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Sustainable Pathogen Management in Specialty Cut Flower Production 

Cut flowers are among the most profitable floriculture products, worth $333 million in annual national sales (NASS, 2023). Cut flower producers include a diverse combination of large-scale international operations growing traditional crops such as roses, chrysanthemums, and carnations, and small-scale local operations growing specialty cut flower crops such as ranunculus, dahlias, and lisianthus, among many others.

The floriculture industry has recently seen an increase in popularity and sales of these crops, with the Association of Specialty Cut Flower Growers boasting over 2,900 members (ASCFG, 2024), up from only 500 in 2014. However, soilborne diseases caused by bacteria, fungi, and oomycetes can cause significant losses, estimated to be 50-75% in crops grown directly in field soil, including ornamentals. Soilborne diseases are some of the most persistent, as residues and pathogen structures left in the soil are difficult to destroy and build up over time when there is repeated cropping of susceptible species.

For decades, soil fumigation with compounds such as methyl bromide was common in the management of soilborne pathogens; however, it contributed to ozone depletion and was internationally phased out in developing countries in the early 2000s. As chemical fumigants are no longer the go-to option, techniques such as soil steaming and solarization have returned to grower favor. Unfortunately, a soil steamer costs upwards of $10,000, a significant investment, especially for small-scale growers. Additionally, the depth at which the steam effectively penetrates is not always enough to fully destroy all pathogen material present, which makes it an imperfect management solution. 

Soil solarization, which uses solar energy to raise soil temperatures, is another technique that has been studied for use in weed and pathogen management. Although it does not require the high cost of acquiring a steamer, it needs hot, sunny days, making it unsuitable for moderate climatic regions such as those found in the Midwestern and Eastern U.S.

In the early 2000s, a new technique, anaerobic soil disinfestation (ASD), emerged from studies in the Netherlands. In ASD, the soil is amended with an easily decomposable carbon source, typically a locally sourced agricultural byproduct, which is tilled into the soil at a depth of 4-8 inches, then saturated with water and tarped with an impermeable plastic for 3-10 weeks. During this time, soil microbes digest the incorporated carbon source, depleting the soil of oxygen and releasing toxic byproducts, including gases, organic acids, and volatile organic compounds. These compounds have an antagonistic effect on pathogen populations leading to disease suppression. Although ASD has been largely tested in vegetable production, little to no scientific research has been conducted in ornamental systems. 

Research funded by the American Flower Endowment and the Association of Specialty Cut Flower Growers at the Ohio State University is evaluating the suitability of ASD for soilborne disease management in the production of specialty cut flowers in temperate regions. 

In the first year of the project, we conducted a pilot study in controlled environmental conditions with a fungal and an oomycete pathosystem, Rhizoctonia solani on Zinnia and Phytophthora drechsleri on Gerbera daisy. We evaluated three different agricultural byproducts (tomato pomace, soybean meal, and wheat bran) as carbon sources for ASD in both soilless and soil-based substrates. Our results showed that tomato pomace and wheat bran are suitable carbon sources for ASD and that the technique is effective at controlling Rhizoctonia stem rot in both types of substrates. However, ASD proved to be more challenging to optimize for the control of Phytophthora root rot. 

Based on these initial results, activities in the project’s second year have focused on testing ASD in raised bed outdoor production of specialty cut flowers under natural environmental conditions.

Shows process of preparing the raised beds for ASD, incubating for 8 weeks in anaerobic conditions, transplanting Zinnia plugs into each plot, and observing disease development to test sustainable pathogen management

 Figure 1. Process of preparing the raised beds for ASD, incubating for 8 weeks in anaerobic conditions, transplanting Zinnia plugs into each plot, and observing disease development

An outdoor trial using raised beds was set up at Waterman Farms in Columbus, OH. Experiments tested the efficacy of ASD in suppressing Rhizoctonia stem rot in two different Zinnia cultivars (‘Zesty Scarlet’ and ‘Magellan Cherry’) using tomato pomace and wheat bran as carbon sources compared to an untreated control, which was not amended with any carbon source. At the end of the ASD incubation period, three-week-old Zinnia plugs of each cultivar were transplanted into each plot and rated for disease development up to 6 weeks post-transplant. Disease severity was overall higher in ‘Zesty Scarlet’ plants compared to ‘Magellan Cherry’, reaching 44% and 25% disease severity, respectively, in the untreated control plots. Plants in the carbon-amended ASD plots had statistically lower disease severity compared to the untreated control. Overall, wheat bran was the best-performing ASD treatment with plants that remained healthy throughout the study period. Together with the results obtained in year 1, we can confirm that ASD is a suitable disease management option for Rhizoctonia stem rot management in specialty cut flower production.

Additional experiments are planned in 2024 to continue expanding testing of ASD to more pathosystems, carbon amendments, and incubation temperatures/durations. Our hypothesis is that lower temperatures combined with longer periods of anaerobic incubation will result in significant disease reduction, making this technique appealing for use in more temperate climates.

by Francesca Peduto Hand – Professor, Ornamental Crops Pathology | Department of Plant Pathology, The Ohio State University