Epidemiological Characterization of Lettuce Drop (Sclerotiniaspp.) and Biophysical Features of the Host Identify Soft Stem as a Susceptibility Factor

Abstract

The soilborne fungus Sclerotinia minor was not known to produce sclerotia in the stems of infected and uncollapsed Lactuca standing intact until our observation in a greenhouse in 2017. We investigated lettuce–environment–S. minor interactions in two tolerant and four susceptible Lactuca genotypes to determine putative risk factors and targets for disease control. Symptomatological, pathophysiological, developmental, basal stem biophysical, and microclimate responses (27 variables) of the genotypes were determined under field or greenhouse conditions. Distinct patterns of infection responses were observed between modern cultivars and their primitive or wild relatives. Modern cultivars were susceptible to rapid basal stem and root degradations by S. minor. Oilseed lettuce PI 251246 and wild Lactuca 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 declined rapidly within 1 day postinoculation (dpi) in susceptible plants but remained intact approximately 5 to 6 dpi in the tolerant 11-G99. Stomatal conductance spiked rapidly in 11-G99 plants within 1 to 3 dpi, coinciding with the emergence of fungal mycelia at the crown. A strong negative correlation detected between basal stem degradation severity or collapse, and stem mechanical strength indicated that stem strength-mediated genetic factors determine the outcome of Sclerotinia infections. Soft stem is a prominent lettuce drop susceptibility factor that could be targeted in resistance breeding and provides the prelude for the analysis of the biological basis of plant architecture-mediated resistance to Sclerotinia spp. in lettuce and other hosts.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .