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The Cause of Bronze Wilt of Cotton
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A mysterious cotton disease known as bronze wilt
appeared in the Midsouth and Southeast during the hot summers of 1995, 1996,
and 1998. Similar disease symptoms were seen in peanuts and soybeans.
Bronze wilt flourishes when daytime temperatures are above 95 °F for 2 to
3 weeks. In 1996, this occurred in late June and early July.
“We first saw the dramatic evidence of this disease around the 4th of
July, causing me to think of it as the 'Fourth of July disease,'” says
ARS plant pathologist Alois A. Bell in
College Station, Texas. "That isn't necessarily the case every year, but
there is a significant relationship between high, sustained temperatures and
outbreak of the disease."
In 1996, several commercial seed companies asked Bell to determine the cause of
bronze wilt. The disease, which causes death of feeder roots and deficiencies
of phosphorus and potassium in tissues, took its toll on cotton grown in
Arkansas, Georgia, Louisiana, Mississippi, South Carolina, and Texas.
In 1998, cotton losses in Georgia were valued at nearly $25 million. Even Pima
varieties of cotton grown in Arizona and California showed yield losses that
year.
Bacterial Suspect
Last summer, Texas and South Carolina had relatively cool temperatures during
June and early July, but temperatures exceeded 95 °F during late July and
most of August. High temperatures during this stage of cotton development
caused excessive abortion of bolls and seed embryos.
Bell found two species of bacteria in seed from 24 farms in the upper coastal
area of Texas. Greenhouse studies confirmed that heat stress and the bacterial
species contributed to reduced yields of between 20 and 50 percent and led to
poor fiber and seed quality.
Wilted leaves of affected plants turn reddish to copper just before the plants
collapse. Diseased plant stems turn deep red to maroon. "During the day,
I've placed my hands on the leaves of affected plants and found them warmer
than comparable leaves of healthy plants. The higher leaf temperature is
probably caused by the limited ability of damaged feeder roots to take up water
from the soil," says Bell.
Bell discovered that a new strain of the bacterium Agrobacterium
tumefaciens is associated with bronze wilt and is present in the seed of
all U.S. cotton varieties. He was the first scientist to isolate the newly
discovered strain, called biovar 1, from both seeds and roots of affected
cotton, peanuts, soybeans, and dry beans.
Though this doesn't prove that the bacterium causes bronze wilt, there is a
high correlation between the presence of the bacterium and disease symptoms.
More research is needed to pin down the precise relationships between the
organism and bronze wilt.
Searching the "B Genes"
Bell believes he's on the track of revealing important genetic reasons why some
plants are more susceptible than others to bronze wilt. In several areas of the
Cotton Belt, popular cotton varieties are bred with specific genes that convey
bacterial blight resistance and early fruiting, explains Bell.
The resistance genes used in traditional breeding programs are generally
referred to as "B genes." Three genes—B2, B3, and B7—are in
the genetic background of Tamcot SP37, a Texas cotton variety that has been
used in many breeding programs worldwide. This variety provides the desired
early fruiting, which permits farmers to save on insecticides.
"Breeders who used Tamcot SP37 as a parent for earliness could have
inadvertently separated the B genes from each other. Greenhouse studies showed
that varieties with B7 alone are very susceptible to bronze wilt and develop
high Agrobacterium populations in roots.
"Cystic fibrosis in humans is a good parallel for explaining what may be
happening genetically in cotton," says Bell. "Plants with one
dominant and one recessive gene remain healthy, but when they have two
recessive genes they develop bronze wilt. It's the same in people carrying one
recessive gene and one dominant gene for cystic fibrosis—they are only
carriers for the disease. But if two carriers have children, one-fourth of
their offspring will likely have two recessive genes and be stricken with the
disease," explains Bell.
A genetic test—like the ones available for sickle cell anemia or cystic
fibrosis—will need to be developed to further confirm these speculations.
Bell is looking for genetic markers to use in tests for identifying seed stocks
that may carry the genes for susceptibility to bronze wilt.
For now, there is no cure for bronze wilt. Avoiding use of highly susceptible
varieties and minimizing heat stress may be the best means of prevention. One
study suggests that farmers should avoid using nitrogen fertilizer alone
without other nutrients.
Other researchers in the Southern Plains Agricultural Research Center in
College Station, Texas, are collaborating with Bell to look for fungal and
bacterial biocontrols. Such biocontrols may, at best, reduce the severity of
disease, as will attempts to correct phosphorus deficiencies in the
plant.—By Linda McGraw,
Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program (#303)
described on the World Wide Web at http://www.nps.ars.usda.gov.
Alois A. Bell is in the USDA-ARS
Cotton Pathology Research Unit, Southern Plains Agricultural Research
Center, 2765 F&B Rd., College Station, TX 77845; phone (409) 260-9518,
fax (409) 260-9470. |
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"The Cause of Bronze Wilt of Cotton" was published in
the November
2000 issue of Agricultural Research magazine.
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