Funky Phytophthora

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Dr. Janna L. Beckerman of Purdue University shares why Phytophthora is such a nuisance and what growers can do about it.

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March 26, 2019

A plant renowned for its familiar beauty, bleeding heart had never been reported to be a host of one of the most problematic diseases out there — Phytophthora. But Dr. Janna L. Beckerman and her colleagues discovered a hybrid species of the disease had infected that plant with its dangling, cartoonish heart-shaped flowers.

Beckerman, professor of botany and plant pathology at Purdue University, called the discovery “the weirdest thing” and also most disturbing she has seen happen with the disease. Then she and her colleagues ran more studies.

“What we found is that the parent species of Phytophthora couldn’t infect the bleeding heart,” Beckerman says. “But the hybrid species — their offspring — was able to infect the bleeding heart.”

Common types of Phytophthora that can infect plants in the greenhouse include Phytophthora nicotianae (also known as Phytopthora parasitica), Phytophthora drechsleri and Phytophthora tropicalis, Beckerman says. Symptoms include the appearance of nutrient deficiencies, wilting and leaf spots.

But these funky Phytophthora — usually funky in a bad way, Beckerman repeatedly stresses — hybridize to create new species, as she has discovered with bleeding heart. That hybridization, along with a slew of other factors, creates huge issues for growers. However, Beckerman says there are steps growers can take to fend off Phytophthora’s funkiness.

Traveling by land, sea and air

Phytophthora has a proficient ability to spread and maneuver, which made it a head-scratcher 150 years ago for the researchers tasked with originally identifying it, Beckerman says. They believed it to be a fungus, and they discovered it swimming. But there was only one problem: Fungi can’t swim.

Phytophthora is actually an oomycete, which means that it’s not a true fungus — sometimes we call them ‘water molds,’” Beckerman says. “And one of the really funky things about them is that they have a stage where the spores actually swim, and if it’s overly wet, it allows them to swim and spread even greater disease.”

But if that weren’t problematic enough, Beckerman notes that Phytophthora can also spread by growing through soil by little threads called hyphae, producing durable spores and overwintering.

Phytophthora and iron chlorosis on petunia in the landscape
Photo courtesy of Dr. Janna L. Beckerman

Killing plants across the planet

Problems associated with Phytophthora have been widespread, Beckerman says. One species of Phytophthora caused the Great Famine in Ireland in the 1840s. And she notes that a different species of Phytophthora is responsible for the destruction of plant, and subsequently, animal, life in the Jarrah Forest in Western Australia.

“Jarrah is a eucalyptus forest in Australia, and [Phytophthora cinnamomi] has been charged with being responsible for the devastation of 80 percent of the plant life in the Jarrah,” Beckerman says. “And as a result, it’s actually been implicated in the extirpation and possible extinction of many birds and mammals.”

What’s more, Phytophthora ramorum, the cause of Sudden Oak Death, has infected dozens (approaching 100 last count) and killed tens of thousands of plants in California, Beckerman says.

Hybridizing Phytophthoras have caused even more issues. “In fact, there is a disease in Great Britain called alder decline, which is a cross of two fairly common Phytophthoras, and their demon baby — their demon spawn — ended up creating this alder epidemic,” Beckerman says.

Ripe conditions

Not all hosts respond the same to a single Phytophthora species, and a host that is resistant to one species of Phytophthora often isn’t resistant to other Phytophthora species, Beckerman says. She gives the example of one type of vinca (“Cora” series) that is resistant to one Phytophthora nicotiana but not others.

“In a planting bed, you can have different species of Phytophthora, so you plant this Phytophthora-resistant vinca, thinking you’re not going to get disease, and then the plant pathology lab comes back and says, ‘You have Phytophthora,’” Beckerman says.

Because growers have many different plants in a single greenhouse, and because those plants come from different parts of the world — such as South America, South Africa and various parts of Asia — Beckerman says many different “potential Phytophthora species” can come together and allow for hybridization to occur.

Phytophthora on vinca
Photo courtesy of Dr. Janna L. Beckerman

Fighting off Phytophthora

Phytophthora may sound like something out of a frightening film — causing a little greenhouse of horrors. But many movies have happy endings, and this situation should end well for growers, too — or be prevented altogether — if they take the right steps, Beckerman says.

Because of its ability to thrive in water, Phytophthora can also be avoided by preventing the accumulation of standing water and overwatering, and by using appropriate and well-drained growing media, Beckerman says.

And although not many varieties are resistant to Phytophthora — and some are only resistant to some types and not others — Beckerman suggests growing tough varieties such as Dusty Miller.

As with many other diseases, application of fungicides is also effective in preventing Phytophthora, Beckerman says. Growers can add granular fungicides in potting mix if they have had issues with the disease on certain crops, such as vinca and petunia.

“Now, one of the nice things about Phytophthora is that fungicide resistance isn’t [as much of an] issue in Phytophthora in ornamentals as it has been in vegetable crops,” Beckerman says. “But we want to keep it that way, and that means you want to make sure that you rotate regularly.”