First report of Phytophthora taxon × salinaslettuce (Subclade 8b hybrid) causing stem and basal rot in lettuce in North America.

Abstract

More than 55% of U.S. lettuce (Lactuca sativa L.) production is in California, with Monterey Co. being the largest producer. Stunted mature romaine 'Valencia' and 'Stomper' and iceberg 'Frazier' lettuces with wilted outer leaves were collected from five commercial fields in Monterey Co. in spring 2023 and 2024. Brown internal stem and crown lesions progressed into sunken cavities and plant collapse. Incidence was approximately 5 to 75%. Margins of discolored stem and crown tissue were surface sterilized and plated on PARP-CMA (Jeffers and Martin 1986) and colonies resembling Phytophthora were recovered. Papillate sporangia ranged from 42 to 67.5 × 25 to 45 μm (avg. 56.1 × 37.0 μm, n = 30) and a length/breadth ratio of 1.4 to 1.7 (avg. 1.5). Globose, intercalary or terminal chlamydospores, 20 to 45 µm in diameter (avg. 32.6 µm, n = 30), and oospores, 17 to 29 μm in diameter (avg. 23.9 μm, n = 30) formed on 10 to 12-day-old cultures. Sequences from the two primary barcodes, ITS and COI (Robideau et al. 2011) were obtained from isolates collected from five different host cultivars and locations, finding identical multi-locus genotypes. ITS chromatograms contained six double-peaks, indicating interspecific hybridization. COI found the lettuce isolates forming a clade with the provisional P. taxon castitis, in Subclade 8b where hybrids are common (Bertier et al. 2013). If P. taxon castitis ITS is used as one parental ITS haplotype, the other haplotype is 1 bp different from P. lactucae and P. pseudolactucae ITS, suggesting one is the other parent of this hybrid taxon, provisionally introduced here as P. taxon ×salinaslettuce with accessions PQ427275-9 (ITS), and PQ424946-50 (COI). Pathogenicity assays were conducted on lettuce cultivars 'Bondi', 'El Guapo', and 'Valencia' (seven-week-old; 473-ml pots; 5 pots each for 'Bondi' and 'El Guapo', and 2 pots for 'Valencia'). Phytophthora inoculum of isolate 5411 was prepared as described in Hao et al. (2019) with oat seed replaced with long grain rice. Ten milliliter inoculum was added into two 6-cm-deep holes on opposite sides of the main stem. A non-colonized mixture was added to an equal number of control plants. All plants were maintained in a growth chamber with a 12-h photoperiod at 20°C/18°C. After one week, 'Bondi' and 'El Guapo' inoculated plants were stunted, and after two weeks, older leaves were chlorotic and wilted. After three weeks, 80% of the plants collapsed. Dark brown internal lesions were seen along the stem, crown, and tap roots of the collapsed plants after four weeks. 'Valencia' inoculated plants were stunted with no internal tissue discoloration observed over the same period. No symptoms were observed on any control plants; P. taxon ×salinaslettuce was reisolated and confirmed via ITS sequence from symptomatic stems, crowns, and tap roots of 'Bondi' and 'El Guapo', as well as non-symptomatic feeder roots of all three cultivars. No Phytophthora was recovered from the controls. This is the first report of P. taxon ×salinaslettuce detection on lettuce; P. taxon castitis has been isolated from strawberry and carrot in Sweden and Canada (Bertier et al. 2013), while P. lactucae and P. pseudolactucae have only been reported on lettuce in Greece and Japan (Elena et al. 2006; Rahman et al. 2015). Plans are underway to confirm the hybrid status and parentage of P. taxon ×salinaslettuce using genomics. This emerging pathogen may cause severe economic losses in CA lettuce production during the winter and spring growing seasons.