Scorch
(Photo courtesy of Pest
Alert, Volume 10, Issue 3, June 8, 2001,
South Dakota Cooperative
Extension Service)
Scorch in irises is a devastating condition that
puzzled amateur and professional gardeners for many years. It has been
identified in all types of irises, both bearded and non-bearded, and in all
parts of the country. Scorch is characterized by the presence of brown
leaves, leaves not easily pulled away from the clump, and no noticeable foul
smell. Central leaves, beginning at leaf tips, wither and die
back. Affected leaves may turn reddish brown, giving the disease its name.
The rhizome remains firm, but the cortical roots rot, leaving only the
central water-conducting tissues in tact. While some plants can recover,
others will die. The malady was often misdiagnosed and no specific
cause of the disease could be determined. However, in September 2000, an
issue of Tall Talk reported that the cause of scorch had been found.
With the cause identified, it should now be possible to track how scorch is
spread and determine the most effective treatment.
A research team from the Botany Department of the
University of Iowa discovered that the diseased plants showing symptoms of
scorch all contained particles of a
mycoplasma-like organism (MLO). These organisms, which are somewhat like a
bacterium without a cell wall, were only found in the phloem of plants with
scorch symptoms. This type of organism is found in some other plant diseases,
and generally can’t be grown outside of living plant cells or transmitting
insect tissue. The fact that MLOs cause scorch suggested that both antibiotics and heat
could be used to cure scorch. The research team found that subjecting
scorched rhizomes to a temperature of 104 degrees F, for three to four days
completely cured the treated rhizomes. Re-sprouted plants showed no sign of
the disease under a microscope. Scorch does not appear to be particularly
contagious since it will occur only in scattered spots of large plantings. It
has also been confirmed that scorch cannot be transmitted by soil, water, or
direct contact between plants. (Compiled with information from various
sources, including "Scorch Identified," by John Burton Hamilton,
appearing in Tall Talk, September 2000, p. 28)
Suggested Treatments: The temperature and time
required to kill the MLO responsible for scorch can easily be obtained in an
incubator or a food dehydrator. Placing the rhizome in a closed greenhouse or
cold frame should also work in a sunny week. Another suggested treatment is
digging up the rhizome and letting it cure in the sun on asphalt paving for
about a week. Fortunately, the heat does not kill the irises, but only the
MLOs. At present, the suspected agents are aphids, so control of these
insects could also limit the disease.
Another suggested treatment comes from well-known
irisarian Paul Black, who experienced a devastating incidence of scorch (up
to 70%) in his own plantings. Mr. Black noted that most scorch appeared
during times of rapid growth, and that it seemed to occur most often in
clean, well-kept gardens. As treatment, Mr. Black began incorporating the
fungicide Terrachlor (PCNB) into the soil and practically eliminated the
disease. (A.I.S. Bulletin 252, 32)
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