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Rhizome Rot
Cross-section through a
rhizome showing discolored, rotted tissue
(Photo by Iain MacSwann, 1972, An Online Guide to Plant Disease
Control,
Oregon State University Extension Service)
Perhaps the single most prevalent threat to the
Southern bearded iris garden is rhizome rot. Our humid, hot, rainy conditions
are the perfect setting for the onset of this devastating malady. Conditions are most favorable for rhizome rot in warm,
wet weather, but rot may continue even when conditions are cooler or dry. Two
specific causal agents of rhizome rot can be identified: bacterial soft rot
(Pseudomonas spp.) and Sclerotium rot (Sclerotium rolfsii).
Bacterial Soft Rot: Soft rot often appears
first at the base of the leaves where they join the rhizome, characterized by
yellowing and dying of the fans. This is accompanied by a soft, wet,
foul-smelling degradation of the rhizome. Fans may separate easily from the
rotting rhizome or they may topple over. The bacteria that causes this is
present in most soils and enters the iris through an injury such as insect
damage or cultivation injury. Soft rot is more likely to begin when rhizomes
are buried when planted. Overwatering, overcrowding and overfertilizing can
also contribute to onset of the disease. Regardless of the cause, the sooner
soft rot is detected, the more successful your treatment will be.
Bacteria can enter through
wounds, causing onset of rot
(Photo courtesy of Missouri
Botanical Garden PlantFinder)
Sclerotium Rot: The symptoms by which to
recognize Sclerotium rot are very similar to bacterial soft rot, with one
exception. In Sclerotium rot, a white, cottony fungus is visible on the
surface and in the soil around the infected plant. Also, small brown resting
structures (sclerotia) reside in the surrounding soil. Sclerotium rot affects
many types of plants, and like soft rot, the bacteria that cause this
condition may be found pre-existing in the soil. A wet, warm environment
favors infection of the plant with this disease. While the cause of this type
of rot is different than soft bacterial rot, the recommended treatment is the
same.
Prevention: Because wet conditions
lead to rot, plant your irises high in raised beds and well-prepared soil.
Good air circulation is your best ally in preventing rot, so strip off old
leaves as they die, thin out clumps every few years, and water only
minimally. Remember, irises are semi-desert plants, and they can survive on
very little water. While they may turn yellow and look a bit unsightly during
the hottest months of summer, they will come back green in the fall.
When cultivating or working near your iris
garden, be careful not to cause damage to the rhizomes. When injury does
occur, such as when thinning, use a 10% bleach solution or other
disinfectant to treat the wound immediately. Allow the rhizome to dry
completely before replanting. Using PCNB when planting is another excellent
prevention tool available to the Southern gardener. For more information on
this method of prevention, please see the potting and planting suggestions
elsewhere on this website.
Treatment: If, despite all your
prevention efforts, rot does occur, there are a number of ways to deal with
it. All are most effective with early detection and complete removal of the
mushy tissue. If rot is detected in an established clump, dig it out and scrape
out the rotted area. Then, choose one of the following remediation methods:
1) soak in a 10% bleach solution for 30 minutes, 2) dust with a granular
cleanser like Comet, or 3) dust with garden sulfur. Allow the
clump to dry completely before replanting so that the damaged parts scab
over. Any of these treatments work to help dry the wound, kill the
bacteria, and create an alkaline environment that helps prevent further
bacterial growth. While you may lose part of the clump, with thorough
treatment, there is a chance of saving at least part of it. If the rot
occurs in a new plant, or if you are unable to immediately dig an
established clump, you may elect to treat it in place. Again, scrape out
the rotted area, then drench the rhizome with one of the three remediation
methods listed above. Allow the rhizome to dry for a few days before
recovering it with dirt. Be careful to disinfect all tools after treatment.
Keep a watch out for signs of recurrence. Left unchecked, bacterial soft
rot can destroy an entire clump within just a few days. For severe cases of
rot, you may want to employ a chemical such as Aliette for treatment.
For greatest effectiveness, combine one of these
methods with a systemic chemical treatment in the soil (such as PCNB)
before replanting. As with all chemicals, read the directions carefully and
follow them. Do not apply near vegetables or plants that will be eaten.
Wear rubber gloves and long pants during the application.
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