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 leafspot diseases in full-shade (2007-2008), full-sun (2007-2008) and partial-shade (2009-2010) environments in McMinnville, TN, USA. Ten cultivars [ÔAmi PasquierÕ, ÔAyeshaÕ, ÔBlue BirdÕ, ÔForever PinkÕ, ÔFuji WaterfallÕ (ÔFujinotakiÕ), ÔMiyama-yae-MurasakiÕ, ÔSeafoamÕ, ÔTaubeÕ, ÔTricolorÕ and ÔVeitchiiÕ] 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.

Differentiation ofCorynespora cassiicolaandCercosporasp. in leaf-spot diseases ofHydrangea macrophyllausing a PCR-mediated method

“…macrophylla, 18 H. macrophylla ssp. serrata, and three hybrids between the two subspecies) following methods described in Mmbaga et al (2012). In brief, ca.…”

“…Although several fungal pathogens are associated with hydrangea leaf-spot diseases in the southeastern USA, Corynespora cassiicola and Cercospora sp. were previously found to be the most frequently isolated pathogens of the leaf-spot disease complex on garden hydrangea (Mmbaga et al 2012). Although other pathogens such as Phoma exigua, Myrothecium roridum, Botryotinia fuckeliana (Ananmorph Botrytis cinerea), Glomerella cingulata (Anamorph: Colletotrichum gloeosporioides), Glomerella acutata (Anamorph: Colletotrichum acutatum) and Alternaria alternata were isolated at low frequency, these co-occurring pathogens can contribute to the disease complexes and further complicate disease diagnosis and resistance evaluations (Mmbaga et al 2012).…”

“…is the best-known, leaf-spot disease of hydrangea (Sinclair et al 1987;Hagan and Mullen 2001;Williams-Woodward and Daughtrey 2001;Hagan et al 2004;Mmbaga et al 2009;Vann 2010;Mmbaga et al 2012). As a group, fungi in the genus Cercospora are nearly universally pathogenic and are among the most prevalent and destructive plant pathogens on a wide range of hosts in almost all major dicot families, most monocot families, and even some gymnosperm and fern families (Pollack 1987).…”

“…Although some isolates of Corynespora cassiicola have exhibited host specificity (Dixon et al 2009), most isolates have a wide host range and more than 50 ornamental plants have been reported as hosts of Corynespora cassiicola (Ellis 1957;Ellis and Holliday 1971;Farr et al 1989;Chase et al 1995). Both Cercospora and Corynespora leaf-spot diseases tend to have annual outbreaks once the pathogen has become established in an area (Hagan and Mullen 2001;Hagan et al 2004;Zaher et al 2005;Schlub and Smith 2007;Mmbaga et al 2012). When the two pathogens co-occur in a disease complex, it becomes difficult to determine the main cause of the disease because neither fungus readily sporulates in culture (Mmbaga et al 2012).…”

“…Both Cercospora and Corynespora leaf-spot diseases tend to have annual outbreaks once the pathogen has become established in an area (Hagan and Mullen 2001;Hagan et al 2004;Zaher et al 2005;Schlub and Smith 2007;Mmbaga et al 2012). When the two pathogens co-occur in a disease complex, it becomes difficult to determine the main cause of the disease because neither fungus readily sporulates in culture (Mmbaga et al 2012). Furthermore, morphological similarities between Corynespora spp., Alternaria spp., and Helminthosporium spp.…”