First report of Globisporangium irregulare causing wilt and root rot on greenhouse-grown common sage in the United States

Abstract

In February 2024, common sage plants (Salvia officinalis) in a commercial greenhouse in Randolph County, North Carolina were submitted to the NC State University Plant Disease and Insect Clinic. Sage plants in multiple trays were observed to have symptoms of wilting, chlorosis, and root rot. Plants had necrotic roots and necrotic areas on leaves, and eventually entire plants collapsed. Symptomatic root segments were rinsed in sterile deionized water, blotted dry, and placed on CMA-PPP selective medium (Eckert and Tsao 1962). Three isolates, each from a different symptomatic plant, were transferred to acidified potato dextrose agar. The morphological structures from each isolate matched the description of Globisporangium irregulare (formally Pythium irregulare) reported by van der Plaats-Niterink (1981). DNA samples were extracted from mycelium mats of the three isolates using the DNeasy Plant Mini Kit (Qiagen, Germantown, MD). The cytochrome oxidase subunits I and II (COI and COII) (Martin 2000; Robideau et al. 2011) and internal transcribed spacer (ITS) regions 1 and 4 (White et al. 1990) were amplified. The amplicons were sequenced and analyzed by BLASTn in NCBI GenBank. The isolates were confirmed to be Globisporangium irregulare with ≥98% identity match and 100% query coverage to type isolate sequences with the following accession numbers HQ708678, GU071760, and HQ643595 for COI, COII, and ITS, respectively. Isolates from this study were deposited in GenBank under accession numbers PP965484–PP965486, PP965487–PP965489, and PP965682–PP965684. To complete Koch’s postulates, ten 14-day-old sage plants were inoculated with 20 mL of 1 × 10 4 oospores/mL of G. irregulare (isolate NC_65624), following a modified protocol (McGehee and Raudales 2021). Individual plants were grown in 5.7 × 5.7 × 6-cm cells cut from six-cell tray inserts filled with a soilless peat-perlite substrate. Plants were maintained indoors on growth carts at 26°C, 50% relative humidity, and a 14-h photoperiod. Wilting occurred 5 days after inoculation, and 60%–70% of inoculated plants showed reduced growth, wilting, and root rot after 14 days. Ten non-inoculated control plants remained symptomless. G. irregulare was recovered from symptomatic roots from all inoculated plants using the method mentioned previously. The experiment was conducted twice. To my knowledge, this is the first report of G. irregulare infecting common sage. G. irregulare has been reported on other herbs such as cilantro (Infante et al. 2018) and clary sage (Reeves et al. 2021). Recognizing G. irregulare as a pathogen of common sage is important for effective disease management and reducing losses in the greenhouse. Preventing root rot caused by oomycetes in greenhouse production systems requires consistent sanitation protocols, avoidance of over-fertilization and over-irrigation, and responsible use of fungicides to minimize disease risk.