Hausbeck working in the greenhouse
Mary Hausbeck

As part of her research for a graduate degree in horticulture, Mary Hausbeck was conducting a geranium cultivar evaluation when Pythium became a problem in the greenhouse.

Spraying silver ethyl sulfate on the foliage to prevent petal shatter in the seeded geraniums appeared to make the problem worse: within days after the foliar application, plants infected with Pythium died. Other plant pathologists at Michigan State University were being contacted about similar issues in local greenhouses.

Hausbeck started researching whether the application of silver ethyl sulfate did prompt an enhanced Pythium infection and her interest in the disease has continued across the decades. Now, as a distinguished professor and extension specialist at Michigan State University, Hausbeck continues researching the damaging fungus.

Greenhouse Management: What new discoveries have been made since you started studying Pythium in the 1980s?


Mary Hausbeck: Each year, my lab runs new trials to determine if there are new fungicides or biocontrol products that can be helpful.

There have been a lot of new fungicide products released to address the group of water molds, including Pythium. We’ve made big advances with products to control Phytophthora and downy mildew, but they are “B-team” players that are not as good as older products like Subdue and Truban. So, there is still work to do.

Recently, we sampled and tracked Pythium problems across some of the major floriculture crops and were able to determine which Pythium types were affecting these crops and which of the Pythium types tend to become resistant to fungicides.

GM: What are some of the challenges to treating Pythium?

MH: The first challenge is to make sure the problem is accurately diagnosed. We also don’t have as many “new” products that are highly effective against Pythium root rot, which can lead to overuse of the products that have been available for many years, which leads to another challenge of Pythium developing resistance if fungicides are overused. Timing treatments is an important consideration because once the infection is established, it’s more difficult to turn the treatment around.

GM: How can greenhouse growers manage the disease and prevent it from spreading?

MH: For root rot pathogens like Pythium, keeping the headhouse and production areas clean is very important.

Pythium is very good, actually, at staying in greenhouses, in soil particles, in the dust of a greenhouse. Due diligence needs to be done between crops when there are opportunities to do some deep cleaning.

Growers have to think about sanitation as a two-step process: the first step is making sure that the surfaces are clean, which means using a pressure washer, picking up and sweeping up materials, dislodging soil and plant residue from plant benches and other structures, and hosing them down. The second step is sanitizing those clean surfaces because we know that surfaces can appear clean and still harbor some of these really resistant propagules. For that second step, we talk about time of contact. We want as much duration of contact with the sanitation material and the surface, and that means we’re not going to go in with a light spray, we’re going to go with a dousing where that surface is going to remain wet with the sanitizing agent for as long as we can manage. Increasing the time of contact with any pathogen that’s residing on that surface increases the likelihood that that pathogen is going to succumb to the sanitation agent. A light spray that dries immediately isn’t going to do it.

GM: How does Pythium affect bedding plants in the greenhouse before they enter the landscape?

MH: These root rots can make the plant less vigorous, making establishment more difficult. If plants are under stress due to disease, the stress associated with transplanting and the landscape environment may reduce the quality and aesthetics of the plantings. The plants might not grow as large as they should or flower as profusely.

GM: What should growers know about the plants that they sell to landscapers, which may be subject to certain diseases once they’re in the landscape?


MH: It’s helpful for landscapers to know that some of the newer fungicides are highly effective and long-lasting against pathogens [and] applying certain fungicides to plants while in the greenhouse, prior to being placed in the landscape, can help provide protection in the landscape.

I like the idea of growers treating for Pythium, applying a drench of a good Pythium product before those geraniums and other ornamentals are sent out of that greenhouse for establishment in the landscape bed.

GM: If we talked again in five years, what would be the ideal advances you’d like to discuss in Pythium research and treatment?

MH: I’d like to be able to say that we have new options for growers either along the lines of traditional chemistry or biopesticides that have a reduced impact to the environment. In a perfect world, we’d have a program where growers could choose a more traditional program or a hybrid program that allows them to do some [chemicals] but also take advantage of the biologicals. Maybe we can get there in the next five years.

The author spent a decade working for a greenhouse grower before becoming a freelance journalist.