Exploring the host range of Sclerotinia sclerotiorum in herbaceous ornamental plants

Abstract

Sclerotinia sclerotiorum is a necrotrophic fungal plant pathogen known to infect over 400 species of plants from 75 families. Many popular herbaceous flowering plants are susceptible. Infested flower beds often suffer significant plant loss each year, reducing their ornamental value significantly. Identification of disease resistant plants would be a useful management tool. The objective of this study was to evaluate species of herbaceous ornamental plants with no reported history of white mold susceptibility for potential resistance to S. sclerotiorum. Plant species included Portulaca grandiflora, Pentas lanceolate, Scaevola aemula, Impatiens hawkeri, I. walleriana, Caladium xhortulanum, Canna xgeneralis, and Colocasia esculenta, and the ornamental grasses Pennisetum glaucum, Setaria italica, Juncus inflexus, Carex flagellifera, Isolepsis cernua, and Acorus gramineus. Zinnia elegans x angustifolia ‘Profusion White’ served as a susceptible control. Disease response was evaluated under field conditions and in controlled environments. Direct inoculation of below ground storage organs of some species was conducted to determine the pathogen’s ability to survive and grow in these plant parts. The effect of temperature and wounding was evaluated in ornamental grasses through inoculation of plants grown at 13, 16, 19, or 22C with and without wounding. Detached leaves of representative ornamental grasses were inoculated and stained to determine the pathogen’s ability to penetrate leaf tissue. In field trials, disease incidence was quantified as the number of plants per bed visibly infected with S. sclerotiorum based on signs and symptoms and disease severity was recorded as percent canopy missing due to death of plants within the bed. In controlled environment studies, lesion length was used as a measure of disease severity. Portulaca grandiflora, P. lanceolata, and S. aemula were highly susceptible in a controlled environment but displayed reduced disease severity in field conditions compared to susceptible controls. Impatiens hawkeri and I. walleriana displayed abscission of diseased plant tissue as an unusual resistance response. Caladium xhortulanum was susceptible to S. sclerotiorum. Petioles, leaves, and corms developed a watery soft rot. Colocasia esculenta was resistant to infection. Sclerotinia sclerotiorum infected only wounded or senescent tissue of this species and did not result in significant symptoms under any conditions. Canna xgeneralis was partially resistant to the pathogen. Although canna petals were readily infected, infection of petioles was restricted to small necrotic lesions. Neither infection progressed to the main stem or resulted in plant death. Acorus gramineus was susceptible in field and growth chamber environments, regardless of wounding and temperature. Juncus inflexus, C. flagellifera, and I. cernua were resistant with minor to no symptom development in field and controlled environment studies. The reaction of P. glaucum and S. italica to inoculation varied under different conditions. Disease severity increased as temperature decreased and with wounding. Microscopy studies of ornamental grasses supported results from the field and growth chamber trials. Mycelial penetration of leaf tissue occurred 24 hours after inoculation (hai) for the susceptible entry A. gramineus, 48 hai for the species with intermediate resistance P. glaucum, and was not observed at 24, 48, or 96 hai for the resistant plant entry J. inflexus. This study indicates that I. hawkeri, C. xgeneralis, C. esculenta, J. inflexus, C. flagellifera, and I. cernua have resistance to S. sclerotiorum and could be utilized in an integrated disease management program for infested landscape beds.