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

Authors: WANG, YU-CHEN - UNIVERSITY OF CALIFORNIA - COOPERATIVE EXTENSION SERVICE; KOSTER, JACK - UNIVERSITY OF CALIFORNIA - COOPERATIVE EXTENSION SERVICE; ROONEY-LATHAM, SUZANNE - CALIFORNIA DEPARTMENT OF FOOD AND AGRICULTURE; BLOMQUIST, CHERYL - CALIFORNIA DEPARTMENT OF FOOD AND AGRICULTURE; BELISLE, WEI - CALIFORNIA DEPARTMENT OF FOOD AND AGRICULTURE; BOURRET, TYLER

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2024
Publication Date: 11/21/2024
Citation: Wang, Y., Koster, J.T., Rooney-Latham, S., Blomquist, C.L., Belisle, W.H., Bourret, T.B. 2024. First report of Phytophthora taxon ×salinaslettuce (Subclade 8b hybrid) causing stem and basal rot in lettuce in North America. Plant Disease. e-ISSN:1943-7692. https://apsjournals.apsnet.org/doi/10.1094/PDIS-10-24-2155-PDN.
DOI: https://doi.org/10.1094/PDIS-10-24-2155-PDN

Interpretive Summary: The Salinas Valley of California produces more than half of all lettuce grown in the U.S., earning the nickname “the salad bowl.” In 2023 a lettuce root disease emerged across the region. The cause of this disease was identified as a previously unknown hybrid fungal pathogen. Unique DNA sequences that can be used to identify this pathogen were identified. This new pathogen was also given a name to facilitate communication by future researchers and regulators. With the causal agent of this emerging disease identified, mitigation and control efforts, which have the potential to save growers million of dollars in deferred crop losses and pesticide costs, can quickly proceed.

Technical 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’ lettuces with wilted outer leaves were collected from a commercial field in Monterey Co. in April 2023. Brown internal stem and crown lesions progressed into sunken cavities and plant collapse. Incidence was approximately 5%. 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 conspecific with the provisional P. taxon castitis (Supplementary Fig. 1), a member of 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 (Supplementary Fig. 2), suggesting one is the other parent of this hybrid taxon, provisionally introduced here as P. taxon ×salinaslettuce. 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 each for “Valencia”). Phytophthora inoculum was prepared as described in Hao et al. (2019) with the oat seed replaced with long grain rice. 10 ml of 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 3 weeks, 80% of the plants collapsed. Dark brown internal lesions were seen along the stem, crown, and tap roots of the collapsed plants after 4 weeks. ‘Valencia’ inoculated plants were stunted with no internal tissue discoloration observed over the same period. No symptoms were observed on any control plants. Phytophthora 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 sp. was recovered from the controls. This is the first report of Phytophthora taxon ×salinaslettuce; P. taxon castitis has been isolated from strawberry and carrot in Sweden and Canada (Bertier et al. 2013) and Calendula in Japan (Supplemental Fig. 1), 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 ongoing to confirm the hybrid status and parentage of P. taxon ×salinaslettuce with genomics. This emerging pathogen may cause severe economic losses in CA lettuce production during the winter and spring growing seasons.