Ornamental nurseries rely on a large quantity of high-quality water to irrigate a wide range of ornamental crops with diverse water needs.  Over a year period, a containerized nursery in California can consume about 2.5 million gallons of water per acre (Pitton et al., 2018).  This high-water demand contrasts with the current scenario of reduced water availability in agriculture due to climate change, water costs, which vary depending on location, and regulatory measures.   To overcome these challenges, some producers reuse captured irrigation run-off water (recycled water) as an irrigation source.  This practice, as shown in the Payoff of Recycling Nursery Water article from this issue, creates value for producers as it reduces water expenditures significantly, especially in locations where water from utilities or irrigation districts is expensive, and additionally, it improves sustainability.  Nevertheless, the adoption of this practice can be hindered by the presence of waterborne plant pathogens, particularly oomycetes, which can recirculate in the irrigation system.  These organisms have the ability to spread and reproduce in water, thus increasing the risk of pathogen infection in ornamental crops.

Oomycetes such as Phytophthora, Pythium, Phytopythium species are recurrent challenges in the nursery industry, causing root rot diseases in a wide host range of ornamental plants.  In particular, Phytophthora species can cause severe above ground symptoms including wilting, die-back, leaf necrosis (browning), leaf chlorosis (yellowing) and sudden plant death (Figure 1). 

It is important to note that these above ground symptoms can be also caused by abiotic factors like water-stress.  To determine if the foliar symptoms observed in plants are caused by biotic (pathogens) or abiotic (environmental) factors, it is important to check the plant roots. Plants infected with Phytophthora and other oomycete pathogens exhibit root rot: brown discoloration, affected roots often soft, outer root cortex may easily come off, and in extreme cases, reduction of total root ball (Figure 2).  All these pathogens that cause root rot diseases can become more prevalent when recycled water is used as irrigation source without implementing a water disinfestation treatment coupled with pathogen monitoring to assess treatment efficacy.

Waterborne pathogen monitoring studies conducted in containerized nurseries have shown that water recycling increases oomycete pathogens prevalence in the nursery irrigation system.  Pathogen prevalence in water from a nursery in southern California was evaluated from 2015 to 2017 (Redekar et al. 2019).  The nursery had an irrigation system that consisted of a main source of untreated municipal water, coming from the Colorado river aqueduct, which was stored in a retention pond.  The retention pond also received run-off water from the nursery that was previously filtered and treated with chlorine dioxide.  Irrigation water at the nursery was a combination of treated and untread water coming from the retention pond.  The study showed that recycling water in the nursery was enriching for pathogens in the irrigation system, as the detection frequency and abundance of pathogenic oomycetes were highest for retention, irrigation and run-off water, compared with the incoming untreated municipal water.  This study did not specifically address the effect of seasonality on the risk of higher oomycete pathogens prevalence, but it found that oomycete richness was reduced with high soil temperatures (Redekar et al. 2020).  In 2020, oomycetes were monitored at a nursery in northern California that recycles water but does not apply any water disinfestation treatment, consistent with a general lack of information on treatment efficacy at the commercial scale in the state.  The study identified over 14 pathogenic Phytophthora taxa from stored, irrigation and run-off water (Figure 3).  At least seven of those taxa were also recovered from roots of symptomatic plants grown at the nursery (Del Castillo Múnera et al. 2024). Thus, recirculating pathogens in the irrigation system can be a source of infection for nursery stock.

Can the prevalence of plant pathogens in recycling water be reduced?

Nurseries from other states that recycle water have shown similar results to the ones in California.  Adoption of water disinfestation treatments can be effective at reducing prevalence of waterborne pathogens, though seasonality impacts efficacy.  Water monitoring from a nursery in Oregon, showed that recycling water enriched plant pathogenic oomycete species, with highest oomycete abundance in winter (December-February) compared to summer (June-August).  To manage oomycetes in recycled water, this nursery implemented a chlorination treatment that proved to be very effective at eliminating pathogenic oomycetes as it reduced their recovery by up to 85% (Redekar et al. 2019).  In the Mid-Atlantic, pathogen monitoring at two containerized nurseries that recycled water (Del Castillo Múnera et al. 2025) and implemented a chlorination and a slow sand filtration treatment showed that there was a higher risk of oomycete contamination in the fall and spring and that slow sand filtration and chlorination treatment reduced oomycetes by 85% and 75% respectively. Nevertheless, there was recovery of oomycete pathogens in the irrigation water after any of the treatments were applied, suggesting the need to improve both treatments in order to achieve better treatment efficacy.   Taking these studies together, even though recycling water can increase the risk of oomycete diseases in the nursery, it is important to identify the seasonality in which there is a greater risk for oomycete abundance, and effective water disinfestation treatments to reduce oomycete prevalence in recycled water systems.

What water disinfestation treatments are available?

There are several water disinfestation treatments that can be adopted to reduce pathogens in water including chemical (chlorine, copper, ozone, hydrogen peroxide) and physical (filtration, heat, UV radiation) (Raudales et al. 2014). The implementation of a water disinfestation treatment depends on the nursery infrastructure, layout, water volumes that need to be treated, targeted pathogens, crops grown (i.e. phytotoxicity of certain chemical treatments), and financial resources, among other factors.  To get more information about available water treatment options, consult the Resource Innovation Institute,  Clean Water³, and Back Pocket Grower.  These online resources provide tools, fact sheets, and comparisons of efficacy among water disinfestation treatments.  In terms of research, the effect of water disinfestation treatments on oomycetes at the community scale have been explored in Oregon and Maryland but not in California nurseries.  Nursery growers in California could benefit from this type of study, as it would provide information on treatment efficacy on nursery settings and evaluate effectiveness not only targeted at single pathogens, but at a community scale across different seasons. 

Do you recycle water or are interested in doing so?  Here are some recommendations 

Recycling water is an effective practice to reduce costs of water inputs and improve water use and nursery sustainability.  Nevertheless, this practice increases the risk of recirculating oomycete pathogens into nursery systems.  To adopt this practice, growers should consider the following:

• Monitor water for waterborne pathogens in the nursery.  Monitoring can be done by deploying baits (rhododendron leaves, pears) (Figure 4) at different points of the water treatment system of your nursery. It is ideal to monitor the water before and after treatment to evaluate treatment efficacy. This video shows how to monitor for waterborne pathogens.
• Monitoring should be done regularly.  Studies show that pathogen prevalence can be affected by seasonality.  Identifying the times in the year in which there is a higher risk of pathogen infection can inform when is the best time to apply plant protectant products, or consider other water sources.
• If you are interested in monitoring pathogens in your irrigation system, contact the farm advisor in your area and the Del Castillo Lab.

References

• Del Castillo Múnera, J., Frankel, S., Rooney Latham, S., Blomsquist, C., Woods P., Frederickson-Matika, D. and Green S. (2024). Early detection of Phytophthora in nurseries and traded plants in Europe and elsewhere: The California case study. American Phytopathological Society (APS) Pacific Division. Phytopathology. In press.
• Del Castillo Múnera, J., Beaulieu, J., Redekar, N.R., Delgado, C., Eberhart, J., Parke, J., Hasselhoff, S., Hu, M., and Swett C.L (202X). Evaluating oomycete pathogen and community responses to chemical- and slow sand filtration-based water treatment strategies to enable water recycling in nursery production. Plant Disease  (accepted).
• Pitton, B. J., Hall, C. R., Haver, D. L., White, S. A., & Oki, L. R. (2018). A cost analysis for using recycled irrigation runoff water in container nursery production: A Southern California nursery case study. Irrigation science, 36, 217-226.
• Raudales, R. E., Parke, J. L., Guy, C. L., & Fisher, P. R. (2014). Control of waterborne microbes in irrigation: A review. Agricultural Water Management, 143, 9-28.
• Redekar, N. R., Eberhart, J. L., & Parke, J. L. (2019). Diversity of Phytophthora, Pythium, and Phytopythium species in recycled irrigation water in a container nursery. Phytobiomes Journal, 3(1), 31-45.

• Redekar, N. R., Bourret, T. B., Eberhart, J. L., Johnson, G. E., Pitton, B. J., Haver, D. L., ... & Parke, J. L. (2020). The population of oomycetes in a recycled irrigation water system at a horticultural nursery in southern California. Water Research, 183, 116050.

   

Johanna Del Castillo Múnera is an Assistant Professor of Cooperative Extension at UC Davis. She can be reached at jdelcastillo@ucdavis.edu.