Stomp out root rot

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