Root Rot
Earlier in the season, we were concerned about a predicted drought.
Now it seems that heavy rains are more common. As they say, if you don’t
like the weather, just wait and it will change. Excessively wet soils
have led to many cases of rotted roots on annuals, perennials and nonwoody
plants in the landscape. We’ve had more problems with established
plants because roots were initially injured by drought. Plants may be
stunted or low in vigor, may grow slowly, or may wilt easily on a warm
day. These problems always become more visible in hot weather because
the lack of healthy roots causes a more rapid decline of plants, which
is very noticeable to the homeowners. The foliage may turn yellow to
brown and drop prematurely, usually starting with the older leaves and
moving up the plant. The severity of the root rot depends on the fungal
pathogen, the susceptibility of the host plant and the soil and moisture
conditions.
If a root rot is suspected, remove the plant from the ground carefully,
place it in a bucket of water and carefully move the plant up and down
in the water to dislodge the soil. Then examine the roots for indications
of rotting. If roots are washed too vigorously, all of the rotted tissue
will be washed off, often leaving a white root interior that appears
healthy, but close examination will show that such roots are much thinner
than healthy white roots. A healthy plant has numerous white roots that
appear fibrous. Roots of a diseased plant show various degrees of water
soaking and usually are some shade of brown or black, both externally
and internally. The discolored roots are often soft and mushy, while
healthy roots are firm. There are many root-rot pathogens, but the major
root-rot fungi that are encountered in Illinois landscapes are Rhizoctonia,
Fusarium, Pythium and Phytophthora. In a very simplified
scheme, we can group the first two fungi as those causing a dry rot,
often with a reddish pink cast, to affected roots. Pythium and
Phytophthora can be grouped as the types causing a soft, brown-to-black
rot of roots.
Prevent root rots from becoming a problem in our gardens by using sound
horticultural practices. This includes use of healthy transplants, proper
site preparation to provide good water drainage away from roots, use
of balanced fertilizer and rotation on the garden plantings for 2 or
3 years with unrelated plants to help prevent the buildup of pathogens
in one area. It is also important to remove crop residue at the end
of the season to help reduce pathogen survival. Once a pathogen is identified,
try to find and use resistant varieties when available. Even if all
of the above practices are followed, root rot may still occur. Fungicides
are available to control the major groups of fungi discussed here. The
fungicides protect plant stems and roots not yet affected but do not
“cure” infected plants. Their use seems most significant in
cases where a root rot is discovered in a flower bed and the goal is
to preserve remaining healthy plants to the end of the season.
Source: Nancy Pataky, University of Illinois
August
2000
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