Modifying Rootstocks to Fight, Prevent Pierce’s Disease

Scientists Help Grapevines Double-team Pierce’s Disease

By Patrick Cavanaugh, Farm News Director

 

The glassy-winged sharpshooter is a flying insect that vectors a fatal grapevine malady known as Pierce’s Disease. Scientists are conducting volumes of research to fight this insect and reduce disease infection on vines. Moreover, scientists are studying methods to prevent the spread of Pierce’s Disease on winegrapes in California entirely.

California grapevine rootstock
California grapevine

Ken Freeze is the outreach coordinator for the California Department of Food and Agriculture (CDFA) Pierce’s Disease Glassy-winged Sharpshooter Board (PD/GWSS) as well as an account director at Brown-Miller Communications. On behalf of the PD/GWSS Board, Freeze communicates with winegrape growers who pay the Board an assessment to conduct research. Freeze explained ongoing research and how research funds are being spent.

FIGHTING PIERCE’S DISEASE

Could introducing pectin into grapevines, help prevent that bacteria, Xylella fastidiosa, from entering the vine? Freeze said, in a recent field trial, “scientists found five different genes that when placed in a rootstock, put either a molecule or a protein up into an unmodified scion that in one way basically stops the bacteria from moving around,” he said.

“For instance, one of the genes comes from the pear,” said Freeze. “I think we’re all familiar with pectin. A lot of plants produce pectin naturally; grapevines are not one of them. By modifying a rootstock with this pear gene, when the bug comes and injects the bacteria into the plant, the pectin literally gums it up and it can’t move around.”

An unmodified plant injected with bacteria by the glassy-winged sharpshooter, turns to another strategy. “When the bacteria population reaches a certain point, [the plant] releases a protein that causes the bad bacteria to stop moving around. However, in grapevines, it is too late when that signal comes; the plant is already dead. By modifying the rootstock to automatically generate that same protein when the bacteria enters the plant, the bacteria shuts down,” said Freeze.

winegrapes fight Pierce's Disease
Winegrapes

Freeze said scientists are seeing that these strategies are working quite well. “What scientists are actually doing now is stacking these genes, two by two, in the rootstock. Now each root will produce two different ways to shut down the bacteria. If for some reason in the future the bacteria figures out a way to overcome one of those ways, chances are it won’t figure out how to overcome both of them.”

PREVENTING PIERCE’S DISEASE

Dr. Andy Walker, a UC Davis professor and geneticist in the Department of Viticulture and Enology, has been working for years on rootstock that could block Pierce’s Disease from entering the vine. “He’s already released 14 different varieties to the Foundation Plant Services as well as three different rootstock that are resistant to Pierce’s Disease,” said Freeze. “The best varieties will be released to nurseries probably next year. Then from there [nurseries will] basically start growing them and taking orders from winegrowers.”

Freeze noted that other field trials are continuing. “We have field trials for a benign strain of Pierce’s Disease. It is like giving plants a smallpox vaccination, only it is the bacteria. That is on its way to commercialization. In the future, you might actually order your new vines from the nursery pre-infected with the bacteria that would normally cause [Pierce’s Disease]. But in this case, it will not cause it,” said Freeze.


CDFA Pierce’s Disease Glassy-winged Sharpshooter Board (PD/GWSS)

CDFA Pierce’s Disease Glassy-winged Sharpshooter Board (PD/GWSS) Interactive Forum

CDFA Pierce’s Disease Control Program

UC Davis Department of Viticulture and Enology

UC Davis Foundation Plant Services

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Winegrape Rootstock Trials

Winegrape Rootstock Trials for Pest Resistance and Vine Productivity

By Patrick Cavanaugh, Deputy Editor

Larry Bettiga, a viticulture farm advisor with UC Cooperative Extension, Monterey County, is working with county growers on winegrape rootstock trials to increase vine productivity.

“Several things have happened,” noted Bettiga,“we are replanting vineyards on former vineyard lands, where a build-up of soil pests already exists. Farmers used to grow a lot of beans and tomatoes in the Valley, so we’ve had a lot of root-knot nematode populations from past cropping patterns.”

“We’ve recently seen ring nematode populations developing at multiple vineyard sites,” Bettiga continued. “With the loss of more effective fumigants, and then the loss of post-plant-type nematicides, the use of nematode-resistant roots is becoming more critical to the success of replanting these vineyards. We have hopes that Andy Walker, a UC Davis viticulture professor and grape breeder, is going to supply us with some better options than we currently have.”

“We have a site where we are comparing five new rootstocks that were released from UC Davis with a number of our standard rootstocks. We are just starting that work, so obviously we have to look at them for several years to get a good feel for how those stalks will fit in comparison to what we are now using,” he noted.

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Replanting Trees as Harvest Ends

David Doll on Replanting Trees

By Laurie Greene, Editor

As harvest comes to a close for many tree crops, the time for replanting trees is swiftly approaching. David Doll, a UC Cooperative Extension farm advisor in Merced County, said that if California receives significant rain this year, the replanting process in orchards would be more difficult.

“If we are potentially coming into a wet winter, it’s going to provide challenges in establishing new orchards,” Doll said. “In the case of heavy rainfall, it’s important to keep a few things in mind and plan accordingly. First, if we’re doing any type of soil modification, we need to get a little bit of moisture to help the soil settle.”

Doll said second step is ‘pulling’ berms—the small hills or walls of dirt or sand in an orchard created to divert rain and irrigation water from the tree trunk. He explained, “We want to pull them before the soil gets too wet. We don’t want to walk into a heavy soil field, such as clay or clay loam, and pull berms because in doing so do, we would actually slick that soil over and have to deal with compaction and future issues with the orchard.”

“Third, when we start planting our trees,” Doll said, “it’s important to make sure that we dig a proper hole with wet soils.” Doll warned if you don’t spend the time to dig a hole,  you can ‘glaze’ the soil or form a crust on the sides of the holes, particularly in clay soils, leaving a hard, compact surface that is impenetrable to young roots. He advised to fracture or scratch glazed soil on the sides of the hole with a shovel or rake before filling in to ensure proper root growth.

Doll also said that when planting, the graft union—the point on a plant where the graft is joined to the rootstock—needs to be kept aboveground. “Countless times I’ve seen people plant the graft union below the ground,” said Doll. “Or they’ll plant the tree, pull up a berm, and actually put the graft union below the ground. Keeping the graft union about one hand’s width above the soil line will ensure the graft union remains aboveground as the tree settles.”

“Lastly, if machine planting in very wet clay loam soil, clods [lumps] and air pockets may form,” Doll said. “That’s problematic. The same thing also may occur with hand planting. It’s important to make sure the planters are digging a properly-sized hole and the roots need to be sufficiently covered. The soil needs to be broken down and then replaced around the tree. Finally, to ‘tank’ the tree, apply about 4-5 gallons of water after replacing the dirt to reduce the air pockets and allow the tree to get a good, solid start.”

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