The bacteria that causes Pierce's Disease, Xylella fastidiosa, colonizes a plant over a period of time causing it to weaken and die.
"It can be a matter of a few years or more quickly, but plants tend to stop producing before they die, so growers will pull them out of a vineyard," Cobb said.
Cobb and his team of researchers zeroed in on the "matchstick" effect of Pierce's Disease. Plants suffering from the disease drop their leaves, but the petiole - or stem that connects a leaf to the vine - remains, resembling a matchstick. This occurs over the length of the vine no matter where the initial insect injection occurred, Cobb explained.
"We've been looking at that area because we think that is a very important indicator of Pierce's Disease, but it also indicates that something is going on there," he explained.
The xylem of a plant is like a pipe with a spring in it which transports nourishing water to various parts of the plant. The bacteria that causes Pierce's Disease, Xylella fastidiosa, moves through the plant in this way.
"What is happening is that the bacteria is actually able to degrade and move through these very thin parts of the cell wall between the xylem elements," Cobb said. "You can actually see them in the 'pit membranes' that are the borders between adjacent cell walls."
The researchers focused the electron microscope at 100,000th of a millimeter along the pit membrane. The membrane normally blocks larger particles from passing through the pits that are located in the xylem, but high-level photographs show the bacteria breaking down the membrane in order to get through the plant.
Cobb said the study examined syrah and cabernet sauvignon plants because they have been known to be impacted by Pierce's Disease. In the field, they selected leaves that were still viable but had some "scorching" or water stress which indicated the disease was pres
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Texas A&M AgriLife Communications