Epidemiological characterization of lettuce drop (Sclerotinia spp.) and biophysical features of the host identify soft stem as a susceptibility factor

Authors: MAMO, BULLO ERENA - University Of California; Eriksen, Renee; Adhikari, Neil; Hayes, Ryan; Mou, Beiquan; Simko, Ivan

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2021
Publication Date: 11/9/2021
Citation: Mamo, B., Eriksen, R.L., Adhikari, N.D., Hayes, R.J., Mou, B., Simko, I. 2021. Epidemiological characterization of lettuce drop (Sclerotinia spp.) and biophysical features of the host identify soft stem as a susceptibility factor. Phytofrontiers. 1(3):182-204. https://doi.org/10.1094/PHYTOFR-12-20-0040-R.
DOI: https://doi.org/10.1094/PHYTOFR-12-20-0040-R

Interpretive Summary: The production of lettuce is affected by several biotic and abiotic factors. Among the biotic factors, lettuce drop is one of the most economically damaging diseases worldwide. The disease is caused by two closely related species of Sclerotinia and lettuce is highly susceptible to both species. The soilborne fungus Sclerotinia minor was not known to produce overwintering sclerotia in the stems of infected lettuce but were observed in a greenhouse in 2017. We investigated lettuce–environment–Sclerotinia spp. interactions in two tolerant and four susceptible lettuce genotypes to determine putative risk factors and targets for disease control. The modern cultivars were susceptible to rapid basal stem and root degradations by S. minor. The oil-seed lettuce the wild lettuce were resilient to degradations and significantly deterred mycelium emergence and symptom development, but sclerotia formed to a significantly higher height in their stems. Photosynthetic efficiency was rapidly reduced in susceptible plants within 1-day post-inoculation but took ~5–6 days in the tolerant lettuce. A strong negative correlation detected between basal stem degradation severity and the stem mechanical strength indicated that stem strength-mediated genetic factors determine the outcome of Sclerotinia infections of the host. Soft stem is a prominent lettuce drop susceptibility factor that could be targeted in resistance breeding. This is the first report of sclerotium production by S. minor in infected lettuce stems representing a new phase in the disease cycle.

Technical Abstract: The soilborne fungus Sclerotinia minor was not known to produce overwintering sclerotia in the stems of infected Lactuca but were observed in a greenhouse in 2017. We investigated lettuce–environment–Sclerotinia spp. interactions in two tolerant and four susceptible Lactuca genotypes to determine putative risk factors and targets for disease control. Symptomatological, physiological, developmental, basal stem biophysical, and microclimate responses (27 variables) of the genotypes were determined under field and/or greenhouse conditions. Distinct patterns of infection responses were observed between modern cultivars and their primitive/wild relatives. The modern cultivars were susceptible to rapid basal stem and root degradations by S. minor. The oil-seed lettuce PI 251246 and the wild L. serriola 11-G99 were resilient to degradations and significantly deterred mycelium emergence and symptom development, but sclerotia formed to a significantly higher height in their stems. Photosynthetic efficiency was rapidly reduced in susceptible plants within 1-day postinoculation (dpi) but took ~5–6 dpi in the tolerant 11-G99. A rapid spike in stomatal conductance was observed in 11-G99 plants within 1–3 dpi, coinciding with the emergence of fungal mycelia at the crown. A strong negative correlation detected between basal stem degradation severity/collapse and the stem mechanical strength indicated that stem strength-mediated genetic factors determine the outcome of Sclerotinia infections of the host. Soft stem is a prominent lettuce drop susceptibility factor that could be targeted in resistance breeding. This is the first report of sclerotium production by S. minor in infected Lactuca stems representing a new phase in the disease cycle.