Crown Gall

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.

 

Fire Blight

One bacterial disease common in the San Francisco Bay Area is fire blight of the Rosaceae family, which attacks fruit trees, including apple, cherry, apricot, and pear. Fire blight is caused by the Erwinia amylovora bacteria. Its common name reflects that an infected tree looks scorched and blackened as though by fire. Note that bacteria are typically host-specific, so that each tree species has its own E. amylovora pathogen variation (pathovar).

This small stand of Pyrus calleryana ‘Aristocrat’ (Aristocrat Pear)
street trees in Dublin, California looks to be infected with fireblight. 

Fire blight bacteria are spread to flower buds and nectaries in early spring, in humid warm conditions, either through splashing rain or pollinators. The bacteria invade the tissue, causing the blossoms to wilt and collapse. A brown sticky exudate is produced from the damaged tissue and spreads to young shoots. The shoots become infected, blackened, and deformed in a characteristic “shepherd’s crook” shape. The bacteria overwinter in cankers that form in the bark and wood. In the spring, when the temperature and moisture levels are favorable, the cankers ooze with bacteria, and the life cycle begins again. The exudate is splashed onto nearby blossoms, or attracts pollinators, which pick up the bacteria and carry it to new blossoms.

Aristocrat pear blossoms, with fireblight symptoms	Characteristic "shepherd's crook" fireblight symptom
 Bud with symptoms of new fireblight infection	Blossoms with no apparent fireblight infection

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The best treatment is prevention, which includes planting resistant fruit trees and keeping trees healthy and vigorous to ward off infection. You can spray the tree in the spring with copper-based applications, designed to provide a barrier that prevents bacteria from reaching the blossoms, but timing is critical.  The IPM has some guidelines and references for determining the best time to spray, typically based on temperature and humidity. Several sprayings may be needed, since blossoms do not all open at once, and conditions favorable to the bacteria may persist over several months. Once infected, pruning infected limbs or branches 8-12 inches below visible injury can help stop the spread of infection. Some experts advocate sterilizing pruning equipment between cuts; others disagree.

To learn more about fire blight, visit the IPM website: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7414.html. You can also search the site for your specific crop: http://www.ipm.ucdavis.edu/PMG/crops-agriculture.html

Plant Diseases – Bacteria

For the next several posts, I’m taking a look at diseases that affect plants in the San Francisco Bay Area, specifically in the Monera (bacteria) Super kingdom. For sources I’m using FiveKingdoms, Essential Plant Pathology, the U.C. Davis Integrated Pest Management (IPM) website, and information from my plant diseases course at Merritt College in Fall 2012.

Bacteria are categorized as prokaryotes; they have small ribosomes surrounding their nucleoids, but lack a surrounding membrane. Bacteria are small, and live in large colonies that can grow unchecked if conditions are favorable. Genes are organized into thin fibrils, and are passed from one cell to another. Scientists have recently discovered that bacteria communicate chemically. They can live in relative isolation, but participate in a phenomenon known as quorum sensing – when they sense chemically they have enough numbers, they may take action as a group. Learn more: http://www.ted.com/talks/bonnie_bassler_on_how_bacteria_communicate.html

Bacteria are hardy, and can survive in extreme conditions. They move in water, and invade plants through natural opening, such as stomata, or through damaged tissue. Bacteria are dispersed through sticky masses. Bacteria are typically spherical (cocci), rod shaped, or spiral shaped, so, to identify them, you need to test for chemical changes rather than simply observe them under the microscope. Bacteria are critical to healthy soil, and most are not pathogenic.

With our dry, Mediterranean climate, the San Francisco Bay Area does not suffer from many bacterial plant diseases. However, with our wet, rainy season, temperate climate, and coastal fog, we do have conditions part of the year that are favorable to bacteria.