First report of Pythium myriotylum causing root rot in Cannabis sativa (L.) in California

In June of 2020 we observed greenhouse grown Cannabis sativa in Sonoma County, CA and Monterey County, CA showing stress symptoms: stunting, leaf chlorosis, and leaf senescence, when moved to flower production conditions. We uprooted symptomatic and healthy plants and observed disease symptoms only in symptomatic plants: reduced root mass, reduced root hair density, and necrosis. Roots and growth substrate samples were taken from infected and healthy plants for further analysis. Approximately one gram of soil was added to 20.0 mL deionized water and 5.0 mL of the resulting slurry was spread on water agar plates. Plates were rinsed free of soil after 24 hours of incubation, then incubated for an additional two days. Diffuse mycelial growth was observed on all soil plates from symptomatic plant pots and not healthy plant pots. Four subcultures were transferred to V8 media and grown for three days. Roots with brown lesions and healthy roots were surface sterilized by soaking in 0.1% sodium hypochlorite for five minutes, rinsed in sterile deionized water, and two-centimeter segments plated on V8 agar. After 24 hours mycelial growth was observed growing from the cut ends of the lesion roots and not the healthy-looking roots. Four subcultures were transferred to V8 and grown for three days. Mycelium from water sample isolates and root isolates were collected and DNA extractions performed using Quick DNA Fungi/Bacterial Kit (Zymo Research Irvine, CA, USA), then PCR amplified using ribosomal internal transcribed spacer (ITS) primers ITS100/ITS4 as described by Riit et al. and cytochrome oxidase I (COX) primers OomCox-Levup/OomCox-Levlo as described by Robideau et al. (Riit et al., 2016; Robideau et al., 2011). Amplicons from all eight isolates for each region were Sanger sequenced and the found to be identical, and consensus sequences deposited in Genebank with accession numbers MW436422 and MW448569 for ITS and COX sequences, respectively. Observation of cultures under light microscope revealed morphological characteristics congruent with P. myriotylum (Watanabe, 2002). Two V8 cultures isolated from roots were cut into approximately 2-3mm2 pieces and transferred to a one-liter flask of water. A negative control using clean V8 was also prepared. The flasks were placed on a rotary shaker and incubated at 150 rpm at ambient temperature for 48 hours. The resulting suspensions for zoospore and control treatments were observed under a light microscope and motile zoospores present in the water suspension from P. myriotylum cultures only. The zoospore suspension was then divided into six equal portions and applied to the soil of six C. sativa rooted cuttings in one-gallon pots. The control slurry was added to two C. sativa rooted cuttings. All plants were grown in a controlled environment for 28 days with 16-hour photoperiod. All plants were then removed from their pots and roots observed for symptoms. Plants that were treated with zoospore suspensions had tan to brown lesions on significant numbers of roots, and reduced root hair density compared to the plants treated with the control V8 agar suspension. Roots samples from all eight plants were then surfaced sterilized in bleach as previously described and five root sections from each plant plated on V8 media. After 48 hours mycelial growth was observed from root sections from P. myriotylum zoospore treated plants and not control plants. DNA extraction and PCR amplification for ITS and COX amplicons was performed from one representative culture for each plant, Sanger sequenced, and aligned with the previous sequences. All ITS and COX sequences were identical to the original sequences from greenhouse samples. P. myriotylum may cause root rot in C. sativa in greenhouse cultivation.