Crown gall is another bacterial disease common in the San
Francisco Bay Area. Crown gall is caused by the Agrobacterium tumefaciens bacteria,
and is named because masses of galls grow on the roots or root crown of
infected plants. Crown gall has a wide host range including 93 families of pome
and stone fruits, nuts, ornamental plants, and vines and canes. It was first
studied in grape vines in Italy by Fridiano Cavara in 1897. Lack of vigor and
poor growth above ground, may indicate a plant is infested with crown gall
below ground. You can dig carefully around the root crown to look for
evidence.
The A. tumefaciens bacteria can live in soil as a
saprophyte (living on nonorganic matter) for a short time, and then invades a
host plant through a wound. The bacterium transfers a piece of its plasmid DNA
into a plant cell; the genetic code integrates with the plant cell’s genome.
Genes on the tumor-inducing (Ti) plasmid cause the cell to divide over and
over, forming a tumor. The tumor, or gall, provides shelter and a food source
for the bacteria, which live and multiply between the cells of the gall. When
the gall deteriorates or breaks off in the soil, the bacteria are released back
into the soil. Galls typically form on plant roots, but may grow above ground
on trunks and stems.
Bacterial gall growing on root |
Crop rotation is an important management tool to break the disease
cycle. Other measures include planting crown gall resistant plants, treating
the roots of transplants with A.
radiobacter (an antagonistic strain), and removing infected plants. Some
experts recommend disinfecting tools used for pruning and grafting, but others
dispute its effectiveness. To learn more, about crown gall, visit the
IPM website: http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/crowngall.html
and http://www.ipm.ucdavis.edu/PMG/GARDEN/FLOWERS/DISEASE/crowngall.html.
I was fascinated to learn that this mechanism of using the
Ti plasmid of A. tumefaciens to introduce genetic code into a plant’s
genome is used to introduce desirable genes into plants and provides the
foundation for genetic engineering. This method is used to breed plant strains that are resistant to specific diseases, accelerating the process of selective breeding that humans have been
practicing since the beginning of cultivation. This is controversial for many, but lifesaving in areas of the world, like Africa, where plant diseases devastate food crops.
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