Vertical mulching is a technique used to help drain heavy compacted soil around the roots of trees. Use a power drill with a 2-inch bit to drill holes 12 to 18 inches deep from just beyond the large woody roots to the drip line. Fill these holes with pea gravel or sand or a mixture of both. This will help water to drain and promotes the development of feeder roots.
Most root rot develops when roots are waterlogged for hours in the late spring and summer.
Do not overwater plants. Allow the soil around the plants to dry a few inches below the top before you water again. Do not water the base of shrubs and trees.
Add leaf litter or compost to heavy soils to help them drain.
Keep the crown of the plant dry; remove some of the mulch or soil around the base, but do not expose the roots.
If conditions are dry in the late fall, thoroughly water evergreen plants that will hold their leaves or needles through the winter.
Fungicides are ordinarily used to treat fungal infections of the leaves, trunks, stems, branches and leaves.Treating fungi living in the ground is more difficult.
Horticulturalists at the University of Wisconsin recommend fungicides containing etridiazole, metalaxyl, PCNB, propiconazole and thiophanate-methyl. They recommend biological control agents Gliocladium, Streptomyces and Trichoderma.
No universal rules apply. You need to know precisely which of the many fungi have infected the roots of your plants. Contact your county extension agent for advice on how to diagnose your root rot and which fungicides, if any, you should consider using.