Diseases and pathogens in ornamental plants

by Stephen Wagner

When you Google Francesca Hand, you’ll find a plethora of results – nursery management, Ohio Lawn Care Association, Master in Plant Health Management, Pretty Plants Doc, ornamental plant pathology. In a 151-page book titled “Plant Pathology in Ohio,” we learn that she is double degreed (B.Sc. and M.Sc.) in agricultural sciences and technologies and Ph.D. in plant pathology, all from the University of Florence, Italy.

In 2013, Hand was appointed assistant professor in the Department of Plant Pathology at Ohio State University, with responsibility for research, Extension and teaching in turf and ornamental crop diseases. She uses conventional and molecular techniques, combined with greenhouse and field studies, to investigate disease epidemiology, biology and ecology of plant pathogens in an effort to improve disease control strategies in economically important crops.

Hand recently served as a virtual presenter for Greenhouse Growers Day, courtesy of the Penn State Extension, to discuss ornamental plant diseases. “There’s warm temperature inside,” Hand said, “because that is conducive to new growth in plants. When we look at the greenhouse environment there are certain factors that are important to the extent of disease development, things that influence the amount of disease and the type of pathogens that we can find. The first is a closed environment. The water vapor that is transpired by the plants tends to accumulate in the greenhouse, making the greenhouse a very humid environment. Usually it’s very wind-free; there is very little air exchange between the inside and the outside. When you put these factors together – warmth, humidity and very little air flow – they are conducive conditions for disease development.”

Another factor is host density – lots of different plants grown in limited space. That, in turn, means that some plants are being grown in sub-optimal conditions. If there is a disease problem (a pathogen), it would be easy for that strain to be transmitted from plant to plant because of plant crowding. When one crop exits the greenhouse and another comes in, Hand noted growers do a good job of cleaning and sanitizing growing benches at the beginning of the season, but it’s hard to keep that up due to overlap during the season.

“In some cases, propagation is being done at different operations, or you buy vegetative material from outside,” Hand said, which can impact the operation on the inside. “At the root level, we have a variety of pathogens that are naturally present, usually in poly mixes and in soil … but they are able to colonize in soilless media as well.” Those pathogens are the water molds and fungi groups and cause root rot diseases. The pathogens invade the root systems, killing the cells and tissue in the roots. When tissue is necrotic, no new roots are produced from those dead cells. Hand said if you have a more mature plant with a bigger and more developed root system, symptoms may not mean plant death. She prefers to call it crop debilitation or compromised uniformity.

The most common pathogen in greenhouse production is pythium (pythium root rot), Hand said. Pythium is a water mold, and the feet of the fungus gnat are frequently a vector for their transmission. “They really like wet environments because they produce some types of spores that are able to swim in water,” said Hand, “so when there’s a lot of water in the media, the pathogen thrives and can cause a great deal of damage.” Those particular qualities bring the pathogen down to a level of commonality, affecting petunias, geraniums, pansies and poinsettias. All varieties of the pythium pathogen are equally aggressive.

“Controlling fungus gnats and algae is important. Keeping hose ends off the ground to avoid picking up the residue is also important,” she said, adding that if you do have a problem with pythium, it’s probably a good idea not to recycle those containers because you always carry the risk of range of use from the pathogen in production.

Another common pathogen in greenhouse production – one that is hard to manage once it’s established – is Thielaviopsis basicola, the fungus that causes black root rot. There are certain plants that bear watching for black root rot like calibrachoa, pansy, petunia, vinca and poinsettia. This is a pathogen that has very specific environments that influence disease development: high soil moisture content and a high pH. “Any time you have a pH of 6.2, you can have problems with this organism,” Hand explained. “Because the pathogen produces a very hardy spore, one that’s hard to control once it’s established, and it particularly likes to stick around on re-used flats from crops that were sick.” It’s also important to select a proper poly mix and to keep the pH to 5.5, if possible, but definitely below 6.0.

From there, Hand moved to Rhizoctonia, a soil-borne fungus. “It lives in the soil in the poly mix. It doesn’t really affect the root system,” she said. “It’s more of a crown rot-type of pathogen. It attacks the interface between the soil and the above-ground plant.” It can attack plants at any stage of production in the greenhouse; it’s also a big deal in the landscape, in flower beds and even on perennials and shrubs. As far as greenhouse crops, it’s particularly a problem on vinca, begonia, impatiens, gerbera daisy and geranium. “This is more of an issue in warmer climates. It prefers soils that are moist but also have high oxygen levels,” Hand said. “The type of damage that the pathogen causes is enough to kill the plant even though the root system may be completely healthy.”

2021-03-02T10:11:25-05:00March 2, 2021|Grower, Grower East, Grower Midwest, Grower West|0 Comments

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