You’ve tasked crews to scout for pests and diseases. But are you monitoring your crops for another potential problem – one that may be perpetrated by your neighbor? The predicament in question is herbicide drift, and pundits say it’s not a question of if you’ll experience drift, but when.

A common source of herbicide drift damage to nursery crops comes from two chemicals commonly used in ag, Dicamba and 2,4-D.

Drift can occur as particle drift or vapor drift, according to Tanner Delvalle at Penn State Extension. Particle drift occurs when small spray droplets move long distances due to wind. When a pesticide volatilizes or evaporates into the atmosphere and moves off site and damages non-target plants, it’s known as vapor drift. It’s important to know that drift, especially vapor drift, may travel more than 1 mile.

Row crops are not the only source of drift. For example, if your nursery is close to a golf course, a park, a maintained road, a rail line, or a residential development, it’s likely you’ll experience drift injury.

Steve Black, founder of Raemelton Farm in Frederick County, Maryland, experienced significant drift damage from a nearby property in 2017. It was early spring at his B&B nursery and trees were just starting to leaf out, the crew had stopped digging and was transitioning into planting and monitoring for pests. At first, Black and his crew noticed some oaks that “just didn’t look right,” he says. A local extension researcher visited the nursery and thought it could have been from a late frost. But these trees broke bud after the area’s last frost, Black recalls. Next, another extension professional observed the trees and thought it looked like herbicide damage, consequently asking Black what his crew sprayed. But that particular row hadn’t been treated with any herbicides since the previous summer. Then Black and his scouting crew noticed other trees that looked odd, such as red maples with “droopy” leaves and conifers with that same droopy characteristic, but in the leaders. Soon it became apparent that for some species of trees, the odd growth patterns were happening across the entire property. But for others, it appeared only at one end of the farm, what Black called a “spatial gradiation.” For the oaks, there was damage of some sort on every single specimen on the property. The trees closest to the drift source were in their second year of production.

Different species of trees have different levels of sensitivity. The dogwoods closest to the drift source had obvious damage, but that obvious signs dropped off the farther away from the source.

“And there is no remedy for systemic herbicide exposure except to water the trees and try to keep them healthy otherwise,” Black says.

After a handful of experts, including extension agents, university researchers and a for-hire scout, concluded it looked like herbicide damage, Black informed the Maryland Department of Agriculture (MDA), which sent a representative to take samples and begin an investigation. MDA acquired the spray records from everyone in the area (including Black’s own records), and they identified a nearby property that used Dicamba. Unfortunately, the operator’s spray records were not complete, to say the least. The investigation found that the farmer used concentrations and spray tips were specifically prohibited by the label.

Black contacted the farmer and explained the damage to his trees, and the farmer said he needed some time to think about the issue. Next, Black received a letter from the farmer’s lawyer who said to take it up with the farmer’s insurance company.

The insurance company was ready to settle, but the amount covered only about 10% of the damages.

“In my situation, the sprayer’s insurance policy considers herbicide drift as farm pollution and their policy was capped at a very small number,” Black explains. “So, the insurance company offers me this very small number and says once I accept it, I have to agree not to pursue any more damages. They put me in the position of either taking the settlement, which is not enough to cover the losses, or take legal action. I can’t think of anything worse than suing your neighbor. It’s so far afield from who I am as a person and how I was raised.”

Data collection

As soon as Black noticed the damage, he documented every detail in hand-written notes and photos.

“At moment one when you suspect a drift event, you open notes and start a diary with dates, times and specific observations,” Black says. “Everything you do, every person you talk to, that information goes in the diary.”

Although your state ag official will collect samples, Black says to hire a third party to collect foliar and soil samples for testing, including areas where damage looks its worst, where it’s not as obvious and from asymptomatic areas.

And take photos of everything, he advises.

“I was out with the camera and a ruler, and I'm holding branches up to the ruler to monitor the growth patterns and the lack of growth. I documented the leaves that first came out – the ones that were so damaged,” Black says. “And then I documented the leaves as they became twisty and tiny. As the growth progressed over the summer, the leaves got closer and closer to looking ‘normal’ normal again. I had indicative photos of every single species on the farm. And because I used my cell phone, those photos were geo-referenced and dated. It became this monstrously, huge collection of photos, but ones that I needed to help assess the damage. All the data I collected were contemporaneous notes, which beat the ‘he said vs. I said’ arguments.”

Plus, Black keeps inventory and growth records for each tree, so he knew how much every Quercus sp. grows each year, for instance.

Black hired Hannah Mathers, owner of Mathers Environmental Science Services in Ohio, to survey the trees and provide confirmation that his crops were damaged by herbicide drift. Mathers has been conducting studies on herbicide drift and nursery crops because of the lack of published information on the topic.

Analyte tests are critical when it comes to a drift event, Mathers says.

“In my work, I’m trying to correlate the ppm you get from the analyte test with the injury I’m seeing,” she adds. “And certain growth stages are more affected than others, and certain cultivars are more affected than others. That’s why the analyte tests are so important.”

And the sooner tests are conducted, the better your data will be.

“If it’s been six months from the drift event and we get a result of .2 ppm of a chemical, that’s not reflective of what actually happened,” she says.

In her research, Mathers has observed twisted leaves with Group 2 herbicide drift. When Group 4 herbicides drift on to nursery crops, she’s observed abnormal stem and leaf growth such as downward twisting, parts of leaves fused together, loss of apical dominance and abnormal vein formation that curls around itself.

Besides the technical expertise, Black needed an expert that could help calculate the dollar amount assigned to the drift event.

Black’s attorney hired Stephen Ford, who consults as an expert witness in litigation involving agricultural damages. He’s also an ag economist and a farmer.

“When it comes to figuring out dollar amounts for losses, you need someone who understands production nursery economics and someone who can come up with a proper calculation and a defendable number,” Black says.

The grower who incurred damages has to come up with a dollar amount that makes their situation whole, but “don’t ask for the moon and the stars when you’re not really due the moon and the stars,” Ford says.

On the other hand, don’t sell yourself short. There are a lot of costs involved that growers may not have considered, he adds.

“In some cases, there may be a cost of delaying the harvest, the cost for carrying over the crop for another year, or the cost of a delayed subsequent crop,” Ford explains. “A grower may decide to sell the damaged product or sell it as a less than optimal time, which lowers his market price. There’s also the possibility that a buyer finds out about the damage and labels the grower as an unreliable supplier and you lose their business. And make sure you’re keeping track of all your time and your staff’s time that’s dealing with this issue. Know how many man hours you’ve spent on this event.”

Even if you’re not taking legal action, all these costs are necessary to understand and track if you’re presenting them to the offender or an insurance company.

“Know the costs of doing business so you can come to a solution and a resolution,” Ford says.

Sometimes your own records are what makes or breaks the case in a drift event.

Black noticed some discrepancies in the weather conditions that were noted in the farmer’s spray records when he reviewed them with his attorney. Black had his own meticulous spray records, but he needed more proof beyond the he said/he said argument.

The day the farmer allegedly sprayed, Black hosted a tour group on his property that had a professional photographer in tow. The camera tagged each photo with geolocation and time/date stamps. When Black obtained the photos, it was obvious that the sky was cloudy, it was windy (trees and people’s hair indicated the wind direction) and the ground was wet. What was apparent in the photos did not match up with the applicator’s spray records.

“Besides your own records, you need as many sources of data that you can possibly collect when you’re trying to remedy spray drift from somewhere outside your own property,” Black advises.

Most of Black’s crop eventually recovered, but he lost an entire year of growth and still had to pay all the production and management expenses of caring for the trees, along with extending production time in the field.

“With the oaks especially, it was like they just stood still for a year,” Black says.

There were a few trees that had to be culled when they developed cankers and bark splits, but it wasn’t a large amount.

“Even though the experts we talked to said the trees would eventually be fine the following year, I was fully prepared to send checks to customers if the trees didn’t leaf out. I made sure we could do that, but it could have been a business-ending decision,” he recalls.

It was a tough lesson and one that still leaves him with a pit in his stomach.

“It killed me to have to take legal action but the damages were huge and I have a responsibility to everyone who works here to keep the company afloat,” he says.

Lessons from the field

After going through a drift event, Black wishes more people in the industry talked about this issue and how it affected their nursery.

“If I had known about other people’s experiences with herbicide drift, it would have made my handling of it much better because I was figuring it out from a blank slate,” Black says. “That’s why I think it’s important for people to talk about this issue and help fellow growers through it when it happens to them.”

Besides the reactive lessons, Black learned there are proactive ways to deal with a drift event.

First, have open communication with your neighbors and know what they’re growing and spraying.

“Make sure all of your adjoining landowners understand you produce ornamental crops that are sensitive to herbicides,” Black says. “I send out a letter every spring reminding our neighbors that we grow an ornamental crop and they are sensitive to herbicides.”

Next, register your property at Drift Watch (www.driftwatch.org).

Drift Watch provides a directory of state sensitive crop registries and sells field signs that you can place along your property. Black registered his farm, including his USDA-certified organic trees.

“Professional applicators are supposed to check to see if there any sensitive crops in the area,” Black says. “This includes companies that spray the sides of the highways for weeds. Those are not targeted sprays, so if you’re the next farm over from the road crew, you’re going to get drift.”

Each one of Black’s applicators is a licensed applicator, and the nursery keeps scrupulous records, he says.

“With a drift event, you have to first prove you didn’t cause the damage yourself by showing your spray records with your neighbors and investigators. These are contemporaneous notes and have much more validity than someone’s recollection.”

Scout your crops regularly so you’ll know at the earliest opportunity when you’ve had a drift event.

“If you don’t realize it until two weeks after the drift event, you may not be able to detect the herbicide in the soil,” he explains.

Black also added herbicide drift detection plantings along the perimeter and of the property and inside the farm, including grapes, forsythia and some legumes like soybeans and sunflowers.

“Grape foliage will show Dicamba and 2,4-D symptoms basically the next day after a drift event,” he says. “What you use in those detection plots may vary by state, so ask your extension agent what to plant.”

Everyone in the field has a phone with a camera, and now when a neighbor is spraying, someone on the crew takes a picture of the sprayer, takes a photo of a larger view of the area of the nursery, checks the weather app and takes a screen shot of it to record temperature, wind speed and other factors.

“They have those photos in case we ever need them. It only takes a few seconds to do it and we have the record. 2017 was a scary year, but we’re now loaded for bear if it happens again,” he says.

Another lesson Black wants to drive home: As a nursery grower, there is no problem coexisting with ag. But it’s important to be a good neighbor and keep the lines of communication open.

Photos by Steven Jeffers and Linus Schmitz

Each year, the Horticultural Research Institute picks a handful of projects from a competitive batch of grant applications. One of the 12 projects picked to receive funding in 2021 is “Early detection of Phytophthora spp. on nursery-grown ornamental plants.”

Steven N. Jeffers is the lead researcher on the project. He’s a professor of plant pathology at Clemson University, where he’s part of the Extension horticulture program team within the Plant and Environmental Sciences Department of the College of Agriculture, Forestry and Life Sciences.

Jeffers has been studying Phytophthora species for many years. In his other role as Extension Specialist, he collaborates with the South Carolina Department of Plant Industry, USDA-APHIS and the USDA Forest Service to help minimize the impact of Phytophthora on ornamental crops and trees in the Southeastern U.S.

“Phytophthora has been a problem in nurseries forever because once you get them it’s nearly impossible to clean them up unless you take everything out and sanitize your benches and your soils to get rid of overwintering and surviving inoculum,” Jeffers says.

In 2015, Jeffers began to hear from local growers whose lavender plants were dying in the fields. The oils and products from this perennial are in high demand, and it was named the 2020 Perennial Plant of the Year by the National Garden Bureau, which only boosted its appeal. Lately, his lab has become a hub for lavender samples with root disease.

“We have samples from every corner of the country, coast to coast and border to border,” Jeffers says. “Because it’s become such a popular plant, we have a lot of people trying to get into the lavender growing business. But the problems they get in the field, we’ve traced back to the nurseries.”

Diseases caused by Phytophthora species are a constant and substantial problem in U.S. nurseries and greenhouses with few sustainable management options. While the use of fungicides can be effective temporarily, the best long-term solution is sanitation. Growers need to identify and eliminate sources of inoculum early during plant production. The goals of the HRI project are to evaluate methods for efficiently detecting Phytophthora spp. on nursery-grown plants and investigating factors that may affect detection.

“What we’re doing with lavender growers — and it extends to the nursery business — is we want them to get their plants tested before they plant them out in the field,” Jeffers says. “Once you put a diseased plant in the field, you’ve infected that field and made a problem that’s going to be there for many years to come.”

Cleanliness: next to godliness

“Sanitation is the name of the game,” Jeffers says. “But it’s easy to say and hard to do.”

These organisms can persist in soil for years, capable of survival and overwintering through adverse conditions. They also produce a modal swimming spore. These pathogens are very common in irrigation systems that recycle irrigation water.

So what sort of preventative measures can a nursery take? First, start off with clean plugs, a clean bench, and clean potting mix. That’s not too bad for the first batch of plants, but constant production makes it tough to maintain the necessary sanitation level. Phytophthora and Pythium are easily splashed around from plant to plant, and overhead irrigation systems many outdoor nurseries use are notorious for splashing inoculum, Jeffers says.

If you’re recycling irrigation water that includes runoff coming out of pots, you have a chance of infecting or re-infecting plants you irrigate. Swimming zoospores of Phytophthora and Pythium are readily carried in that runoff water, and if the pathogen travels to a retention pond, it may end up back on the plants.

In addition to sanitation, Jeffers and his lab manager, Linus Schmitz, have been evaluating detection methods. They’re encouraging growers to send samples of their plants to be tested. They’ve also been improving the testing process to make it so growers don’t have to sacrifice so many plants to provide a representative sample.

A better way to test

The first step to detecting whether the pathogen is living in your nursery is to test early in the production cycle. Jeffers and Schmitz have been working on a way to test entire trays of plants in a non-destructive manner, which means testing without damaging the plants. As scientists, they’d like to work with the largest possible sample. But how many plugs does the grower want to sacrifice?

Jeffers has growers send a picture showing the size and configuration of their tray. His team then figures out how many plugs will be necessary for a representative sample. Currently, they shoot for 25-30%. Next, they have the growers pick up the selected plugs, cut off the bottom quarter-inch of the roots, then stick the plugs right back in the cell.

This method doesn’t harm the plant; in fact, it may stimulate new root growth. Jeffers has found that the bottom of the plug is the most likely hiding place for Phytophthora zoospores.

Next, the growers place the root clippings in a Ziploc bag and send them to the clinic. Jeffers and Schmitz will test those root clippings and tell the grower if Phytophthora is present in that tray or not — hopefully before the grower has planted them in the field. Jeffers and Schmitz are collaborating with the U.S. Lavender Growers Association to trial this sampling procedure.

When on-site, they may use another method to detect the pathogen: the pour-through test. They place a pan under a tray of plants and pour water through it. It essentially washes the spores out of the plants and into the tray below it. Next, they assay the leachate from the tray. The team has had some success in a preliminary basis with that type of sampling and plans further tests at several area garden centers and nurseries.

“Right now, we’re pretty comfortable that both those sampling methods work,” Jeffers says. “We need to fine-tune them and figure out how accurate they are. We’re also learning that these fungicides, which are really good, effective products, really throw a wrench in our works.”

Proper fungicide use

Growers can protect healthy tissue from becoming infected by using fungicides preventively. Another part of the HRI research project is to find out which fungicides commonly used for Phytophthora mask detection of an active pathogen, and document how long that inhibition effect lasts.

“All of the detection methods we’re working on to rapidly find these things in a non-destructive manner will be all for naught if our growers are using a lot of these fungistatic fungicides [that mask detection],” he says.

For more: Horticultural Research Institute, www.hriresearch.org; https://www.clemson.edu/extension/horticulture/index.html

Agricultural Research Service (ARS) scientists and their Oregon State University (OSU) collaborators have developed a highly detailed genetic way to trace the spread of Agrobacterium, one of the world's most important bacterial plant pathogens. Their research was published in Science (http://bit.ly/agrobacterium_track).

Agrobacterium causes crown gall disease in fruit trees (apple, cherry, berry, walnut), woody ornamentals and shrubs including roses, shade trees, herbaceous perennials, and grapevines, in all affecting more than 100 greenhouse and nursery species valued at $16.2 billion in annual economic activity in the United States. It also is particularly problematic in hydroponic crop growing.

What gives Agrobacterium its virulence is the presence of plasmids inside the bacterial cells. Plasmids are autonomously replicating DNA molecules that have become part of the bacterium but are not essential to the physiology of the cells. These plasmids have genes that give Agrobacterium the unique ability to transfer a portion of the plasmid into plant cells and genetically reprogram the host to cause disease.

These plasmids also have genes that allow Agrobacterium to transfer the entire plasmid horizontally from one bacterium to another rather than only vertically as in parent to offspring. Once acquiring a harmful plasmid, a previously benign strain of Agrobacterium can become a novel pathogen line. This ability has made control of the pathogen and tracking of an outbreak very difficult.

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To develop their tracking system, the researchers first had to figure out the actual evolution and classification of the plasmids found in Agrobacterium. The plasmids come in two main classes: Tumor inducing (Ti) and Root inducing (Ri), neither of which are directly genetically related to Agrobacterium.

Before these studies, the accepted scientific view was that the frequent transfer of genetic information among plasmids and large amount of genetic variation among Agrobacterium species made drawing evolutionary relationships practically impossible. Without such information, it is not possible to track disease outbreaks.

Surprisingly, the ARS/OSU team discovered Ti and Ri plasmids are all descended from just six and three lineages, respectively.

They analyzed whole genome sequences of more than 1,500 strains from the Rhizobiales order, to which Agrobacterium belongs, which showed different lineages of Agrobacterium emerged independently and at different times in the past.

"What we found was that following the transmission of plasmids themselves between bacterial cells was key to tracking disease outbreaks" says OSU postdoctoral scientist and computational biologist Alexandra J. Weisberg. She is co-advised by plant pathologist Niklaus Grunwald with the ARS Horticultural Crops Research Unit in Corvallis, Oregon and professor Jeff Chang with OSU. "Armed with this extensive genetic sequencing information about how to classify plasmids and Agrobacterium, we could infer both how bacteria move among nurseries and how the plasmids move among bacteria," Weisberg says.

Having whole genome sequences of Agrobacterium allowed linking nursery outbreaks on the basis of having strains with the same genome and plasmid sequences, the same genome sequence but different plasmid sequences, or different genome sequences but the same plasmid sequences, Weisberg explains.

The researchers were able to track at least seven cases in which global distribution of plants contributed to the widespread transmission of a single Agrobacterium strain-plasmid combination. In one case, they tracked one nursery that produces plants for wholesalers that may have served as a kind of patient zero source for many outbreaks. Strains of the same genotype-plasmid combination were later identified in two other nurseries in another part of the world.

With the ability to separately analyze the bacteria from the plasmid, they found many cases in which plasmid transmission perpetuated disease spread. For example, the researchers found one strain/plasmid combination that was collected in 1964. Plasmids with the same sequences were identified in strains collected 30-40 years later in very different parts of the world.

Understanding the genetic basis for how pathogens like Agrobacterium evolve and diversify, especially in agricultural settings, provides a new foundation for determining their spread, at high resolution, and assessing risks of future outbreaks, Weisberg says. It allows researchers to determine whether agricultural and horticultural practices are spreading bacteria/plasmid combinations or plasmids by themselves and in turn, will help growers use appropriate strategies to limit spread.

"The strategy has the potential to be applied in tracking other diseases in plant and human/animal epidemiology and even tracking food safety disease outbreaks," she says.

For more: ARS Horticultural Crops Research Unit, http://bit.ly/ARS_Corvallis; Oregon State University, College of Agricultural Sciences, agsci.oregonstate.edu.

Kim Kaplan is a public affairs specialist at ARS, ars.usda.gov.