First Report of <i>Golovinomyces bolayi</i> Causing Powdery Mildew on Lettuce ( <i>Lactuca sativa</i> ) in Texas

Greenhouse lettuce is a growing industry valued at 2.5 billion in the world. Powdery mildew is one of the major fungal diseases in lettuce in controlled environment agriculture (CEA) (Hernandez 2025). Lettuce production in CEA in Texas is increasing with the establishment of the most extensive greenhouse production facilities (KXAN 2023). In May 2024, typical powdery mildew symptoms were observed on multiple lettuce varieties, with 40% disease incidence, grown on a nutrient film technique hydroponic system at the Texas A&M Research and Extension Center in Dallas. Powdery mildew covered 50% of the leaf surface. Ten diseased plants were collected using a randomized sampling method from different areas of the facility and analyzed at the Texas Plant Disease Diagnostic Laboratory in College Station, TX. Morphological examination revealed abundant hyphae and conidia on the upper and lower leaf surfaces. Hyphal appressoria were indistinct or nipple-shaped and solitary. Conidiophores (n=30) were hyaline, erect, and 78 to 110 μm long, arising from the upper surface of hyphal mother cells or the lateral surface. Foot cells were cylindrical, measuring 40 to 52 × 8 to 15 μm, followed by 1 to 3 shorter cells, and formed conidia in chains. Conidia (n=87) were hyaline, ellipsoid to ovoid, doliiform or subcylindrical, and 22 to 35 × 10 to 12 µm. Conidiogenesis was formed in the Euoidium-type. Chasmothecia were not observed. The morphological characteristics were consistent with those of Golovinomyces bolayi (Braun et al. 2019). Previously, G. bolayi was named Erysiphe cichoracearum f. intybi and is now considered synonymous, belonging to the G. orontii complex (Braun et al. 2019). The identity of the pathogen was further confirmed through sequence analysis. DNA was extracted from conidia and mycelia using Zymo Fungal Bacterial Kit (Zymo Research Corporation, Irvine, CA. The internal transcribed spacer (ITS) region was amplified using primers ITS5 and ITS4 (White et al. 1990), and the 28S region was amplified using nested PCR with primers ITS1/TW14 and PM28/PM28R (Bradshaw et al. 2020). BLASTn analysis of the 517-bp ITS sequence showed more than 99% identity to G. bolayi accessions AB427188.1 (516/517), AB769463.1 (516/517), AB769447.1 (517/517) (Uchida et al., 2009; Braun et al. 2019; Takamatsu et al. 2013) and the 554-bp 28S large subunit regions showed 100% identity with G. bolayi accessions MT060301.1 (554/554), AB769446.1 (554/554), and NG075340.1 (554/554) (Mieslerová et al. 2020; Takamatsu et al. 2013). The ITS and 28S region sequences were deposited in GenBank under accession numbers PV041439 and PV041440, respectively. Based on morphological data and sequence analysis, the fungus was identified as G. bolayi. Pathogenicity tests were performed and repeated two times on six 14-day-old plants of L. sativa cultivar ‘Green Sweet Crisp’ by gently shaking infected leaves from diseased plants above more than 20 leaves of the healthy plants to let conidia fall. Six additional control plants were not inoculated. The plants were maintained in a growth chamber at 22 ± 1°C with a 16/8 h day/night period with 40 ± 5% relative humidity. Signs similar to those observed on initial disease plants were observed 8-10 days post-inoculation. The pathogen was confirmed using morphological and molecular methods as described above. All non-inoculated controls remained symptom-free, fulfilling Koch’s postulates. G. bolayi is widespread on L. sativa and has been reported in many countries, including the USA (Braun et al. 2019; Mieslerová et al. 2020). To our knowledge, this is the first report of G. bolayi causing powdery mildew on lettuce in a greenhouse production system in Texas. The discovery of this species may necessitate a re-evaluation and refinement of current powdery mildew management practices, especially in a closed agriculture environment.