Francesca Peduto Hand

Photo courtesy of francesca peduto hand

Editor’s note: The e-GRO can be found at bit.ly/Phytophthora-e-gro

Greenhouse Management: Tell me about Phytophthora chrysanthemi — how was it discovered and how is it different from other Phytophthora?

Francesa Peduto Hand: I described this for the first time in 2017. We received a sample from a public garden in Ohio and the plants were exhibiting classic symptoms of root rot from Phytophthora — necrotic root systems and rotting of some of the branches. But there was a parting of the veins that we don’t normally see with Phytophthora. It then tested strongly for Phytophthora. So we started some cultures and realized that it was growing very slow at 25° C, basically room temperature. That’s common for the Phytophthora species we work with. So we started doing more research and found that there was Phytophthora described on mums in Croatia and Japan from a few years early that matched. They had incubated the sample at a much higher temperature, so we did that and the culture started growing very rapidly and filled out the plate. This had not been described in the U.S. before. And [in September 2022], Margery Daughtrey received a sample from a nursery in New York, and based on the plant veins, that it could be Phytophthora chrysanthemi. It ended up being an unconfirmed diagnosis, but she suspected that it could have been. So we decided to put out the e-GRO and let growers know about this.

GM: How do you differentiate this strain vs. others?

FPH: You can’t really differentiate these based on just symptoms. It would have to be sent to a lab and cultured out. The pathogen has specific traits that we look at to differentiate it within other Phytophthora species. It wouldn’t change much from a grower’s standpoint in terms of what you can do about it [to know what it is]. There hasn’t been any fungicide testing done against this pathogen because there’s just been one unconfirmed case. But there are effective management options for Phytophthora and we can assume that they would work against Phytophthora chrysanthemi as well. But, again, it hasn’t been tested.

GM: What is it like working on something like this?

FPH: It’s really exciting to find a new pathogen. Growers don’t like to hear this, but as scientists, we are always fascinated by new pathogens and new opportunities to figure things out about these very important pathogens within the industry. So it was exciting to get pulled into this. Our diagnostic clinic was the one that received the sample in the first place and then they contacted me as the [Ohio] state specialist, and we worked with a representative from the state department of agriculture. And when you hear the name ‘Phytophthora’, I don’t want to say that people freak out. But it is an important name to put out there. There was a lot of concern around it, but it has been limited as of now.