Evaluation of Hydrangea macrophylla for resistance to leaf-spot diseases

Abstract

Garden hydrangea (Hydrangea macrophylla) is a popular ornamental plant that can be devastated by leaf-spot diseases. Information is needed to determine susceptibility of commercial cultivars to leaf-spot diseases. To address this need, 88 cultivars of H. macrophylla were evaluated for their resistance to leaf-spot diseases in full-shade (2007-2008), full-sun (2007-2008) and partial-shade (2009-2010) environments in McMinnville, TN, USA. Ten cultivars ['Ami Pasquier', 'Ayesha', 'Blue Bird', 'Forever Pink', 'Fuji Waterfall' ('Fujinotaki'), 'Miyama-yae-Murasaki', 'Seafoam', 'Taube', 'Tricolor' and 'Veitchii'] were rated resistant (R) or moderately resistant to leaf spot under each of the three environments. In 2007-2008, approximately 51% of the cultivars were rated R in full shade, but only 5% were R in full sun. In 2009-2010, only 1% of the cultivars were rated R in partial shade. Although environmental parameters including temperature and rainfall influence disease severity and host reaction, a shaded environment was least favourable for leaf-spot disease development, which demonstrates that establishing hydrangea in shaded environment can be an effective tool along with cultivar selection for managing leaf-spot diseases on hydrangea. Six pathogens, Corynespora cassiicola, Cercospora spp., Myrothecium roridum, Glomerella cingulata (Anamorph: Colletotrichum gloeosporioides), Phoma exigua and Botrytis cinerea, were associated with leaf-spot diseases of garden hydrangea. Of the leaf-spot pathogens, C. cassiicola was most frequently isolated (55% of all isolates), followed by Cercospora spp. (20%) and other pathogens (25%). Because symptoms attributed to each leaf-spot pathogen were similar, cultivars were selected for resistance to multiple leaf-spot pathogens. © 2011 Blackwell Verlag GmbH.