Bedding Plants: Your Worst Enemies
Have you ever wondered why different groups of plants get different diseases? If you have only been growing one type of plant, such as bedding plants, you might think all plants get the same diseases. But since I have switched around from cut flowers to foliage plants to bedding plants, woody ornamentals, herbs and herbaceous perennials, I have noticed some general differences.
In this new series, I will cover each group of plants, identifying the disease spectrum that is a type of fingerprint for many groups. I am hoping that this will get some of you thinking about your crops and that what you have to prevent is not as daunting as the entire spectrum of diseases that can occur on plants.
Some of the elements that affect which diseases attack a group of plants include : type of propagation material, production conditions (greenhouse vs. nursery), utilization (landscape vs. potted house plant), seasonal timing (winter vs. summer) and longevity of the crop (turnover in the final use site). Overall, we have to consider the value of the crop and what inputs are financially feasible. We would like to avoid all problems but preventative strategies are not viewed as cost-effective for all growers.
Bedding plants have the characteristics of primarily being produced from seeds; they are grown in mass (both in propagation and the landscape); and they are fast-growing and exposed to overhead irrigation or rainfall throughout much of their life. Finally, they are not very expensive on a per plant basis relative to crops like potted flowers (mums, poinsettias and orchids).
Why are seed-borne pathogens so common among bedding plants?
Ornamental and vegetable seeds are produced in open fields of the crop where plants are irrigated overhead and/or exposed to rainfall. Wet leaves promote fungal and bacterial diseases. The crop is closely planted to favor high seed set, making spread of fungal and bacterial pathogens quite easy. Flowers themselves are often attacked by the fungi and bacteria that then contaminate seeds. The fact that many fungicides can jeopardize seed set makes their use under these conditions less feasible. Finally, while we often see vegetable seed treated after harvest with effective fungicides or bactericides or even treated with hot water containing disinfestants (like bleach or peroxide) this is not a routine practice by ornamental producers. Again – one can ask why. The answers are a combination of the value of the seed vs. the relatively small volume. In addition, treating seed can reduce its viability which is not an option for ornamentals that are often very expensive.
Seed-borne pathogens include fungi like Alternaria, Cercospora and Colletotrichum and bacteria including Pseudomonas and Xanthomonas. Xanthomonas leaf spots are more common in the summer due to the increased overhead irrigation or rainfall and the increased temperatures. Pseudomonas spp. can also increase in the summer and may be found on impatiens, salvia, basil, marigold and ranunculus. All of these leaf spots can be a problem in the landscape as the plants fill in and leaves stay wet overnight. Overhead irrigation and crowding can lead to an outbreak of leaf spots during production at any time. It is not uncommon to find more than one pathogen involved in an outbreak of leaf spot or blight.
Why does damping-off occur?
The answer to this one could not be more obvious — at least to me. If you start with a seed, it can be attacked immediately, leading to damping-off. A wide variety of soil-borne pathogens including Fusarium, Phytophthora, Pythium and Rhizoctonia are recognized for attacking seeds. However, did you know that Alternaria, Botrytis and Colletotrichum can also cause damping-off on bedding plants? Seed germination is an especially vulnerable step in bedding plant production. The advent of the plug industry has taken away some of the risk of germinating your own seeds and transplanting them. Concentrating all of the risk at a plug producer allows a high degree of expertise to develop in plug production. One concern is that the use of fungicides to prevent damping-off has been demonstrated to reduce percentage or speed of seed germination. This is minimized by producers who rely on cultural and biological prevention.
What does overhead irrigation do to bedding plant diseases?
Flats of plugs are necessarily irrigated from overhead due to the mass of tiny plants that must be watered a little bit many times a day. Even under these conditions, relatively few bedding plants are subject to Botrytis blight during production. Once in the landscape, however, Botrytis can start, as flowering plants are a big target for this ubiquitous pathogen. Since the majority of bedding plants are flowering plants, Botrytis is always a concern. Botrytis often attacks flowers and other less robust tissues and easily spreads throughout a planting since the mass of plants are meant to completely cover the bed area.
Why do landscape plantings keep dying out?
When landscapes are replanted they are usually not treated in any way regardless of their condition. This is partially due to cost of treatment and availability of treatment options, but especially public critique of these treatments. The most common recurring problems to consider are soil-borne fungi including Phytophthora, Pythium and Rhizoctonia and even downy mildew. The first three are often very broad-based in their appetite for bedding plants and easily move from one crop to the next regardless of how related the crops might be. There are really very few bedding plants that will not suffer when planted into a landscape that has been used for years and has become contaminated with these fungal pathogens.
Treatment with granular fungicides and biologicals may help preventatively, but once an area is infested with Phytophthora it is not easy to clean it up with anything except fumigants, which is not an acceptable method for landscapes. Application of fungicides is also not very effective, making crop rotation and variation more critical in a landscape. The other group of pathogens that can be carried over in the soil is downy mildew fungi. While not many of our ornamental downy mildews are known to produce oospores (long-lasting resting spores like weed seeds), it is always a possibility, as we have proof in many other crops that this can and does occur. At least in this case, switching to another type of bedding plant is a real option to avoid continued losses. For instance, the impatiens downy mildew does not attack anything but impatiens and using another crop will stop this problem.
Summary
Bedding plants do have a specific disease profile including:
-Seed-borne pathogens including a few fungi and bacteria
-Damping-off
-Overhead irrigation in propagation, production and the landscape leading to leaf spots
-Contamination of landscape beds with soil-borne fungi
Once you know what you are facing, you can take the appropriate preventative steps. Preventing everything (even those things that are less likely) is not a sustainable way to grow plants.
Bedding Plants: Your Worst Enemies
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