White mold, caused by the necrotrophic fungus Sclerotinia sclerotiorum (Lib.) de Bary, causes stem rot, crown rot, wilt, and death of many common herbaceous ornamental plants. Relatively little research has been done on management of white mold and resistance to this disease in ornamental crops. Plant varieties from four genera of widely available annual bedding plants with no reported history of white mold susceptibility were evaluated for potential resistance to S. sclerotiorum. All plants were inoculated with three isolates of S. sclerotiorum and evaluated for disease severity in a controlled environment and under field conditions. Portulaca grandiflora Hook. ‘Sundial Scarlet’, P. grandiflora ‘Sundial White’, Pentas lanceolata Forssk. ‘Graffiti Pink’, and Scaevola aemula R. Br. ‘Whirlwind White’ were highly susceptible to white mold in the controlled environment but had significantly lower disease incidence and severity than Zinnia elegans ×angustifolia ‘Profusion White’, the susceptible control, under field conditions. Impatiens hawkeri W. Bull. ‘Sonic Red’, I. hawkeri ‘Sonic Amethyst’, I. hawkeri ‘Sonic White’, I. walleriana Hook. f. ‘Blitz 3000 Red’, I. walleriana ‘Blitz 3000 White’, and I. walleriana ‘Blitz 3000 Rose’ displayed abscission of diseased plant tissue as an unusual resistance response. Plants from all four genera that were evaluated became infected with S. sclerotiorum to a lesser extent than susceptible controls under field conditions and could be used as part of an integrated disease management program for white mold in ornamental plantings.
Sclerotinia sclerotiorum (Lib.) de Bary is a fungal pathogen that causes stem rot, crown rot, wilt, and death of many common annual flowering plants. Infested flower beds often suffer significant plant loss each year, and the identification of disease resistant plants would be a useful management tool. Caladium (Caladium ×hortulanum Birdsey), canna (Canna ×generalis L.H. Bailey), and elephant ear [Colocasia esculenta (L.) Schott] were evaluated for potential resistance to S. sclerotiorum. Plants grown in field conditions in Minnesota in 2012 and 2013 were inoculated through the application of sorghum grains colonized by S. sclerotiorum. The number of plants infected and percent of canopy dieback were recorded weekly for 3 months. The susceptibility of leaves, flowers, and below ground storage organs was also examined through direct inoculation of plant tissue with a mycelial plug of the pathogen in controlled environmental conditions favorable for disease development. Symptoms and progression of the infection were recorded after 24 days. Symptoms of infection on all three species were similar in field and controlled environments. Caladium plants were susceptible to S. sclerotiorum. Petioles, leaves, and corms developed a watery soft rot. Elephant ear was highly resistant to infection. Sclerotinia sclerotiorum infected only wounded or senescent tissue and did not result in significant symptoms under any conditions. Canna had partial resistance 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. This study indicates that canna and elephant ear have resistance to S. sclerotiorum and could be used in an integrated disease management program for infested landscape beds.