In 2021 and 2022, wilt symptoms were observed in lettuce (Lactuca sativa L.) fields in Yuma County, Arizona (AZ). Incidence was < 1% at all locations. Symptoms included stunting, yellowing and wilting of outer leaves. As disease progressed, outer leaves wilted and turned necrotic. In advanced stages, tap roots turned brown-gray, with few feeder roots. The crown remained intact until the plant collapsed. Symptomatic romaine and iceberg plants were collected from two sites. Necrotic roots were washed and then plated on amended corn meal agar (PARP) (Kannwischer et al. 1978). After 2-3 days, slow growing, coenocytic, submerged mycelia grew from these pieces. In culture, profuse oogonia formed with diameters of 30-39 (avg. = 33.7) µm and spiny protuberances (5-8 [avg. = 6.4] µm long) with thickened bases. Oospores were spherical and aplerotic, with diameters of 25-32 (avg. = 27.8) µm. Lettuce with identical symptoms from the Salinas Valley, California (CA) were also tested and similar isolates were recovered. Pathogenicity was tested using six AZ and one CA isolates. Inoculum was grown on barley seeds moistened with sterile distilled water, autoclaved three times (at 24 h intervals), then inoculated with colonized agar plugs and incubated at 20°C. Inoculum was used after two weeks. For each isolate, 12 3-week-old iceberg (cv. Speedway) and romaine (cv. Del Sol) plants were inoculated by placing 3-4 colonized barley seed next to the roots of the potted plants. Plants were maintained in a greenhouse at 24-26°C (daytime high) with ambient light. After eight days, all inoculated plants exhibited chlorotic lower leaves that later wilted. Both feeder roots and taproots showed brown-gray discoloration and were necrotic. Microscopy showed the presence of spiny oogonia in inoculated roots. Symptoms caused by the AZ and CA isolates were indistinguishable from each other. Isolations from necrotic tissue resulted in colonies morphologically identical to the original isolates. Twelve control plants inoculated with uncolonized barley seed developed no symptoms. DNA was extracted from all seven AZ and CA isolates pre-inoculation, and AZ isolate 2 recovered from both lettuce types post-inoculation for molecular characterization. The internal transcribed spacer (ITS) and cytochrome C oxidase subunit 2 (COX II) were amplified for the above isolates using primer sets ITS1/ITS4 (White et al. 1990) and FM66/FM58 (Villa et al. 2006), then sequenced. ITS of pre- and post-inoculated isolates for AZ (OQ054806 and OQ054807) and CA (OQ564388) matched 1078/1078 bases of Globisporangium uncinulatum (syn. Pythium uncinulatum; AY598712.2) with 99.8% identity. There were two SNPs in COX II for AZ isolate 1 (OR069239); all other isolates pre- and post- inoculation for AZ (OR069240 and OR069242) and CA (OR069241) uniformly matched 533/535 bases of G. uncinulatum (KJ595385.1) with 99.4% identity. Based on these molecular and morphological data, the isolates were identified as G. uncinulatum (Blok and Van Der Plaats-Niterink 1978; Van Der Plaats-Niterink 1981). To our knowledge, this is the first report of G. uncinulatum on lettuce in AZ. Designated as Pythium wilt, this disease is reported on lettuce in The Netherlands (Blok and Van Der Plaats-Niterink 1978), Japan (Matsuura, et al. 2010), and CA (Davis, et al. 1995). Arizona is an important lettuce growing region; if this disease becomes more prevalent, lettuce production in this region could be negatively impacted.
Keywords: Globisporangium uncinulatum; Lettuce; Pythium wilt.