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Invasive Weed Threatens Pistachio Orchards


Native to California, a troublesome plant species has been rearing its head in recent months, seeking new fertile ground to take root and thrive, often threatening productive pistachio orchards in the San Joaquin Valley.

Alkaliweed is a perennial plant long known to populate along irrigation ditches and roadsides, especially in coastal areas of the state. But over the last several years the weed has revealed its invasive nature, seeking new ground to grow its extensive and often deep root system in farm fields and orchards.

And worse, because this plant, which weed specialists are now considering to be an invasive weed, is defying chemical control management and rebounding from applications of a host of the most effective known herbicides, leaving the specialists scratching their heads on how to stop the threat of it spreading.

“We’re more or less starting at ground zero on this plant. There has been very little research and very little even written about it, so we are starting at ground zero in an attempt to understand how it works and why it resists management as well as it does,” Kurt Hembree, University of California Cooperative Extension (UCCE) Weed Management Farm Advisor in Fresno County, told Western Farm Press.

“Only recently have we viewed this as an invasive weed species. It’s not a common plant. A number of states including California are aware of it, but lately we are seeing it move into areas around pistachio orchards in the San Joaquin Valley and attempts to control it have mostly failed so far,” he added.

Known to populate saline soils with adequate moisture, the plant’s ability to adapt rapidly to new areas is one of its alarming qualities. In some cases, alkaliweed has completely taken over pistachio orchards after first spotted just two years earlier. Repeated applications of postemergence herbicides have only yielded minimal control.

James Schaefer is a second-year graduate student at California State University in Fresno and also works at UCCE in Fresno County. With a degree in agronomy, he has focused on alkaliweed for his master’s degree project. Together, Hembree and Schaefer are in the process of writing the book on the plant’s biology and ecology.

“It is a tough plant to control. While traditional chemicals seemed to suppress the plant for a few weeks, it bounces right back, in some cases seeing regrowth within weeks of chemical application,” Schaefer recently said in a podcast.

He said his research started on the basis of helping growers control the weed, “taking a step back and looking at the bigger picture, like how the plant works, it’s biology and ecology, to help us develop control strategies.”

“What is its level of sensitivity to daylight and shade as well as water moisture and being found in a lot of alkaline and saline soils? Does this play an influence on hosting this plant?” Schaefer asked.

Hembree says studies are currently underway to look at growth characteristics like response to salinity, light, and moisture.

“We realize now that the primary spread of this weed is by seed transmission through irrigation canals and natural runoff channels and into streams, and it is a hardy seed. Once it is distributed in new host soils, it quickly begins to take hold and develops what can be a deep root system that makes it even more difficult to control,” Hembree advised.

He noted that his team is currently working with a pistachio orchard where the weed took hold in a localized area.  Over the course of a two-year period, the team has agreed, it has “taken over the orchard” despite aggressive control efforts. He says they are seeing more of its invasive qualities and that it is concerning researchers and farmers alike.

Hembree says they have tried multiple modes of herbicide action in trials and pistachio growers have “thrown just about every burn down chemical at it. They have only served to suppress the weed. Over two or three weeks the plant will regenerate and send up new shoots from its extensive root system.

“Burn down products have not been effective at all,” he warned.

Hembree said researchers have been working with several pistachio growers in the Valley and last week promoted a new survey they hope more growers will take time to complete concerning their experiences and efforts with aklaliweed. He explained his team is hungry for data about the plant and hopes that grower input might provide new insights into the plant. Growers can and are encouraged to participate in the survey online at  https://bit.ly/2HWuhNW.

“We would like to hear from everyone that has experienced this weed. We have some reports of alkaliweed in cotton and tomato fields as well as nut orchards and we need more data to help us extend our research to get ahead of this weed that threatens California agriculture,” Hembree noted. “We’re now looking to discover more about its biology like what are its strengths and what are its weaknesses. The truth is, we simply don’t know a lot about this plant, so we are trying to expand our knowledge and need fresh data.”

He said assistance is already being offered by other farm advisors and UCCE professors like Brad Hanson and other California State University graduate students. Also working with the team is Anil Shrestha, a weed science professor who chairs the California State University-Fresno Department of Viticulture & Enology. Contributing to the team, as well, are Dr. Lynn Sosnoskie, agronomy and weed science advisor for Merced and Madera Counties.

“We greatly appreciate the help of our colleagues and the growers who are already contributing to our research, and we encourage all others with experience with this plant to take the survey and help us move forward in finding better solutions in what could be a major fight with a truly invasive plant species in the years ahead,” Hembree said.

He warned, the first signs of alkaliweed sprouts occur in March and by the end of May it will produce flowers and seed, “a pretty rapid developing plant.” He noted that the plant grows all summer long right up to December in some areas. From a spreading standpoint, he said this plant probably falls within the top five most active, so it moves downstream quickly.”

“Our current thought is that because the plant enjoys sunshine, it targets pistachio orchards because of open canopies, especially in areas with saline soils and adequate moisture,” Hembree said.

He advises all California growers to watch for early signs of alkaliweed and to contact their local farm advisors if they suspect they have developed the plant in their fields or orchards.

Source: Logan Hawkes, Western Farm Press

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