Water scarcity continues to challenge California’s floriculture industry, leading many nurseries to rely on recycled irrigation water as a supplemental or primary water source. Although this approach conserves water, it also increases the risk of recirculating plant pathogens, particularly oomycetes such as Phytophthora, Pythium, and Phytopythium. These organisms move easily in water and can initiate new disease cycles each time crops are irrigated. Because many nurseries lack the infrastructure for water treatment, growers often depend on routine fungicide drenches, which may not fully contain pathogen spread.

To better understand the risks associated with recycled water, UCCE advisors and UC researchers began a monitoring effort in partnership with three floriculture nurseries in San Diego County. Since mid-2024, monthly sampling of recycled irrigation ponds has been conducted using baiting methods, and symptomatic plants have been evaluated to determine whether the same pathogens appear both in water and in crops. Throughout this project, we aim to identify oomycete species most commonly present in recycled water and symptomatic plants, evaluate potential seasonal patterns, and provide growers with information that can guide more effective disease management under a recycled water scenario. 

Are oomycetes present in recycled water?

Since June of 2024, we have monitored at 10 different time points for the presence of oomycete pathogens in recycled water that is used for irrigation of ornamental crops in three different nurseries. In this newsletter article, we will share the results obtained from two nurseries. At each nursery, oomycete baits, which consist of pears and rhododendron leaves, were deployed in retention ponds (Figure 1) that contained a mix of municipal water  and recycled run-off irrigation water.

Nursery 1

This is a large-scale nursery growing ornamental crops mainly in containers on weed mats on the ground, and few crops are grown on raised benches. Irrigation is overhead and runoff is collected in an unlined pond with multiple pumping stations and smaller reservoirs. Recycled water is sometimes filtered before reuse to remove larger debris particles. From water samples, processed through pear or rhododendron baiting (Figure 2), a total of 10 oomycetes were recovered (Figure 3). 

Higher pathogen diversity, based on number of species identified, was higher from January to April.  The most prevalent species recovered over time include Phytophthora (P.) honggalleglyana, Phytopythium (Pp.) litorale, and P. riparia. These species have been reported as inhabitants of water bodies and riparian areas, and their pathogenicity in ornamental crops grown in the facility sampled needs to be further examined. None of these species have been reported as devastating ornamental pathogens. 

Nursery 2

This is a large-scale nursery growing ornamentals in containers on weed mats on the ground. Plants are overhead irrigated and runoff is collected by the weed mats into a large concrete reservoir. Recycled water is filtered to remove larger debris. From water samples, processed through baiting, a total of 8 oomycete species were detected. In this nursery, a greater diversity of species was not particularly observed by season, but in the months of September, April, May and July a higher number of species were detected, with P. honggalleglyana, Phytopythium sp., and P. parsiana as the most common species overtime (Figure 4).  Further work determining the species identity of the isolates identified as Phytopythium needs to be carried out, as well as further evaluation of their pathogenicity in ornamental crops grown in California. 

Taken together, results from collection ponds in the two nurseries sampled show that oomycetes are present in recycled water, and their diversity and abundance vary across nurseries and throughout the year. Nursery 1 exhibited strong seasonal peaks (January to April), while Nursery 2 had more consistent year-round recovery.  These preliminary results suggest that integrated management practices should be kept throughout the year.  Evaluating if the oomycete species found in recycled water are also present in the crops grown at each facility will help to further establish the risk of the presence of these species found in the retention ponds.

Are oomycetes recovered from water also infecting symptomatic crops?

Since February 2025, symptomatic plants including Gerbera, Lavender, Bellis, Lupinus, Dianthus, Calibrachoa, Monarda, Osteospermum, Pansy, Petunia, and Verbena exhibiting root rot, leave and branch necrosis (Figure 5) were sampled.

From Nursery 1, Pythium (Py.) aff. coloratum was recovered from Osteospermum exhibiting root rot and wilting symptoms, and Py. aphanidermatum from symptomatic Poinsettia (Figure 6). In terms of common species found in water and in symptomatic plants, Pythium aff. coloratum was the only common oomycete found in both sources. No other oomycetes were found from the plant samples collected in this nursery. 

From Nursery 2, at least 16 plants from different species exhibiting root rot symptoms were sampled. Oomycete species were recovered from Foxglove, Pansy, Bellis, French Lavender, Monarda, Lupine, Verbena, Delphinium, Salvia, Phlox, among others (Figure 5). From these plants, 11 oomycete species were recovered, with a greater number of species present from plants sampled in July and August.  Some of the known pathogenic species recovered include P. niederhauserii, P. cryptogea, P. cactorum and Globisporangium (a.k.a Pythium) irregulare (Figure 7).  Other oomycete species recovered that need further pathogenicity confirmation include Py. aff. coloratum, Pp. helicoides, Py. myriotylum, Pp. litorale and P. honggalleglyana.  In this nursery, 5 oomycete taxa (P. honggalleglyana, Py. aff. coloratum, Pp. helicoides, Pp. litorale and Phytopythium sp.) were commonly found in water and symptomatic roots, during the same two months-window, suggesting recycled water can be a potential source of pathogen spread in these ornamental crops. 

Takeaways from this short survey:

• Recycled water harbors oomycete species, including several known pathogens of ornamental crops.
• Although it varied by nursery, oomycetes were present throughout the whole year.
• Similar oomycete isolates have been recovered from both irrigation runoff ponds and symptomatic plants, suggesting a possible link. However, further molecular and morphological characterization is needed to confirm species identity.
• Water sampling has been conducted over the past year, whereas surveys of symptomatic plants began only six months ago. A longer monitoring period is necessary to strengthen associations between water- and plant-associated isolates. Continuation of the monthly monitoring will also help determine whether there is a seasonal pattern in the recovery of oomycete species.
• Pathogenicity tests are essential to determine whether the recovered isolates are capable of causing disease.