Evaluation of Annual Bedding Plants for Resistance to White Mold

Abstract

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.