New yield-robbing berry pests send growers, researchers scrambling for solutions

By Renee Stern

Contributing Editor

Expanded blueberry plantings have exposed growers to unfamiliar problems, including a pest that’s moved in from neighboring citrus groves and a mysterious syndrome researchers have dubbed blueberry fruit drop that causes affected plants to lose most or all of their berries.

In California, growers have had to contend with citrus thrips, while researchers in the Pacific Northwest are working to figure out the cause of the strange yield-robbing malady.

Old pest, new host

Tender new blueberry shoots in California’s Central Valley provide a post-harvest feeding ground for citrus thrips, with potentially devastating impacts on the next season’s fruit. The pest wasn’t known as a problem in blueberries until growers began planting fields near the area’s citrus groves, says David Haviland, a University of California Cooperative Extension entomology farm adviser in Bakersfield.

New growth that emerges after harvest in late June and early July will produce fruiting wood for the following season.

Citrus thrips feed on those shoots during summer months, leaving them stunted and shriveled.

“There are no biological controls or cultural controls,” Haviland says.

Some cultivars, such as Star, are highly susceptible to thrips damage. But, he says, “There’s no magic variety that can avoid it.”

He’s tried high-pressure washes to knock thrips off plants with limited results. Work on fungal pathogens to kill the pupal stage in the soil is still in early phases.

A trio of weapons

Growers can target the pest with post-harvest sprays of Assail (acetamiprid) and spinosyn-based Success or Delegate. Also showing promise is Movento (spirotetramat), which was registered earlier this summer.

However, “Citrus thrips have a well-documented history of developing resistance,” Haviland says.

Some growers require more than six sprays per season to keep the thrips in check, he says.

But, Haviland says, “The best three products are different modes of action, so they should hold up for a whole season. That’s about as good as you’re going to get with a resistance-management program.”

Assail has a medium residual effect. But Movento, while providing almost no knockdown, lodges within sprayed bushes to provide long-term control as plants grow. In trials last year, Movento showed up to four weeks of residual control, he says.

Thrips populations can reach high levels during harvest, when spraying isn’t possible. Haviland recommends scheduling a knockdown spinosyn spray immediately after harvest, followed two weeks later by Movento to keep the pest in check.

“It’s something we need to control,” says Blake Ueki, farm manager at Lagomarsino Farming in Tulare, Calif. Two post-harvest spinosyn sprays have been enough to ward

off any damage to company fields.

Without that effort, numbers could build to as many as 10 per leaf. “You don’t want to get that high,” he says.

Losses vary from field to field, and determining their extent is difficult, says Manuel Jimenez, a UC farm adviser in Tulare. Damaged fruiting wood represents only potential fruit.

But affected branches must be severely pruned, setting them back a year to regain lost ground, he says.

Mysterious fruit drop

Meanwhile, researchers are trying to determine why scattered blueberry bushes in fields around the Pacific Northwest suddenly lose most or all of their berries close to harvest.

“You’ll see a mature bush that should have 15 to 30 pounds of fruit and it has only a couple of berries,” says Bob Martin, research plant pathologist for the U.S. Agriculture Department’s Agricultural Research Service in Corvallis, Ore.

Affected bushes are identified easily at that point, even from a distance, Martin says. Without the fruit load of their neighbors, those plants stand up to 3 feet taller.

“It can’t be mistaken for anything else,” says Lisa Wegener, a pest and disease consultant

working with the British Columbia Blueberry Council in Abbotsford.

But spotting symptoms before fruit set is tricky. Leaves of affected plants display red lines—in a different pattern from what develops in some varieties infected with blueberry scorch virus—and blossoms may develop a pink cast.

“But that’s hard to spot and it’s not strikingly obvious,” she says.

So far reports have centered primarily on a single cultivar, Bluecrop, but Martin is reluctant to rule out others. Bluecrop may be more susceptible, showing extreme symptoms, with other cultivars affected in less-noticeable ways, such as poor fruit set. Grafts from affected Bluecrop bushes onto other cultivars should provide some answers.

In search of the cause

Analyses of affected plants have turned up at least one unfamiliar virus, but Martin isn’t certain that’s the entire answer. A similar virus has been identified in other plants and regions, including sickly tomato plants from California, Mexico and Mississippi, offering more areas for study.

How the disease spreads also remains a mystery. Movement within a field is slow and seems to take a radial pattern outward from an infected plant. That, he says, suggests a soil vector such as nematodes. If so, controlling the problem could prove difficult, Wegener says.

Researchers are planting new Bluecrop bushes in tubs of soil from affected fields as well as clustering healthy and infected plants in test plots to try to induce transmission and learn more.

The only common element noted so far is the Bluecrop cultivar.

“The sites are on different soils and a range of climates and grower management,” says Mark Sweeney, berry industry specialist with B.C.’s Ministry of Agriculture and Lands in Abbotsford.

Aphid-management programs to control scorch and other diseases may be slowing the spread of fruit drop, Sweeney says.

Wegener’s research is set up in a screenhouse, with potential to eliminate aphids from the suspect list. If test plants still develop symptoms when aphids are absent, soil-borne vectors such as nematodes could be more likely culprits.

The researchers suggest growers report incidents of fruit drop to add to their data and then remove affected plants to help limit any spread.

“The most important thing is monitoring,” Sweeney says. “Walk the fields and get (infected bushes) out of the fields as early as possible.”

“It’s a big mystery,” Wegener says. “But this season will give us some answers.”

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